View Full Version : Strange TXV setup
There have been several recent threads that have made me think about this setup in a new light.
The system has 3 boxes each with holdover plates, the refrigerant passes through each one in turn and running temps in each box are:-
Freezer -10 to -12C (2 plates at -20C)
Refrigerator 2 to 4C (plate -6C)
Salad Chiller 4 to 8C (plate -4C)
But the TXV is inside the freezer and the bulb is inside the Chiller so once the system's down to temps one must assume that the TXV will be sensing the diaphragm temperature and not the bulb's at some instant. Maybe as the system cools down further on cycle the bulb see temps lower than than the freezer?
However, it works, but is very susceptible to charge.
I've already got the message that it may be better to get the TXV say into the chiller but why does it work as it is? Any ideas
Chef
Oh - its R134 and running Dx = 125PSI and Sx = 2PSI
mad fridgie
06-03-2010, 05:29 AM
You are indicating that the evaps plates are in series?
You have single TXV and the bulb is on the outlet of salad evap, you have the superheat set very high?
I'm thinking there must be more to the story, some means of controlling the temps of the warmer plates.
MF - the plates are indeed in series with with one TXV and the bulb is on the outlet pipe of the chiller but I cant really determine the SH as it may be the diaphragm thats trying to control it?
Gary - There is no temperature control of any plate but it is a holding system so it is run for about 1 hour and all the plates freeze, each one has a different ratio of ethylene glycol so it freezes at a different temp and the plates get smaller relative to the box size as the box temps are higher. It is a manual timer and switched on when the system looks like it needs some more cold - say 60 mins in the morning and 30 mins at night.
Chef
External equalizer or internal?
Internal equaliser - TF2 with #4
3410
The simple diagram shows the basic layout but the desuper, condenser, accumulator, dryer, sight glass not shown.
The liquid line to suction exchanger is inside the chiller but the bulb is between it and the chiller plate
Chef
Interesting. If we assume the charge has migrated, then the TXV should be at minimum flow.
Interesting. If we assume the charge has migrated, then the TXV should be at minimum flow.
Hmmm... possibly the incoming liquid helps warm the diaphragm, preventing migration.
Interesting. If we assume the charge has migrated, then the TXV should be at minimum flow.
But there is the rub, the system sucks itself down like a big dog.
If the freezer is at say -6 at startup around the diaphragm (as it is at the top of the freezer) and the suction goes down to 2 to 4 PSI the valve should open but there is a large pressure drop through the system, there is pressure tap just after the TXV and one on the compressor but I cant find the data that I measured a while ago - seem to remember about 8PSI.
Also the TXV is set to a very low SH to get best performance. Seems like it may be working somehow from the diaphragm temp alone?
I am just trying to think out loud here, what if the diaphragm stays roughly constant and the valve meters the refrigerant through pressure drop changes in the evap? As the flow rate increase the pressure increase and so closes the valve a little:rolleyes:
Never heard of a TXV working like that before though?
Chef
mad fridgie
06-03-2010, 07:28 AM
Just a silly question first "Accumulator"
for clarity is a suction accumulator,
do you have a liquid reciever?
What do you mean" but is very susceptible to charge."
As gary asked internal or external equalised?
Hmmm... possibly the incoming liquid helps warm the diaphragm, preventing migration.
It's a Danfoss with the liquid line at the bottom so the liquid heat might not easily get past the colder output port of the valve up to the diaphragm, also feels pretty cold
Just a silly question first "Accumulator"
for clarity is a suction accumulator,
do you have a liquid reciever?
What do you mean" but is very susceptible to charge."
As gary asked internal or external equalised?
Your right it is a receiver and not an accumulator, there is no suction accumulator.
The receiver is also quite small - the type you find on a car A/C system.
The system seems to work more like a cap tube system or orifice system in that a few puffs of gas can move the frost line along the liquid/suction exchanger and vise versa. So when the system is settled in and cycling we check the exchanger and add a little gas to optimise its pull down times.
Internal equalisation.
Chef
It's a Danfoss with the liquid line at the bottom so the liquid heat might not easily get past the colder output port of the valve up to the diaphragm, also feels pretty cold
The diaphragm would be sensing the freezer temp, not the plate temp nor the saturation temp.
mad fridgie
06-03-2010, 07:45 AM
If in doubt go external equalised (what is your evap design pressure drop) This can cause hunting of the valve and prematurely close the valve!
I suspect that there is no or little work being done in freezer plates.
Comp SST -22C, plates -20C, it is likely that liquid is just sitting in the evap, (like a capillary).
TEV not a problem!
The diaphragm would be sensing the freezer temp, not the plate temp nor the saturation temp.
Exactly, so the question is can the valve control on this temp input? Does not seem right really.
So what happens if the valve is just there as an orifice and the system is actually running on a critical charge basis.
The low suction pressure must be a clue I feel?
Chef
Being internally equalized the saturation temp sensed by the TXV is that of the freezer evap. Assuming migrated charge, the TXV is maintaining a difference between freezer evap saturation and freezer air temp, both of which would change very slowly... giving you a fixed flow, very much like a fixed orifice.
Apparently the various glycol solutions determine the different box temps.
Changing to external equalized without changing the TXV location would be interpreted as higher superheat causing higher refrigerant flow, but the flow would still be fixed... albeit at a higher rate. You would need to remove some refrigerant.
If in doubt go external equalised (what is your evap design pressure drop) This can cause hunting of the valve and prematurely close the valve!
I suspect that there is no or little work being done in freezer plates.
Comp SST -22C, plates -20C, it is likely that liquid is just sitting in the evap, (like a capillary).
TEV not a problem!
External equalised might make it worse because of the TXV's position in the freezer.
The freezer plates get cold and stay cold so we have to assume they are working fine.
So here is a what if idea.
What if the system is critically charged and the freezer takes the lion share of cold, then the fridge and chiller take a little but are more responsible for the systems SH (the gas will still be nicely cold enough to cool these smaller plates) and the temp going to the compressor is about 10 or 15C - its not frosted out of the boxes!!
Chef
mad fridgie
06-03-2010, 08:24 AM
External equalised might make it worse because of the TXV's position in the freezer.
The freezer plates get cold and stay cold so we have to assume they are working fine.
So here is a what if idea.
What if the system is critically charged and the freezer takes the lion share of cold, then the fridge and chiller take a little but are more responsible for the systems SH (the gas will still be nicely cold enough to cool these smaller plates) and the temp going to the compressor is about 10 or 15C - its not frosted out of the boxes!!
Chef
Sorry you mis-understand, when your plates are frozen there is no load to boil the refrigerant (only flow is flash gas, with a little liquid carry over, mass of refirgerant is being held in the freezer plates, if you increase your reciever size (thus charge) then liquid could flow correctly to next plates, then valve would control on superheat.
At the moment you are critical charge.
Being internally equalized the saturation temp sensed by the TXV is that of the freezer evap. Assuming migrated charge, the TXV is maintaining a difference between freezer evap saturation and freezer air temp, both of which would change very slowly... giving you a fixed flow, very much like a fixed orifice.
This seems to be the only conclusion but how strange is this phenomena. The setup must be just so coincidentally right that it appears the TXV does something.
If one adjusts the SH on the TXV the suction pressure changes so one assumes the valve is functioning as a TXV but in fact it may be just changing the size of the orifice.
Now thats quite interesting.
So what is the best solution out of this problem. Although I call it a problem but it has worked faultlessly for many years and now its seems not be what it should be ie a TXV controlled system it would be nice to set it right.
Chef
I suspect the different glycol mixtures are what makes it all work regardless.
Moving the TXV and going with external equalizer would freeze the plates faster and more closely control the SH returning to the compressor... and the charge would no longer be critical.
On the other hand, it seems to be working fairly well as a fixed orifice system.
Sorry you mis-understand, when your plates are frozen there is no load to boil the refrigerant (only flow is flash gas, with a little liquid carry over, mass of refirgerant is being held in the freezer plates, if you increase your reciever size (thus charge) then liquid could flow correctly to next plates, then valve would control on superheat.
At the moment you are critical charge.
I get the point about the freezer plates being the controlling part and the suction would drop quickly if we did not turn it off, but the whole point seems to be diaphragm is stuck in the freezer and so the bulb will never get used in the control.
I know I will have to move the TXV to another location to make it work and as you say get a bigger receiver.
Or maybe just leave it as it is and continue to do manual control? Nah - too much of a challenge really.
Chef
mad fridgie
06-03-2010, 08:51 AM
I get the point about the freezer plates being the controlling part and the suction would drop quickly if we did not turn it off, but the whole point seems to be diaphragm is stuck in the freezer and so the bulb will never get used in the control.
I know I will have to move the TXV to another location to make it work and as you say get a bigger receiver.
Or maybe just leave it as it is and continue to do manual control? Nah - too much of a challenge really.
Chef
I dont believe the valve is sticking.
Saying that the system works, why change it!
mad fridgie
06-03-2010, 08:56 AM
From a pure RD perspective, you should only make one change at a time.
If you think you need a bigger reciever, do this first, recharge then test.
This method you can prove 100% where changes happen, if you try all together, I am sure everything would be sorted, but will only guess what you have fixed. human natures says what you think must be true!
I suspect the different glycol mixtures are what makes it all work regardless.
Moving the TXV and going with external equalizer would freeze the plates faster and more closely control the SH returning to the compressor... and the charge would no longer be critical.
Well, controlling the SH might be a good idea as compressor failures are a bi-annual event probably due to poor suction conditions.
However changing anything is a nightmare in space and a mix of swagelok, brazed, SAE and hydraulic hose fittings makes it a headache. I will have to do some major surgery. Its an inherited system.
Chef
PS I bet it would be very difficult to design a system with the correct glycol concentrations and a TXV acting as an orifice from scratch to work!!!!
human natures says what you think must be true!
Now if we can get the everybody to buy into this it will be a day of days.
Maybe we should call you Confucius.
mad fridgie
06-03-2010, 09:27 AM
Now if we can get the everybody to buy into this it will be a day of days.
Maybe we should call you Confucius.
No justed "CONFUSED":D
Well, controlling the SH might be a good idea as compressor failures are a bi-annual event probably due to poor suction conditions.
You forgot to mention that this system had compressor failures.
So what is the best solution out of this problem. Although I call it a problem but it has worked faultlessly for many years and now its seems not be what it should be ie a TXV controlled system it would be nice to set it right.
So... which is it?... working faultlessly or compressor failures?
From a pure RD perspective, you should only make one change at a time.
If you think you need a bigger reciever, do this first, recharge then test.
This method you can prove 100% where changes happen, if you try all together, I am sure everything would be sorted, but will only guess what you have fixed. human natures says what you think must be true!
If this is to be done one step at a time, the first step should be moving the TXV.
You forgot to mention that this system had compressor failures.
So... which is it?... working faultlessly or compressor failures?
The working faultlessly bit is the fact that it cools everything down and has appeared to working as a TXV controlled system, now we know that is not the case.
For many years it has done what it is supposed to but the compressors fail every 2 or 3 years.
We had always assumed this was because the unit is a car A/C compressor designed for air con duty and not LBP freezer work so it gets pushed quite hard. Now with this new info it makes me wonder if the 'orifice' system could help in the failures. Haven't figured that out yet.
The Sx would be about the same with a TXV and an orifice.
With the orifice the boxes get a gradient of cooling that seems to be just right but with a TXV they will all get sucked down -20C in each plate, that may present a new problem.
Chef
The working faultlessly bit is the fact that it cools everything down and has appeared to working as a TXV controlled system, now we know that is not the case.
For many years it has done what it is supposed to but the compressors fail every 2 or 3 years.
We had always assumed this was because the unit is a car A/C compressor designed for air con duty and not LBP freezer work so it gets pushed quite hard. Now with this new info it makes me wonder if the 'orifice' system could help in the failures. Haven't figured that out yet.
The Sx would be about the same with a TXV and an orifice.
With the orifice the boxes get a gradient of cooling that seems to be just right but with a TXV they will all get sucked down -20C in each plate, that may present a new problem.
Chef
I would install a thermostat in the fridge box to control the system... and have it pump down the system.
You may have to explain this one a little more - a temperature control in the fridge box.
Why this box?
This system is run from a diesel powered genset and that is run when needed - not all the time so using a temp control in the fridge would mean it may try and cycle on when the genset is not running. Thats why its manually controlled - run the genset - cool the boxes - make water - charge batteries - heaters - maybe aircon - pumps and systems - then switch it all off and no more noise. Do it all again 12 hours later!
Its just stuff that Superfridge and Taz24 were talking about on another thread that started all this.
Chef
So here is the synopsis of where the system is at:
TXV acting like a an orifice.
Charge migration to the diaphragm
System is controlled by critical charge
The plates operate at different temps and provide some system stability by using SH to cool the last 2 plates
Compressor failures periodically
Receiver to small
But the condenser is OK - what luck.
So here is a working system thats upside down and its so horribly wrong where to begin.
First I need to understand why it did work and figure out how to put it right?
Chef
You may have to explain this one a little more - a temperature control in the fridge box.
Why this box?
Of the two boxes that are not intended to freeze things, the fridge is closest to doing so.
This system is run from a diesel powered genset and that is run when needed - not all the time so using a temp control in the fridge would mean it may try and cycle on when the genset is not running. Thats why its manually controlled - run the genset - cool the boxes - make water - charge batteries - heaters - maybe aircon - pumps and systems - then switch it all off and no more noise. Do it all again 12 hours later!
Power the liquid line solenoid (through the thermostat) off the genset. When the genset starts the system starts. When the plates are frozen and the fridge temp drops dangerously low, the thermostat drops out the solenoid, pumps the system down and shuts off on low pressure. Do it all again 12 hours later.
Hmmm - not really sure I want to add an automatic start and stop to a system that is on a knife edge. We have no problem with manual control after years of experience looking at the box temps and selecting 20mins more or 30 mins more. Anyway its a Ranco unit so I would have to do a manual reset of the low pressure trip each time anyway - and if I forgot the beers would not be cold - doing what we do that would be crime of galactic proportions.
Now it is about fixing it properly - a new TXV and new location, maybe a LBP hermetic and seperate water cooling loop? But this just goes back to all the plates dropping to -20C and the Grand Fromage will have frozen iceberg lettuce and crunchy strawberries. There is always a limit - this is it.
So dual TXV's? If you have an idea its a good time to say as this system gets pulled tomorrow along with everything else and its the ideal time to do complete
revamp
Chef
mad fridgie
07-03-2010, 06:53 AM
What happens to the compressor, does it seize?
What happens to the compressor, does it seize?
Not always - we only had one seize and that made a mess but others just get really noisy and when it sounds 'ripe to be picked' we replace it. Oil is correct quantity in the compressor and clean so can only assume severe duty. We hear other people also have the same problem running these AC compressors as freezer units.
The reason to use them is they are cheap, buy them anywhere, very compact and light and really powerful - all good signs of premature failure I suppose.
Chef
Hmmm - not really sure I want to add an automatic start and stop to a system that is on a knife edge. We have no problem with manual control after years of experience looking at the box temps and selecting 20mins more or 30 mins more. Anyway its a Ranco unit so I would have to do a manual reset of the low pressure trip each time anyway - and if I forgot the beers would not be cold - doing what we do that would be crime of galactic proportions.
Now it is about fixing it properly - a new TXV and new location, maybe a LBP hermetic and seperate water cooling loop? But this just goes back to all the plates dropping to -20C and the Grand Fromage will have frozen iceberg lettuce and crunchy strawberries. There is always a limit - this is it.
What's stopping the plates from dropping to -20 now?
What's stopping the plates from dropping to -20 now?
I was hoping you would have the answers to this question.
Well I think that the orifice situation causes it.
First the unit is critically charged so has less refrigerant in it than it could withstand if the TXV was fully operational.
Second the orifice makes sure the suction pressure is low ie around 2 PSI - its a fixed valve or nearly a fixed valve.
Now the small amount of refrigerant boils of in the 2 freezer plates and the still very cold gas cools the other 2 plates down. As they are smaller it is possible the gas alone (plus a bit of liquid) will do the job.
The system is stopped when the freezer gets to the desired temperature and so no more cooling is given to the other plates.
This is probably why it is charge sensitive. If the freezer plates are cold and the others not we add gas until all cool at the correct rate. If we add too much gas the last 2 plates get too cold before the freezer is down to temp.
What a bizarre situation.
Chef
mad fridgie
07-03-2010, 08:09 AM
Move TXV bulb to freezer suction, insider the freezer cabinet. Problem one sorted, (run low superheat 2-3C)
Install 2 comps in series (2 stage), lower speed of each compressor.
Bigger reciever
If we add too much gas the last 2 plates get too cold before the freezer is down to temp.
Another important clue.
I would put the medium temp boxes on a separate TXV and EPR.
Move TXV bulb to freezer suction, insider the freezer cabinet. Problem one sorted, (run low superheat 2-3C)
Install 2 comps in series (2 stage), lower speed of each compressor.
Bigger receiver
I will have to think about the bulb in the freezer, it may work and its a good idea. Interesting idea as the rest of the system is still on SH cooling.
2 compressors - easier to just replace them every 2 years or so but if I lower the speed of the compressors the flow rate will fall - more problems here I see, lower gas velocity, oil return poor performance etc.
Bigger receiver I got that already, but only if I can get the TXV to work like a TXV - maybe your bulb idea has merit. Think upon this I will.
Chef
mad fridgie
07-03-2010, 08:36 AM
I will have to think about the bulb in the freezer, it may work and its a good idea. Interesting idea as the rest of the system is still on SH cooling.
2 compressors - easier to just replace them every 2 years or so but if I lower the speed of the compressors the flow rate will fall - more problems here I see, lower gas velocity, oil return poor performance etc.
Bigger receiver I got that already, but only if I can get the TXV to work like a TXV - maybe your bulb idea has merit. Think upon this I will.
Chef
slower speed on only 2 compressors, same system mass flow. (increased volumetric efficiency per compressor)
Moving the bulb to the freezer would solve the charge migration problem.
Then we would see if the other plates are indeed SH cooled.
Another important clue.
I would put the medium temp boxes on a separate TXV and EPR.
Just run this one by me - 2 TXV's?
an EPR why.
Are you sure this will achieve the same thing as we have now with the differences in temp between the plates.
I must admit the charge critical solution is not ideal but it does seem a really good solution and I just cant see how 2 valves is going to replicate it.
All the plates will still get down to -20C
Chef
mad fridgie
07-03-2010, 08:50 AM
Just run this one by me - 2 TXV's?
an EPR why.
Are you sure this will achieve the same thing as we have now with the differences in temp between the plates.
I must admit the charge critical solution is not ideal but it does seem a really good solution and I just cant see how 2 valves is going to replicate it.
All the plates will still get down to -20C
Chef
I think the only problem, is trying to find a small enough EPR, I presume the chiller duty is Watts, not Kilowatts
slower speed on only 2 compressors, same system mass flow. (increased volumetric efficiency per compressor)
If the compressors are in series they must run at he same RPM to get the same flow rate but they will have half the pressure ratio.
If the compressors are in parallel then they will need to be run at half speed and and they will still have the same high pressure ratio.
Its a thought but not sure its the best solution.
Chef
Just run this one by me - 2 TXV's?
an EPR why.
Are you sure this will achieve the same thing as we have now with the differences in temp between the plates.
I must admit the charge critical solution is not ideal but it does seem a really good solution and I just cant see how 2 valves is going to replicate it.
All the plates will still get down to -20C
Chef
I am suggesting two parallel circuits.
The first circuit: liquid line>TXV1>Plate1>Plate2>suction line.
The second circuit: liquid line>TXV2>Plate3>Plate4>EPR>suction line.
The EPR maintains higher temp in Plates 3 and 4.
Moving the bulb to the freezer would solve the charge migration problem.
Then we would see if the other plates are indeed SH cooled.
MF has a good idea here and it may still allow SH cooled plates later in the system and avoid the migration.
So we now have a new fridge term - SH cooled - thats going to be tricky to explain but it is a real phenomena!!
Chef
anna.savvy
07-03-2010, 09:08 AM
Nice representation.
mad fridgie
07-03-2010, 09:09 AM
If the compressors are in series they must run at he same RPM to get the same flow rate but they will have half the pressure ratio.
If the compressors are in parallel then they will need to be run at half speed and and they will still have the same high pressure ratio.
Its a thought but not sure its the best solution.
Chef
Car AC comp have avery short piston stroke (especially swash plate), so pressure difference becomes very important as far as volumetric efficiency.
There is a gap between the piston and the discharge valve, this is filled with compressed gas, on the down stroke (suction) no gas is drawn in until the higher pressure gas expands to meet the suction pressure.
So you actual mass flow through the compressor is likely to be quite low. Low volumetric efficiency.
Going 2 stage, reduces the the pressure drop, increasing volumetric efficiency, thus increasing actual mass flow. So slowing the speed down will bring actual mass flow back to original amount. Slower speed less mechanical stress, lower discharge temps.Increased life!!! and is actually more efficient as far as power goes.
(forgot to say interstage does need cooling)
So we now have a new fridge term - SH cooled - thats going to be tricky to explain but it is a real phenomena!!
A new term for a bad idea. High compressor inlet SH is a real phenomena.
mad fridgie
07-03-2010, 09:17 AM
SH cooling is not new, the term we use is "useful superheat" verses non useful superheat (heat absorbed say down the suction line, and can also can be expressed as suction line pressure drop)
MF - I understand the prblems with the swash plate compressors and the volumetric efficiency thats why thet fail a lot but if you run slower then the volumtric flowrate will drop.
These units are are 140cc per rev so if we slow it down it will be less volume and less mass flow.
The pair of compressors must run at the same speed!
Well actually the first one must run at the same speed and the second one must be smaller and designed to run with the output of the 1st compressor - seems quite tricky but solvable.
Chef
Nice representation.
Good to meet you anna.savvy and thanks for joining in.
Please go ahead and ask or contribute as you feel, sometimes the discussions go quite quickly and we gloss over stuff so please interrupt and ask for clarification - might even help us as well.
Chef
mad fridgie
07-03-2010, 09:30 AM
MF - I understand the prblems with the swash plate compressors and the volumetric efficiency thats why thet fail a lot but if you run slower then the volumtric flowrate will drop.
These units are are 140cc per rev so if we slow it down it will be less volume and less mass flow.
The pair of compressors must run at the same speed!
Chef
Reducing the pressure differential across the compressor will increase the actual mass flow (refrigeration effect), thus to keep the mass flow the same, speed must be reduced on each compressor.
Do the 2 compressor need to run at the same speed,
"no" , but for ease "yes"
mad fridgie
07-03-2010, 09:51 AM
Taking some estimates, using 2 of the same type of compressor (as you are presently using)
Low stage 50% speed of present speed
High stage 30% of present speed.
Reducing the pressure differential across the compressor will increase the actual mass flow (refrigeration effect), thus to keep the mass flow the same, speed must be reduced on each compressor.
Do the 2 compressor need to run at the same speed,
"no" , but for ease "yes"
OK if the volumetric efficiency improves because of the lower pressure ratio the compressor can be run slower by the ratio of the efficiencies at the 2 pressure ratios.
It may be at best 20 %. (probably more like 5%)
So I agree it will go a bit slower. But the second compressor will now be getting gas a higher pressure and so the volume will be lower and it will need to be a smaller compressor. If the first compressor is 140cc per rev the second might be around 50cc per rev. The problem here is there are not many AC car units of this
size? In fact I do not know of any?
Double stage compression with inter cooling is a fascinating subject but I am not sure its right for this application - yet.
Chef
size
mad fridgie
07-03-2010, 10:08 AM
maybe, you could give the volume of each eutectic plate (freezing point for each would be nice), then I can calculate duty.
Also what speed is the comp running at?
how long does it take to freeze (from a warm start)
mad fridgie
07-03-2010, 10:13 AM
OK if the volumetric efficiency improves because of the lower pressure ratio the compressor can be run slower by the ratio of the efficiencies at the 2 pressure ratios.
It may be at best 20 %. (probably more like 5%)
So I agree it will go a bit slower. But the second compressor will now be getting gas a higher pressure and so the volume will be lower and it will need to be a smaller compressor. If the first compressor is 140cc per rev the second might be around 50cc per rev. The problem here is there are not many AC car units of this
size? In fact I do not know of any?
Double stage compression with inter cooling is a fascinating subject but I am not sure its right for this application - yet.
Chef
size
Is the point above based upon adiabatic efficiency, which is correct, but volumetric efficiency, I think you will be surpised, how poor your compressor is actually performing at present working condtions.
maybe, you could give the volume of each eutectic plate (freezing point for each would be nice), then I can calculate duty.
Also what speed is the comp running at?
how long does it take to freeze (from a warm start)
The freezer plates are 5 litres each and freeze at -15 to - 20C, the fridge plate is 5 litres and freezes at around 5 litres and freezes at -6C and the chiller is 4 litres and freezes at -4C
The compressor runs at 1800RPM - no variation whatsoever.
From warm it takes a while to freeze - data to hand shows about 2 to 2 1/2 hours from 32C down to -15C but that is without any box load as the units are cooled down before stuff is loaded - with a full load it could be much longer. Otherwise its a max of 1.5 hours a day to keep everything frosty.
Chef
mad fridgie
07-03-2010, 10:35 AM
The freezer plates are 5 litres each and freeze at -15 to - 20C, the fridge plate is 5 litres and freezes at around 5 litres and freezes at -6C and the chiller is 4 litres and freezes at -4C
The compressor runs at 1800RPM - no variation whatsoever.
From warm it takes a while to freeze - data to hand shows about 2 to 2 1/2 hours from 32C down to -15C but that is without any box load as the units are cooled down before stuff is loaded - with a full load it could be much longer. Otherwise its a max of 1.5 hours a day to keep everything frosty.
Chef
Thanks, doing some very BASIC calcs, I think you are running at 30% volumetric efficeincy (very low)
I would go with the parallel circuits (second TXV, EPR). This is a common configuration where two or more temps are maintained off a single machine. The EPR prevents the higher temp circuit from dropping below its preset value.
And I suspect getting some cooling back to the compressor (lower inlet superheat) will greatly extend its life.
monkey spanners
07-03-2010, 06:49 PM
If they are only running 1.5hrs a day but fail after a year, thats not many hours... about 18 days in fridge years!
mad fridgie
07-03-2010, 10:11 PM
Peak load is around 1Kw,
Comp if 100% volumetric efficient should produce 3.7Kw (at stated conditions)
So actual Vol eff. is 27%, actually lower if you consider that peak load includes pulldown, so when just topping up vol eff. is likely to be 20%ish
SH cooling, is around 100watts, slightly lower than required, indicating that there is indeed a little bit of liquid carry over (not much) Which is likely to boiled off within a very short period in the fridge plate.
So the question is, when the bulb is moved into the freezer and freezer load is 100% satisfied, the TXV is closed to its minimum, will this minimum flow allow sufficent liquid carry over to chiller the next 2 plates.
If not, then a simple change of the orifice to a larger size may do the trick.
(based upon R134a)
I would go with the parallel circuits (second TXV, EPR). This is a common configuration where two or more temps are maintained off a single machine. The EPR prevents the higher temp circuit from dropping below its preset value.
And I suspect getting some cooling back to the compressor (lower inlet superheat) will greatly extend its life.
It seems a reasonable idea, if you have an example or link where I can see how its done it may worth the effort.
We have started to pull out the genset today and replace it but I am not sure if a car compressor will fit the new unit till it arrives. If not I might have to fork out for a hemetic compressor of some type, which one I have no idea yet as it has to be 110v 60Hz alla USA style.
Its the obvious time to do a complete refurb but it will mean designing it without the chance of any trials and testing along the way, always tricky with such a kit of bits to work with.
As monkey spanners said it is about 18 (or 23) days of running for each year of operation which is pretty awful when it gets put like that.
Chef
Peak load is around 1Kw,
Comp if 100% volumetric efficient should produce 3.7Kw (at stated conditions)
So actual Vol eff. is 27%, actually lower if you consider that peak load includes pulldown, so when just topping up vol eff. is likely to be 20%ish
SH cooling, is around 100watts, slightly lower than required, indicating that there is indeed a little bit of liquid carry over (not much) Which is likely to boiled off within a very short period in the fridge plate.
So the question is, when the bulb is moved into the freezer and freezer load is 100% satisfied, the TXV is closed to its minimum, will this minimum flow allow sufficent liquid carry over to chiller the next 2 plates.
If not, then a simple change of the orifice to a larger size may do the trick.
(based upon R134a)
I get about 3.5Kw of cooling as well and compressor seems to use a bit more than 1Kw - may be 1.4 or 1.5 Kw. I looked at the specs for the compressor and at a compression ratio of about 10 it has a volumetric efficiency of just 60% which seems very low and could be why its failing early.
I have no way to test the bulb in the freezer idea as we have just pulled out the genset today- more work I fear.
I have no idea if the plates further down stream will cold if this mod is made.
But just looking at the SH cooling available it seems way to small to be effective and is probably not the best option to rely on for controllable cooling.
Chef
It seems a reasonable idea, if you have an example or link where I can see how its done it may worth the effort.
http://www.sporlanonline.com/90-20.pdf
This might be better:
http://www.sporlanonline.com/90-21.pdf
mad fridgie
08-03-2010, 09:24 AM
With the new information, I would suggest you change the refrigerant to R404a (new hermetic LT compressor rated to -40C), change TEV (ext equ), place bulb in freezer, remove liquidline/ suction line heat exchanger (already enough superheat). Leave system as is.
size comp at 1kw -20C SST 20C SH
Reason, at the end of pull down, SST will drop, so will SH, the lower superheat, will freeze the fridge plates.
Ther freezer will get colder, this is never a problem, the chances of actually reaching -40SST are remote, so equipment all ways in spec. Reliable. Plus system at lower temps still in positive pressure
Gary - The pdf showing the 2 TXV's and the EPR's is very good and explains it. I get the idea now and will have to think on this for while.
BUT
Now I have 2 seperate systems but they are piped as a single system. So the freezer plates have 1/2" and 3/8" pipes in parrellel through both plates and they then join the fridge which has 1/2" pipes and the chiller has 5/8" pipes. So now one TXV will run the Freezer and one the fridge and chiller, lower flows in each one, oil return? Maybe a complete repiping of all the plates - not really a option as the piping is mostly not removeable without a chainsaw, its built in.
Chef
With the new information, I would suggest you change the refrigerant to R404a (new hermetic LT compressor rated to -40C), change TEV (ext equ), place bulb in freezer, remove liquidline/ suction line heat exchanger (already enough superheat). Leave system as is.
size comp at 1kw -20C SST 20C SH
Reason, at the end of pull down, SST will drop, so will SH, the lower superheat, will freeze the fridge plates.
The freezer will get colder, this is never a problem, the chances of actually reaching -40SST are remote, so equipment all ways in spec. Reliable. Plus system at lower temps still in positive pressure
Ah thinking out the box again MF. the R404 has some merit but as we already have to carry R22, R134a and R** another 13Kg gas bottle is a stretch and we can only get the 13Kg bottles here.
The idea of having the freezer ridiculously cold is a nice one but I would have to be very convinced the SH cooling was going to do the job. Move away from the orifice and critical charge and may never work again.
As I said in the post to Gary, most of this stuff is built in ( ie part of the structure) and cannot be removed or easily changed so the solution has to be both technically possible and hopefully elegant.
The dual TXV approach proposed by Gary would be fine except for my reservations over the lower flows in each section.
But keep thinking MF thats what your good at.
Chef
mad fridgie
08-03-2010, 10:49 AM
You can get an LT R22 compressor, similar principles (not much info available, as R22 not used here anymore) Worst case if does not work, you have to move probe, set then high superheat setting.
Only thing i would do is fit a high discharge thermostat.
It will work!
Gary - The pdf showing the 2 TXV's and the EPR's is very good and explains it. I get the idea now and will have to think on this for while.
BUT
Now I have 2 seperate systems but they are piped as a single system. So the freezer plates have 1/2" and 3/8" pipes in parrellel through both plates and they then join the fridge which has 1/2" pipes and the chiller has 5/8" pipes. So now one TXV will run the Freezer and one the fridge and chiller, lower flows in each one, oil return? Maybe a complete repiping of all the plates - not really a option as the piping is mostly not removeable without a chainsaw, its built in.
Chef
Liquid rich mixtures carry oil very well. Superheated vapor does not.
Wherever the line is sloped downhill, moving the oil is not a problem. It simply rolls down the hill.
So... oil return is only a concern in the suction line and only where the oil is being moved uphill.
I tend to think that if oil return has not been a problem in the past with the two plates running only superheated vapor, then it will probably not be a problem in the future, despite the decreased flow.
I can't see the system from here, so you will have to be the judge.
mad fridgie
08-03-2010, 08:03 PM
Option 2 "Double expansion"
New TEV (ex equ), feed fridge plates first, then to freezer plates, install hand expansion valve (needle valve) where present TEV is. "Install pressure port either side of HEV"
TEV bulb on freezer plate outlet.
Set hand expansion valve so inlet pressure is -6C.
Effectively as far as control of the TEV the fridge plates and hand expansion valve) are acting as a distributor (pressure drop) You could use an EPR instead of the HEV, but not sure what will happen over the long term to the valve seat with a continious liquid vapour mix.
Only mod to cabinets is a new suction suction from the freezer.
mad fridgie
08-03-2010, 08:18 PM
There is awide range of semihermetics, that will run down to below -40C on R22 (head cooling fan required), good for transport, no internal springs to worry about, only problem $$$$$$$$$$$$
I get the point about the freezer plates being the controlling part and the suction would drop quickly if we did not turn it off, but the whole point seems to be diaphragm is stuck in the freezer and so the bulb will never get used in the control.
I know I will have to move the TXV to another location to make it work and as you say get a bigger receiver.
Or maybe just leave it as it is and continue to do manual control? Nah - too much of a challenge really.
Chef
Under these conditions, could the TXV be replaced with a manual valve?
Under these conditions, could the TXV be replaced with a manual valve?
Sure... but it wouldn't improve anything.
Sure... but it wouldn't improve anything.
Sure, but setting a TXV in a place sure to create bulb charge migration would be a bit inefficient anyway - too many unknowns. Why go to the expense with a system which must surely be operating well outside manufacturer's design envelope.
Either place the TXV in a sensible location, or change to cap tube, or manual valve.
:)
Sure, but setting a TXV in a place sure to create bulb charge migration would be a bit inefficient anyway - too many unknowns. Why go to the expense with a system which must surely be operating well outside manufacturer's design envelope.
Either place the TXV in a sensible location, or change to cap tube, or manual valve.
:)
This is a very bad design. If we reposition the TXV, or switch to a cap tube or hand valve, this woud still be a very bad design.
I am surprised that it works as well as it does. I am not surprised that it eats compressors.
There have been several recent threads that have made me think about this setup in a new light.
The system has 3 boxes each with holdover plates, the refrigerant passes through each one in turn and running temps in each box are:-
Freezer -10 to -12C (2 plates at -20C)
Refrigerator 2 to 4C (plate -6C)
Salad Chiller 4 to 8C (plate -4C)
But the TXV is inside the freezer and the bulb is inside the Chiller so once the system's down to temps one must assume that the TXV will be sensing the diaphragm temperature and not the bulb's at some instant. Maybe as the system cools down further on cycle the bulb see temps lower than than the freezer?
However, it works, but is very susceptible to charge.
I've already got the message that it may be better to get the TXV say into the chiller but why does it work as it is? Any ideas
Chef
Oh - its R134 and running Dx = 125PSI and Sx = 2PSI
Chef, have you perhaps looked at the sequential cool-down curves for each of these chambers?
I wonder if the original designer may not have used the chamber differential temps & locations to force the bulb to provide a level of progressive control. After a certain point, charge migration should ultimately allow the system to run as an orifice-type.
These temp-time plots could provide some very interesting insights, I'd think.
Chef, have you perhaps looked at the sequential cool-down curves for each of these chambers?
I wonder if the original designer may not have used the chamber differential temps & locations to force the bulb to provide a level of progressive control. After a certain point, charge migration should ultimately allow the system to run as an orifice-type.
These temp-time plots could provide some very interesting insights, I'd think.
Are you assuming this was professionally designed? Seems doubtful.
As I see it, the original intent was to equally feed/freeze all of the plates. It worked out with two different temp ranges because the bulb charge migrated.
Dumb luck.
To save lots of quotes I will just answer all the comments in one go.
The system was originally designed and marketed by a USA company - still in business I believe - as a 2 plate system. The 2 plates in the freezer, this was probably installed by someone else and they put the TXV in the freezer and had some little temperature controlled plastic window things that were supposed to open and close and use cold from the freezer to keep the fridge and chiller cold. (they called it spill over)
Well that was obviously never going to work as the size of the plates was too small for all of the 3 box's.
So we added the fridge plate and kept the chiller with the spill over system. Later we added the chiller plate, sized to keep the temp in the box correct for the salad stuff. Then later to enhance performance the liquid/suction exchanger was added. This helped a lot as the charge could be increased and get better cooling but of course I only know that with the hindsight from this thread.
Then a desuper was added to stop the discharge line melting stuff. The TXV was changed to a Danfoss and with trial and error the #4 was chosen as the best performance. Again hindsight shows it to be an orifice system and thats why it was sensitive to the assembly number.
So in answer to your question - it was not professionally designed and underwent many many changes and tweaks to get it to work and now I understand what is going on (mostly anyway) I agree it is amazing it ever produced cold at all but with all the tweaks we did to get it running over the years it is working.
However it is wrong and needs a proper sort out.
As regards the pull down data - I have detailed pressure, temp and time plots for the freezer only and will post it later, the fridge and chiller are simply verified as being cold enough when the plate is frozen all the way across - simply feel it by hand but this always occurs after the freezer is down to temp ie critical charge.
Was it dumb luck it worked - probably but the tweaking made sure it worked as good as it could.
Compressor failures? not sure yet but LBP application and low suction with high dp across the compressor probably caused it.
So I have a schizophrenic system that has psychopathic tendencies. Brilliant.
Chef
So I have a schizophrenic system that has psychopathic tendencies. Brilliant.
Goes with the part of the world you live in... :D
Then a desuper was added to stop the discharge line melting stuff.
Ahhh... another piece of the puzzle.
Discharge temp is directly related to compressor inlet superheat. High discharge temp indicates high inlet superheat which causes very high temp at the discharge valves inside the compressor. This cooks the oil, destroying its lubricating properties... and that explains the compressor failures.
The desuperheater treated the symptom, not the cause. The cause was/is high compressor inlet superheat.
The second TXV with EPR will allow you to regulate the fridge/chiller temp independent of the freezer temp. The fridge/chiller temp is dictated by the EPR. It pulls down to the desired saturation temp and then stays right there while the freezer pulls down to temp. The cold vapor coming back, no longer being over-superheated by the fridge/chiller plates, keeps the compressor and its oil cool.
mad fridgie
10-03-2010, 06:24 AM
Option 2 "Double expansion"
New TEV (ex equ), feed fridge plates first, then to freezer plates, install hand expansion valve (needle valve) where present TEV is. "Install pressure port either side of HEV"
TEV bulb on freezer plate outlet.
Set hand expansion valve so inlet pressure is -6C.
Effectively as far as control of the TEV the fridge plates and hand expansion valve) are acting as a distributor (pressure drop) You could use an EPR instead of the HEV, but not sure what will happen over the long term to the valve seat with a continious liquid vapour mix.
Only mod to cabinets is a new suction suction from the freezer.
Forgot to say any refrigerant
The desuperheater treated the symptom, not the cause. The cause was/is high compressor inlet superheat.
Yeah I was kind of thinking along this line. The outlet temp is very high for 125PSI condensing pressure, I think a hermetic close the evap output (to stop water dripping) is an idea, also to consider with the EPR. Put both TXV's in the chiller (somewhere?) and it may be possible to change the chiller from 5/8 to 1/2" so helping the flowrates.
Not sure about the suction/liquid exchanger?
Chef
Yeah I was kind of thinking along this line. The outlet temp is very high for 125PSI condensing pressure, I think a hermetic close the evap output (to stop water dripping) is an idea, also to consider with the EPR. Put both TXV's in the chiller (somewhere?) and it may be possible to change the chiller from 5/8 to 1/2" so helping the flowrates.
Not sure about the suction/liquid exchanger?
Chef
The freezer TXV should stay in the freezer. Just move the bulb to the freezer.
I think a hermetic close the evap output (to stop water dripping) is an idea...
I have no idea what this means. Can you rephrase this?
At the moment there is a long run of suction line to the compressor on the engine but if I use a hermetic unit then it can go closer to the evap outlet and so with its new low temp suction will not cover everything in dripping water, even to insulate will help maybe but they always want to sweat somewhere along the awkward suction run.
At the moment there is a long run of suction line to the compressor on the engine but if I use a hermetic unit then it can go closer to the evap outlet and so with its new low temp suction will not cover everything in dripping water, even to insulate will help maybe but they always want to sweat somewhere along the awkward suction run.
That makes sense. Would be much easier on the compressor, too. Will this be a problem for the condenser piping?
I am suggesting two parallel circuits.
The first circuit: liquid line>TXV1>Plate1>Plate2>suction line.
The second circuit: liquid line>TXV2>Plate3>Plate4>EPR>suction line.
The EPR maintains higher temp in Plates 3 and 4.
Gary, is correct standard practice is a split system with engine driven eutectic holding plate.
In addition to dual TXV’s a solenoid is a good feature in liquid line to refrigerator TXV controlled by thermostat located in refrigerator.
On holding plate system, suction pressure regulated by an adjustable low pressure switch set to stop compressor when freezer plates are frozen solid is used to control system.
These systems are generally water cooled so a high pressure switch set slightly above max operating temperature.
I assume the compressor is rated from 6000 to 12000 Btu at 2000 rpm when used in a High Back Pressure application. I also assume you are using a swash plate compressor like one of the Sanden models. In a properly designed refrigeration system these compressors have operated without failure for more than twenty years. Repeated compressor failures on eutectic plate systems are caused by:
Incorrect oil level, The oil cavities of these compressor are designed to hold less than 7 ounces oil, in these mutable plate systems additional oil is needed as oil and refrigerant velocities at low plate freezing temperatures is not adequate. Adding to oil return problem there is often many oil trap areas in eutectic plates.
Normal cycling of a eutectic plate system compressor runs one or two hours per day and off twenty two hours. During off cycle refrigerant and oil migrates into very cold plates remaining there until the next startup. Since there are no restrictions in suction side after TXV liquid in plates accelerates rapidly back to compressor. To reduce the possibility of compressor liquid flooding these systems need a suction line accumulator and TXV orifices that are sized to max plate Btu acceptance rather than compressor max capacities.
The engine drive refrigerator compressors that I know of are operated at an rpm that matches maximum Btu that plates can absorb. On a system where the aggregate of all plates eutectic solution is 15 gallons, normal compressor operating rpm is 900 to 1200 with a max short duty cycle or 1600 rpm.
RLK - many thanks for your info.
The system has a low pressure switch but is normally always manually controlled, even so it does drop out if the suction goes past 0PSI. The high pressure switch is set to allow initial pulldown from warm and is about 225PSI.
It is water cooled and the condenser seems fine as the liquid out is just a few degrees above water temp.
The unit is a TM13 which is similar to a Sanden SD5. It is rated at 14,000BTU at 1800RPM for HBP applications and we calculate we are getting about 11,000 to 12,000 BTU at our conditions. The eutectic capacity is 8.6 USgal in all 4 plates
The max BTU acceptance of the plates is unknown at the moment as the coil arrangement inside the plate is unknown and some are sealed so we can never open it to find out. The only way would be to measure the pipes internal volume once the system is apart but it is an approximate method only.
The compressor failure by oil hold up is possible but whenever we have checked it the oil quantity seems OK for the unit - normally we test about 2 or 3 oz in the compressor which is pretty close to the specs for an SD5. However we always test the oil immediately after a cycle so the unit should have the correct oil returned to it and maybe after many hours of off cycle things change. Dont know the answer to that one.
Also it appears we are running with a SH in the range of 30 to 40C (nominal is 35C) whereas the quoted SH for the compressor is 20C at the rated load.
Gary points out this will cause high discharge temperatures (which we do indeed get) and this in turn is a possible cause for failure.
Poor oil return (unproven but possible) and egg frying hot - looks like a recipe for short lived compressor.
Thanks again for your information.
Chef
^^@RLK: Excellent post. Very informative. :)
mad fridgie
11-03-2010, 05:18 AM
RLK - many thanks for your info.
The system has a low pressure switch but is normally always manually controlled, even so it does drop out if the suction goes past 0PSI. The high pressure switch is set to allow initial pulldown from warm and is about 225PSI.
It is water cooled and the condenser seems fine as the liquid out is just a few degrees above water temp.
The unit is a TM13 which is similar to a Sanden SD5. It is rated at 14,000BTU at 1800RPM for HBP applications and we calculate we are getting about 11,000 to 12,000 BTU at our conditions. The eutectic capacity is 8.6 USgal in all 4 plates
The max BTU acceptance of the plates is unknown at the moment as the coil arrangement inside the plate is unknown and some are sealed so we can never open it to find out. The only way would be to measure the pipes internal volume once the system is apart but it is an approximate method only.
The compressor failure by oil hold up is possible but whenever we have checked it the oil quantity seems OK for the unit - normally we test about 2 or 3 oz in the compressor which is pretty close to the specs for an SD5. However we always test the oil immediately after a cycle so the unit should have the correct oil returned to it and maybe after many hours of off cycle things change. Dont know the answer to that one.
Also it appears we are running with a SH in the range of 30 to 40C (nominal is 35C) whereas the quoted SH for the compressor is 20C at the rated load.
Gary points out this will cause high discharge temperatures (which we do indeed get) and this in turn is a possible cause for failure.
Poor oil return (unproven but possible) and egg frying hot - looks like a recipe for short lived compressor.
Thanks again for your information.
Chef
Not sure your calcs are correct, at rated 7.2C SST, you get 14000BTUs, but you are running at -20C SST, you would be lucky if you are getting 5000BTUs,
Your total load presuming 100% water (of course is not) would be 16100 BTU (allowance in SHC, to cover cabinet)
You said pulldown time 2.5hrs, so averaged duty is 6440 BTU, you would expect this as SST would start higher then drop towards the end, to below 5000BTUs
Very high discharge temp has also been caused by the introduction of the suction to liquid line heat exchanger. ( addition SH on and above what is leaving the cabinets)
It was indicated that major changes were not applicable to this system.
If that is not the case, then 2 seperate TEVs and 2 liquid sol valves, fridge off on plate stat. The temperature of the refrigerant is not important?
The freezer sol valve, off when comp goes off (stops liquid migration) Do not worry about pump down, as trying to set a very low differential on this application would be difficult. Because you are using aphase change, OFF cycle pressure will remain constant. Only when completly melted will pressure rise, to late!
Not sure your calcs are correct, at rated 7.2C SST, you get 14000BTUs, but you are running at -20C SST, you would be lucky if you are getting 5000BTUs,
Your total load presuming 100% water (of course is not) would be 16100 BTU (allowance in SHC, to cover cabinet)
You said pulldown time 2.5hrs, so averaged duty is 6440 BTU, you would expect this as SST would start higher then drop towards the end, to below 5000BTUs
I calculated the cooling using the volumetric flow rate of 0.0042m3/hr and a density of 5.1Kg/m3 to get mass flow of 0.021Kg/s.
Then from the PH diagram get H3 and so H4 and also H1 giving a deltaH in the evap of 150KJ/kg.
Kw=mass flow * deltaH = 3.15KW or 10,754BTU
Dont forget that the rated duty is air cooled condenser and discharge of 16bar so when water cooled the discharge pressure drops and the H3 moves to the left giving more cooling than you can deduce from the curves alone.
Pulldown time as I call it is from warm ie room temp and cycle time the period to cool from cold to very cold. The first is 2.5 hours and the second is about an hour.
Not knowing how much ice is still left in the plate when the cycle time is measured does not really give an accurate measure of the duty.
Anyway if all the ice was to be refrozen then 32litres of water at 333j/g yeilds 10.6Mj and at 3.1Kw that would yake 59 minutes so I think the numbers are quite correct but please check them and let me know if got someting wrong.
Chef
mad fridgie
11-03-2010, 06:11 AM
I calculated the cooling using the volumetric flow rate of 0.0042m3/hr and a density of 5.1Kg/m3 to get mass flow of 0.021Kg/s.
Then from the PH diagram get H3 and so H4 and also H1 giving a deltaH in the evap of 150KJ/kg.
Kw=mass flow * deltaH = 3.15KW or 10,754BTU
Dont forget that the rated duty is air cooled condenser and discharge of 16bar so when water cooled the discharge pressure drops and the H3 moves to the left giving more cooling than you can deduce from the curves alone.
Pulldown time as I call it is from warm ie room temp and cycle time the period to cool from cold to very cold. The first is 2.5 hours and the second is about an hour.
Not knowing how much ice is still left in the plate when the cycle time is measured does not really give an accurate measure of the duty.
Anyway if all the ice was to be refrozen then 32litres of water at 333j/g yeilds 10.6Mj and at 3.1Kw that would yake 59 minutes so I think the numbers are quite correct but please check them and let me know if got someting wrong.
Chef
Moving imperial to metric, (working imperial as you uses BTU and gals)
You do not know how frozen the plate is when you restart the machine. But we do know the state when you start the system.
max weight of water 68LB (not quite true as there will be a minimum 7% air gap to allow for expansion, and reduction for volume of refrig pipes)
Start 100F 1BTU/Lb/F
Freezing Point 32F (not quite true but close enough) 144BTU/F (in real lfe is over varing temps) and would be less as contains an anti freeze
Final 0F 0.5BTU/Lb/F
so 68* 68= 4624
68*144=9792
68*32*.5=1088
Total=15504BTU (rounded up to 16100BTU, in my head lol)
Run time 2.5Hrs
Therefore averaged hourly duty 6440BTU (1.88KW)
At the start SST will be high, so actually duty is likely to be higher than the rated 14000BTUs, dowards the end duty i suspect is below 3000BTUs.
Yes you are correct your SCT is lower as well as the sub-cooling, but your SH is whey higher than design, (there is a bit of to and fro)
Your compressor calculations do not account for volumetric efficiency.(which is an unknown)
The freezer sol valve, off when comp goes off (stops liquid migration) Do not worry about pump down, as trying to set a very low differential on this application would be difficult.
Good idea... but the solenoid valve should be in the common liquid line, preventing migration to all of the plates.
Peter_1
11-03-2010, 06:37 AM
http://www.refrigeration-engineer.com/forums/showthread.php?t=18406&highlight=eutectic+migration+liquid&page=3 post 47
Start 100F 1BTU/Lb/F
Freezing Point 32F (not quite true but close enough) 144BTU/F
Final 0F
Start 100F 0.5BTU/Lb/F
so 68* 68= 4624
68*144=9792
68*32*.5=
Well the plate freezes at -20C so we also need to account for that drop in temp and the boxes also have some thermal inertia that should be accounted for and The Grande Fromage will always be adding some food in the fridges and freezer as well but not too much.
So when I add those numbers up I still get the same result for the cooling effect.
Chef
mad fridgie
11-03-2010, 07:01 AM
Well the plate freezes at -20C so we also need to account for that drop in temp and the boxes also have some thermal inertia that should be accounted for and The Grande Fromage will always be adding some food in the fridges and freezer as well but not too much.
So when I add those numbers up I still get the same result for the cooling effect.
Chef
Only half freezes at -20C, the other half is -6C(?)
But we are not dealing with water, so heat capacities stated are higher than actual, and the actual mass is less than calculated, i sort of allowed for this to cover the boxes, (maybe the Grande Fromage, is killing the calculations? lol)
I am presuming these are only little boxes (3-4 cuft each) Thin metal internal sheet coving some sort of blown insulation (so low thermal mass). Slow thermal inertia, need to run again for an hour.
My figures are correct give or take abit.
Your compressor calculations do not account for volumetric efficiency.(which is an unknown)
OK missed the last part of your calcs post - now I see it all.
Your right about the efficiency so if that is factored in I get 2.3Kw which is getting close to your 1.9Kw so it seems we can agree somewhere around this position.
But it is useful to hammer it out and get closer to the real values as it will be needed later to spec out a new compressor if thats the way it finally goes.
Thanks MF
Chef
mad fridgie
11-03-2010, 07:12 AM
OK missed the last part of your calcs post - now I see it all.
Your right about the efficiency so if that is factored in I get 2.3Kw which is getting close to your 1.9Kw so it seems we can agree somewhere around this position.
But it is useful to hammer it out and get closer to the real values as it will be needed later to spec out a new compressor if thats the way it finally goes.
Thanks MF
Chef
It would be strange if 2 engineers could come up with the same figure, when we actually have no real known facts to work with.:D
Peter_1
11-03-2010, 07:27 AM
Thanks, doing some very BASIC calcs, I think you are running at 30% volumetric efficeincy (very low)
Almost impossible to calculate this: you start with warm plates, compressor will evaporate till its max extreme LP limit (can be +10°C), so a very high COP ...IF...the condenser can handle this high load.
As eutectic temperature drops slowly, load drops, LP drops accordingly and so the COP. So you don't have a linear process but a compressor starting at +10°C going to -15°C LP and flattening a first long time at the eutectic point of the liquid with the highest temperature.
For me, there's no problem to connect them all together - we did it several times - but client (in our cases) must be aware that once the -5°C is frozen (where we already went through the eutectic point) that we will subcool it further down to solidify the plates with a colder eutectic solution. So the -5°C plate will while cooling down for the colder plates become also -10°C. If there's then that moment goods it the -5°C box, then those goods could be frozen.
Connecting them all together has an advantage: you only need once a zone in the last plate for superheating, otherwise you will need one in each plate.
All the copper tubes in the plates we make are placed/bended in such a way that refrigerant flows down so that no oil never can be collected in it. We made 4 plates 2 weeks ago in stainless steel from 2 x 1 m x 6 cm thick, connected to a 3/4 HP on R507
All the applications we have run only 6 to 8 hours overnight while holding then the whole day their cold.
The first day when they charge those up from ambient temperature, a charge of 20 to 24 hours is needed.
We're speaking then of a whole vending van.
http://www.lackiererei-bongers.de/images/bofrost.jpg
We have applications with vans starting at -25°C and ending the day at -20°C (in the van)
The size of the compressor is only important if you need to cool it down within some time margin.
As far as this information could help you further (haven't read all the posts) read also once my link I posted some boxes higher here
Only half freezes at -20C, the other half is -6C(?)
But we are not dealing with water, so heat capacities stated are higher than actual, and the actual mass is less than calculated, i sort of allowed for this to cover the boxes, (maybe the Grande Fromage, is killing the calculations? lol)
I am presuming these are only little boxes (3-4 cuft each) Thin metal internal sheet coving some sort of blown insulation (so low thermal mass). Slow thermal inertia, need to run again for an hour.
My figures are correct give or take abit.
The boxes are about 4 cu ft and they are mainly GRP (fibreglass) with some blown foam insulation. But a pull down from 30C is quite a lot of work and as you say the Grande Fromage always messes up the calculations, sometimes she even puts frozen food in the freezer before it is down to temp - what to do eh!
Chef
mad fridgie
11-03-2010, 07:43 AM
The boxes are about 4 cu ft and they are mainly GRP (fibreglass) with some blown foam insulation. But a pull down from 30C is quite a lot of work and as you say the Grande Fromage always messes up the calculations, sometimes she even puts frozen food in the freezer before it is down to temp - what to do eh!
Chef
How dare SHE!!!!
So we need around 2KW, now you have choose what SST. If you choose -20C I believe you will be well on the top side.
Actual data for copeland is available on R22
"Select 7" free down load
For me, there's no problem to connect them all together - we did it several times - but client (in our cases) must be aware that once the -5°C is frozen (where we already went through the eutectic point) that we will subcool it further down to solidify the plates with a colder eutectic solution. So the -5°C plate will while cooling down for the colder plates become also -10°C. If there's then that moment goods it the -5°C box, then those goods could be frozen.
As far as this information could help you further (haven't read all the posts) read also once my link I posted some boxes higher here
Peter the point about the connected plates going below the set temp is part of the thread earlier and if we did indeed connect them and correct the TXV situation we would as you say get frozen stuff in a chiller. Thats really the crux of the problem, how to keep these boxes at the right temp. Seems dual TXV's is possible solution as well as leaving it as it is and just storing a bunch of compressors under a seat somewhere. Just not that elegant.
I read the post and and am not convinced either way about pumpdown - some good points from both sides of the fence really. Do I want to do it? sounds a bit scary and so maybe not.
As for the coils in the plates - pretty sure they start at the bottom and wind their way up the plate in an S fashion but with many oil trap loops.
Chef
Peter_1
11-03-2010, 09:53 AM
In the chiller section, the temperatures decreases indeed to the same temperature as the coldest plate but as soon the compressor is shut off, the chiller plate rises very fast again to its eutectic point. So when charging during night, a small plate of PU is placed before the plate sometimes.
Or you could fill both up to -15°C and place a small isolated plate (we then use forex for this, this a foamed white plastic plate of 3 to 15 mm thick) before the eutectic plate in the chiller with on top of it a small fan (rectangle 120 x 120 as used in PC's) controlled by a lowcost thermostat and a space on the underside of the forex. The cold is then drawn from behind the eutectic plate only when it's needed and you can control temperature. We've done this several times.
In the chiller section, the temperatures decreases indeed to the same temperature as the coldest plate but as soon the compressor is shut off, the chiller plate rises very fast again to its eutectic point. So when charging during night, a small plate of PU is placed before the plate sometimes.
Now this is a very neat idea - just insulate the main face of the plate against direct contact with the goods and the additional cold past the eutectic temp is lost very quickly, probably in just recooling everything in the fridge/chiller down. It would be quite simple to have a little variable door of PU in front and just change this over a small trial period till it is correct.
I can also teach the Grande Fromage to close the sliding door when she has lettuce and open it if full of Castle Beer.
Not much surgery to the system - good news, controllable with small experimentation - also good news.
Thanks for this.
Chef
Now this is a very neat idea - just insulate the main face of the plate against direct contact with the goods and the additional cold past the eutectic temp is lost very quickly, probably in just recooling everything in the fridge/chiller down. It would be quite simple to have a little variable door of PU in front and just change this over a small trial period till it is correct.
Which brings us full circle, back to a spillover system.
As you can see, there are a variety of ways to do this... and all can be made to work. :)
Then a desuper was added to stop the discharge line melting stuff.
Tell us more about your desuperheater, location, water temps, etc. Possibly we can do something with this.
By your description, I am assuming this is a water cooled discharge desuperheater. It may be far more useful as a suction desuperheater.
Which brings us full circle, back to a spillover system.
Well not exactly - the original spillover did not have fridge or chiller plates installed and this option of a PU board in front of the plate is just to protect the goods in close proximity. This is based on putting the TXV in the chiller so it works like it is supposed to.
The desuperheater is a copper tube in tube (1" inner and 1 1/4" outer) unit mounted 2 meters from the compressor discharge and its output goes directly to the condenser which is a coil inside a PVC housing. About 15 turns in a spiral 4" in diameter. Pipe is 3/8" cupro nickel.
All water cooled and temp is 29C on a cold day and 31C on a warm day.
Then to reservoir, filter/dryer, sightglass, diaphragm valve and 4m of 3/8" pipe to the liquid - suction exchanger. Most of it is lagged to stop the liquid getting any hotter en route.
Chef
Well not exactly - the original spillover did not have fridge or chiller plates installed and this option of a PU board in front of the plate is just to protect the goods in close proximity. This is based on putting the TXV in the chiller so it works like it is supposed to.
The desuperheater is a copper tube in tube (1" inner and 1 1/4" outer) unit mounted 2 meters from the compressor discharge and its output goes directly to the condenser which is a coil inside a PVC housing. About 15 turns in a spiral 4" in diameter. Pipe is 3/8" cupro nickel.
All water cooled and temp is 29C on a cold day and 31C on a warm day.
Then to reservoir, filter/dryer, sightglass, diaphragm valve and 4m of 3/8" pipe to the liquid - suction exchanger. Most of it is lagged to stop the liquid getting any hotter en route.
Chef
The water entering the desuperheater is 29-31C?
Absolutely 29-31C - measured with a very accurate thermometer. The water flow rate is very high so it will not increase in temp by more than .1 or .2C through the condenser and then the desuper.
Chef
Absolutely 29-31C - measured with a very accurate thermometer. The water flow rate is very high so it will not increase in temp by more than .1 or .2C through the condenser and then the desuper.
Chef
Not much we can do with water that warm. It was just a thought. :)
Chef., My hundred plus engine driven holding plate system designs all using the SD 508 compressor have provided knowledge of what works and what does not. If your system is not in balance, performance will be poor and compressor’s service life will be at risk. It is true that compressor’s temperature will be too hot to touch during the first few minutes on first pull down of warm plates. If condenser’s size and its cooling medium are adequate high pressure at compressor will not exceed 135 psi using 134a refrigerant. As plate liquid drops one degree per Btu pound and reaches freeze point ice starts to form on coils lowering load on compressor. When holding plate’s evaporator coils reach a temperature 20 degrees below solution’s eutectic point plate is frozen solid and high pressure may drop as low as 95 psi. The important thing to remember is a holding plate is not a conventional evaporator and it is never at a stable thermo dynamic state, as ice forms on coils first reducing heat transfer. Twenty five years ago it was believed that plates in series did not affect daily compressor run times today most of these systems have more than one TXV.
Something you should think about is would you select a 25 ton condensing unit for 4 one ton air handlers in series, this is what you are doing by running this engine driven compressor at 1800 rpm. Just because you might rate a holding plats heat absorption when frozen at 4000 btu does not mean it will except that rate of heat transfer during a short freezing cycle. The 25 ton will remind you of the design error by frequent cycling and tripping temperature restart overload, your out of balance design will not protect compressor.
Perhaps a combined strategy would work best here.
Dual TXV's with insulative shield/spillover to protect product from freezing during pulldown, thus eliminating the need for the EPR. This should take care of the evaporator end of things.
At the compressor end, proper RPM can be established and steps taken to ensure compressor inlet superheat is not excessive.
RLK - thanks for the thoughts. I suppose the main reason it runs at 1800RPM is the genset is USA style ie 60Hz and so runs at that speed and the manual says to drive the compressor with a 5" pulley, one must assume the original installer followed the guidelines.
The manual also shows multiple boxes with serial piping of a single TXV or where the boxes are more apart it shows 2 TXV's with individual solenoids to allow either box to be used independently or together
We pretty much understand the eutectic solution and how it freezes and that's why we set 2 of the boxes with lower concentrations to control the freeze temps and so the box temps, this works fine.
Also the condenser seems to sized OK as it runs now but that may not be the case when the system is changed to one or two TXV's.
The thing we are trying to stop is the excessive Superheat whilst still controlling the box temps within reason.
Thanks for your input
Chef
Dual TXV's with insulative shield/spillover to protect product from freezing during pulldown, thus eliminating the need for the EPR. This should take care of the evaporator end of things.
This does seem to be the best choice at the moment.
But running one loop only at a time or both loops together seems to put a very wide operating envelope on the whole design. Trying to balance it with any one of 3 possible combinations seems tricky. Maybe its better to run both loops at all times?
I also get the impression a bigger receiver is going to be needed.
Chef
This does seem to be the best choice at the moment.
But running one loop only at a time or both loops together seems to put a very wide operating envelope on the whole design. Trying to balance it with any one of 3 possible combinations seems tricky. Maybe its better to run both loops at all times?
I also get the impression a bigger receiver is going to be needed.
Chef
I assume the 3 combinations you are referring to are:
1. Freezer alone
2. Freezer + fridge
3. Fridge alone
Would there be a circumstance where #3 would be needed?
Peter_1
13-03-2010, 07:06 AM
Just read your comment on RLK's post about the high SH:
you normally select the orifice of your TEV for standard running conditions which will be in your case evaporating at -10°C to -15°C. But at startup, this orifice will be far to small so as you know, you will have high SH at startup and this for a long time, especially with plates linked together in serial.
Advantage of connecting the plates in serial is that you only sacrifice one area in one plate for creating SH.
What we have done already done to improve system performance is selecting the orifice for normal running conditions (in your case -15°C) and installing a suction, accumulator with the bulb of the TEV after the suction accumulator)
When we have more than 20 to 25 m length of copper in the plates, we also install a a TEV with external equalization.
Just read your comment on RLK's post about the high SH:
Advantage of connecting the plates in serial is that you only sacrifice one area in one plate for creating SH.
To save you the time of searching back through this post I will precise the situation.
The TXV is in the freezer and bulb is in the chiller so under normal operation the diaphragm on the TXV is controlling and maybe right at the last minute the bulb may take over but this is not known.
So the TXV is acting like an orifice (or a pressure regulated valve) and the last 2 plates then add too much superheat as the system has to be critically charged to work. This is the stumbling block really.
Is it normal to put the bulb after the accumulator?
I think the high superheat during pulldown from hot is acceptable but we have it each cycle even when cold and thats what is bad.
Chef
Chef
I assume the 3 combinations you are referring to are:
1. Freezer alone
2. Freezer + fridge
3. Fridge alone
Would there be a circumstance where #3 would be needed?
Maybe not but its human behavior if the option is there to use it someone probably will -I think I will just stick with option 2 and not have any problems of "well why can't we just run the fridges?"
Peter_1
13-03-2010, 09:35 AM
......
Is it normal to put the bulb after the accumulator?
I think the high superheat during pulldown from hot is acceptable but we have it each cycle even when cold and thats what is bad.
...OK, that's clear for me now.
Suction accumulator: we do it this way to have smallest allowable SH in the plates itself at startup and we need the SA only when system is on regime when orifice is then too big.
Peter_1
13-03-2010, 09:37 AM
Why not simply placing your TEV - perhaps already proposed in earlier posts - in the chiller room?
And install the bulb of the TEV after the HX to increase system performance and reduce a little bit SH in the plates?
You know what you're doing, so I don't see any problem doing it this way.
Each engine driven holding plate refrigeration system is a one of a kind design. Chef, you referred to a manual who was the author? It sounds like the information for series plates and pulley size came from one of my earlier books on do it yourself boat refrigeration.
Small TXV orifices and low compressor rpm are important for compressor longevity. In my earlier post I recommended TXV orifices should be sized to math thermo conductivity of holding plate evaporator coil and not plate size this is important to reduce liquid fluid back on stat up. In normal operation after the first days run liquid refrigerant and oil migrates into cold plates. On the next compressor start TXV’s are at full orifice opening allowing liquid to be pushed and pulled back to compressor. Typically on cold start up accumulator will frost over and suction fitting on compressor will frost up for a short time. The risk of liquid reaching compressor is going to be greater because of your generator's fixed high rpm. There are hundreds of these systems in charter boats around the world. Standard instructions for operating engine driven refrigeration are to engage compressor at low engine rpm.
Balancing superheat is approached differently on engine driven holding plate systems because of the ever decreasing evaporator coil temperature. To cover plate’s evaporator coil's full temperature range the TXV selected must have a full temperature range of -40F to +40F. A freezer plate’s TXV is going to regulate SH for 80 degree F solution temperature down to -20 degree F, so at what point is the SH adjusted? Best to leave manufacturers valve setting alone and monitor suction pressure and SH until plates are frozen solid.
My own sailboat has three holding plates with each having its own TXV and two thermostats to activate two solenoids in liquid lines in order to control boxes and plates separately. A separate small 12 volt condensing unit also supplies refrigerant to extra coils in the same plates using two additional TXVs, if alternative energy is available.
mad fridgie
13-03-2010, 08:52 PM
I think we need to step back from the tech areas for a short while, we need to look at the process.
1: do we need to ensure that pull down is achieved with 2.5hrs.
2: Which cabinet is used the most.
3: Does Chef, want/can make possible major changes. (what can be completed without destroying the cabinets)
4; What level of controllabilty is required.
From my point of view, the system is presently running short of refrigerant, causing high superheat, and then the SH is made even worse by the Suction/liquid line heat exchanger.
mad fridgie
13-03-2010, 09:03 PM
If major changes to the cabinets is not an option,
I would remove the suction.lquid line heat exchanger.
Install suction accumlator
Increase reciever size
Install liquid sol valve (stop migration)
Slow compressor speed
Install insulating strips on fridge evap plates (reduce effect heat transfer area)
Leave TXV bulb where is.
Why not simply placing your TEV - perhaps already proposed in earlier posts - in the chiller room?
And install the bulb of the TEV after the HX to increase system performance and reduce a little bit SH in the plates?
You know what you're doing, so I don't see any problem doing it this way.
This was discussed earlier to move TXV to chiller but it meant subcooling the chiller down to -15C or thereabout so freezing the foodstuff, but your idea of the PU plate resolves this so it is a very nice option. Bulb after the HX is noted and an SA to protect the compressor. Seems the easiest and best option.
Thanks Peter
Chef
RLK - The author is probably C F Horton.
I understand your comment about reducing the reserve capacity of the TXV to help stop floodback and it will help in providing good control at the lowest flowrates.
As most of the liquid will reside in the freezer plates I would think that the higher temp fridge and chiller plates will reduce the liquid floodback when starting up from cold.
The TF2 is from -40F to +50F so that will be fine and I suppose I will just have to wait and see what SH i get the system is running again, but on the TF it is easy to try new SH settings as the control is on the outside.
So what boat do you have - a Jongert? With a shore assist on the plates it sounds as if you have a good redundancy there and if I could, I would fit one to mine but alas no chance now.
Whats your book called?
Chef
I think we need to step back from the tech areas for a short while, we need to look at the process.
1: do we need to ensure that pull down is achieved with 2.5hrs.
2: Which cabinet is used the most.
3: Does Chef, want/can make possible major changes. (what can be completed without destroying the cabinets)
4; What level of controllabilty is required.
From my point of view, the system is presently running short of refrigerant, causing high superheat, and then the SH is made even worse by the Suction/liquid line heat exchanger.
1 - Pulldown can be 3 to 4 hours if necessary but it means running out of spec at high SH for longer.
2 - Usually all 3 the same - full up thats why I think best to keep it as one series system and split usage. If I need just a freezer can always use one of the other 3 systems available, similarly for fridges.
3 - The TXV can be moved easily, the chiller piping can be changed and the HX can be removed. Bulb position can be changed and a SA is easily added.
4 - It runs on a manual clockwork timer - that will not be changed. Just need it to control it's own SH properly.
MF said
If major changes to the cabinets is not an option,
I would remove the suction.liquid line heat exchanger.
Install suction accumulator
Increase receiver size
Install liquid sol valve (stop migration)
Slow compressor speed
Install insulating strips on fridge evap plates (reduce effect heat transfer area)
Leave TXV bulb where is.
Apart from putting the TXV in the chiller this is pretty much where we are right now.
I am very much indebted to you, Gary, Peter and RLK for bashing through this putting wrong to right.
All I have to do now is source it and fit it:D
Thanks
Chef
Placing 4 coils in series is like placing 4 resisters in series. Parallel paths (dual TXV's) greatly reduces the resistance.
Gary - I know you are keen on the twin TXV setup and I see its merits and also its problems, it would be quite simple to add the spurs necessary to change over to 2 TXV's at a later date whilst the whole repiping is going on. I have the TXV in stock as a spare but not different orifices.
Chef
Peter_1
14-03-2010, 12:02 PM
Placing 4 coils in series is like placing 4 resisters in series. Parallel paths (dual TXV's) greatly reduces the resistance.
Gary, these are very small plates with probably 4 m/12 ft max 1/2" coil inside each plate. So we have max 12 m/36Ft coil, so that much resistance will this not give. You even still can use a TEV with int. eq.
Gary, these are very small plates with probably 4 m/12 ft max 1/2" coil inside each plate. So we have max 12 m/36Ft coil, so that much resistance will this not give. You even still can use a TEV with int. eq.
I am not opposed to keeping the plates in series, just pointing out the downside.
The important points here are to:
1. Prevent the fridge products from freezing.
2. Reduce the compressor inlet superheat.
Chef, I do not understand the high super heat this compressor has been used with one TXV on systems with four plates in series and one system with two plates containing 37 gallons of eutectic solution. Two to three hours running the first day with warm boxes yes but there after one hour or less will freeze plates solid again. For plates to hold over 24 hours good insulation is needed and freezer plates needs one gallon of solution for each cubic ft of freezer and ½ gallon for each cubic ft of refrigerator. I believe that compressor suction and high pressure on second days one hour run along with frosted on lines profiles a holding plates system performance. Again a holding plate is not an evaporator it is a energy storage unit. I would like to see the second day pressures and frost areas recorded every 10 minutes for one hour. Then after a couple of hours at rest record parameters again with valve sense bulb hanging lose from line in box. When doing wide open orifice test monitor compressor and stop compressor if frost forms more than one inch down rear plate of compressor. Here is a two hour sample of readings from a balanced system although your reading will not be the same it is the profile changes that defines refrigerant flow. Typical High pressure every 10 min, 135,125,115,100,90,90,90,90.90,90.90,90. Low suction pressure,22,17,12,8,5.3,3,3,1,1,0.-1. Line areas frosted, TXV outlet line, plate connecting tubes and return line to compressor, 20 min line frosted between plates 1 and 2 and frost on return line to compressor, after 20 min no frost on compressor return line.
The SeaFrost people are well respected in the boat refrigeration industry. I bought a new Watkins 32 sailboat in 1983 and installed a used 1978 Sankyo SD508 compressor and it is still ticking today. I can’t post my web site here but Google my name and you will find my site.
Richard Kollmann.
Mike NZ
15-03-2010, 12:41 AM
There have been several recent threads that have made me think about this setup in a new light.
The system has 3 boxes each with holdover plates, the refrigerant passes through each one in turn and running temps in each box are:-
Freezer -10 to -12C (2 plates at -20C)
Refrigerator 2 to 4C (plate -6C)
Salad Chiller 4 to 8C (plate -4C)
But the TXV is inside the freezer and the bulb is inside the Chiller so once the system's down to temps one must assume that the TXV will be sensing the diaphragm temperature and not the bulb's at some instant. Maybe as the system cools down further on cycle the bulb see temps lower than than the freezer?
However, it works, but is very susceptible to charge.
I've already got the message that it may be better to get the TXV say into the chiller but why does it work as it is? Any ideas
Chef
Oh - its R134 and running Dx = 125PSI and Sx = 2PSI
from the diagram it looks like the higher temp coils are operating as saturated and superheated extensions of the freezer coil, making it possible to control the freezer temp by sensing its "extension"
mad fridgie
15-03-2010, 01:50 AM
Chef, I do not understand the high super heat this compressor has been used with one TXV on systems with four plates in series and one system with two plates containing 37 gallons of eutectic solution. Two to three hours running the first day with warm boxes yes but there after one hour or less will freeze plates solid again. For plates to hold over 24 hours good insulation is needed and freezer plates needs one gallon of solution for each cubic ft of freezer and ½ gallon for each cubic ft of refrigerator. I believe that compressor suction and high pressure on second days one hour run along with frosted on lines profiles a holding plates system performance. Again a holding plate is not an evaporator it is a energy storage unit. I would like to see the second day pressures and frost areas recorded every 10 minutes for one hour. Then after a couple of hours at rest record parameters again with valve sense bulb hanging lose from line in box. When doing wide open orifice test monitor compressor and stop compressor if frost forms more than one inch down rear plate of compressor. Here is a two hour sample of readings from a balanced system although your reading will not be the same it is the profile changes that defines refrigerant flow. Typical High pressure every 10 min, 135,125,115,100,90,90,90,90.90,90.90,90. Low suction pressure,22,17,12,8,5.3,3,3,1,1,0.-1. Line areas frosted, TXV outlet line, plate connecting tubes and return line to compressor, 20 min line frosted between plates 1 and 2 and frost on return line to compressor, after 20 min no frost on compressor return line.
The SeaFrost people are well respected in the boat refrigeration industry. I bought a new Watkins 32 sailboat in 1983 and installed a used 1978 Sankyo SD508 compressor and it is still ticking today. I can’t post my web site here but Google my name and you will find my site.
Richard Kollmann.
Hi Richard, i agree with many of your comments, but if you study earlier comments, you will find that the High Superheat is caused by the system being undercharged and by adding a suction/liquid heat exchanger.
If chef correctly charged, his product within the fridges "froze", he became an expert in balancing his load requirements at the expense of the compressor.
It would be interesting to see the load profile (pull down) as he as 3 different eutectic points.
RLK
The figures from warm are
Dx 150,140, 138, 135, 130, 127, 125, 120, 120 PSI
Sx 8, 8, 7, 6, 4, 3, 3, 3, 2.5 PSI
Temp 30, 28, 18, 9, 5, 2, -1, -4, -9 C
The temp is the freezer box temp 2/3 the way up, all the fridge and chiller plates are frozen and the HX is frosted right to the exit of the suction gas, suction line is weeping to compressor. Times are every 15 minutes.
Cycle data is Dx=130, 120 after 15 mins and stays there
Sx=3, 2 after 15 mins and stays there
run time is 50 mins to freeze all plates.
Mad Fridgie - Almost correct, the TXV is in the freezer and so has charge migration and the only way to get it operate was to critically charge it and allow it to work as if it had a fixed orifice, this is probably why it appears to be low on gas, ie the gas is correct as an orifice system but low if it was a TXV system.
Chef
Mad Fridgie - Almost correct, the TXV is in the freezer and so has charge migration and the only way to get it operate was to critically charge it and allow it to work as if it had a fixed orifice, this is probably why it appears to be low on gas, ie the gas is correct as an orifice system but low if it was a TXV system.
Almost correct. If the system were charged correctly as a fixed orifice, the compressor inlet superheat would not be excessive.
mad fridgie
15-03-2010, 03:18 AM
Hi Chef, I do not believe you have a migration problem with TEV. It has not yet be proven that this is the case. All that has been proven is that there is more than enough SH to keep the valve open. If the valve is open then it is a fixed orifice.
What happens to your system if you add more refrigerant? (exclude freezing of food or has this always been your limiting factor?)
If your SST is -20C, even with a SH setting of 10C, you will still get an outlet temp of -10C.
TEVs do not have a limitless SH setting!
One would expect frost at the compressor. Good!
But if the liquid line is not isolated, then liquid will migrate into the evaps, on start up, a slug of liquid will return, this will cause the TEV to close. Giving the impression of TEV migration.
Hi Chef, I do not believe you have a migration problem with TEV. It has not yet be proven that this is the case.
The valve is at -12C and the bulb is at +4C in the chiller before a cycle startup. I think we all agreed a long way back that the diaphragm will have the charge, this is proof beyond a reasonable doubt for me.
Gary - because it is fixed orifice if we add a little bit more the suction freezes to the compressor and if we run it for a few minutes longer it gets really cold, as there is no control on the SH the liquid will just get closer and closer to the compressor till it floods. So it is about right the we operate at the moment - wrong in many respects though as we now know thats why we want to fix it properly.
Chef
mad fridgie
15-03-2010, 04:00 AM
The valve is at -12C and the bulb is at +4C in the chiller before a cycle startup. I think we all agreed a long way back that the diaphragm will have the charge, this is proof beyond a reasonable doubt for me.
Gary - because it is fixed orifice if we add a little bit more the suction freezes to the compressor and if we run it for a few minutes longer it gets really cold, as there is no control on the SH the liquid will just get closer and closer to the compressor till it floods. So it is about right the we operate at the moment - wrong in many respects though as we now know thats why we want to fix it properly.
Chef
In a normal freezer room the TXV is in the room and so is bulb, when the system goes on defrost, TXV can remain at room temperture, the bulb can warm to a temperature above freezing. These do not migrate.
many systems fit the bulb after a suction/liquid Heat exchanger where the bulb is above the room temperture, these do not migrate.
I see that you are using a no 4 orifice, quite big (and to big), but would balance with your original compressor sizing (excluding changes in voulmetric efficiency) Remember that these valves DO NOT close 100%, (generally between 10-25% of max) so at some point there could be a likely hood that the load reduces below that of the valve minimum closing point. When all fluid is frozen and poor heat transfer is occuring.
No 4 orifice around 4 kw at -20C evap 8 Bar pd,
Given there is a HX after the bulb, if the bulb charge has not migrated, then the TXV should modulate closed and not allow frost back to the compressor.
On the other hand, if the suction is frosted back to the compressor, then the bulb (on the other side of that HX) should be cold enough to take back its charge... in which case it should modulate and prevent the frost at the compressor.
So... why does adding a small amount of charge cause frost back to the compressor?
Also, if adding a small amount of charge frosts the suction line after the HX, then the fridge/chiller plates are NOT being chilled by superheated vapor. All of the plates are fully activated.
mad fridgie
15-03-2010, 06:38 AM
Given there is a HX after the bulb, if the bulb charge has not migrated, then the TXV should modulate closed and not allow frost back to the compressor.
On the other hand, if the suction is frosted back to the compressor, then the bulb (on the other side of that HX) should be cold enough to take back its charge... in which case it should modulate and prevent the frost at the compressor.
So... why does adding a small amount of charge cause frost back to the compressor?
I have had a look on the danfoss site does not give superheat control limits (adjustment) but does give varying detail to open 11C SH to open to design. This then shows that you would expect frost. Also valves do not close 100%, so present valve could be giving 1KW of load at maximum shut down, which likely to be greater than the plate load, thus liquid floodback may occur.
The present orifice matches chefs equivelent compressor duty (which practically is not as high as he predicted) I am not saying it has not migrated, but neither would i presume it was.
Here's the good news:
All 4 plates have been running fully activated without freezing the products in the fridge/chiller.
The TXV can simply be moved to the chiller box and the system should run about the same as it has been running. No insulated shield needed if the produxts have not been freezing.
I have had a look on the danfoss site does not give superheat control limits (adjustment) but does give varying detail to open 11C SH to open to design. This then shows that you would expect frost. Also valves do not close 100%, so present valve could be giving 1KW of load at maximum shut down, which likely to be greater than the plate load, thus liquid floodback may occur.
The present orifice matches chefs equivelent compressor duty (which practically is not as high as he predicted) I am not saying it has not migrated, but neither would i presume it was.
If the orifice is oversized and the bulb charge has not migrated, then the valve should be hunting.
mad fridgie
15-03-2010, 09:29 AM
If the orifice is oversized and the bulb charge has not migrated, then the valve should be hunting.
It can only hunt if there is enough refrigerant, and if the minimum close position of the TEV produces less cooling effect than the load.
Chef has mastered critical charge, thus acting like a fixed orifice. (all 4 plates cgarged)
So the question should be how do you prove if the charge within the TEV has migrated.
It can only hunt if there is enough refrigerant, and if the minimum close position of the TEV produces less cooling effect than the load.
Chef has mastered critical charge, thus acting like a fixed orifice. (all 4 plates cgarged)
So the question should be how do you prove if the charge within the TEV has migrated.
The problem at this point is we can't run the system, make adjustments and take measurements.
The TXV is no doubt cranked wide open. It needs to be adjusted to a central setting for a starting point. We need to know the receiver outlet subcooling, evap outlet superheat and compressor inlet superheat near beginning of cycle and near end of cycle.
At this point all we can do is make assumptions and jump to conclusions.
Chef, is the bulb insulated?
Does the system have a sightglass?
It can only hunt if there is enough refrigerant, and if the minimum close position of the TEV produces less cooling effect than the load.
Chef has mastered critical charge, thus acting like a fixed orifice. (all 4 plates cgarged)
So the question should be how do you prove if the charge within the TEV has migrated.
Frost after the HX would indicate that there is sufficient refrigerant for the conditions. If the bulb is insulated, the coil outlet suction line temp should be low enough for the charge to migrate back to the bulb... and at some point during the cycle the load exceeds the minimum close position flow, in which case it should modulate (and hunt if the orifice is too large)... therefore the TXV is adjusted wide open and/or the bulb is not insulated.
Gary - the last two plates freeze to their design spec and the system is turned off manually when it is ready, if you add a little gas it starts to freeze back to compressor, its not a controlled system its run by feel the and stop the timer thats why it has never got back to the compressor (yet).
The bulb is not insulated but taped on with metal film (think cling film made of metal) and system has a sightglass which is pure liquid at all times. A quick guess at the SC is 2 to 4 dergrees but I am away at the moment and dont have the data to hand.
There are many points I have read and would like to discuss but for a few days the big cyclone is taking up many peoples attention.
I still think its running from a nearly fixed temp in the freezer acting on the diaphragm and the pressure changes are just small changes to orifice size - not control but just change.
Chef
Gary - the last two plates freeze to their design spec and the system is turned off manually when it is ready, if you add a little gas it starts to freeze back to compressor, its not a controlled system its run by feel the and stop the timer thats why it has never got back to the compressor (yet).
The bulb is not insulated but taped on with metal film (think cling film made of metal) and system has a sightglass which is pure liquid at all times. A quick guess at the SC is 2 to 4 dergrees but I am away at the moment and dont have the data to hand.
There are many points I have read and would like to discuss but for a few days the big cyclone is taking up many peoples attention.
I still think its running from a nearly fixed temp in the freezer acting on the diaphragm and the pressure changes are just small changes to orifice size - not control but just change.
Chef
The bulb must be tightly strapped and heavily insulated.
Is the sightglass at the receiver outlet?
Gary - the last two plates freeze to their design spec and the system is turned off manually when it is ready, if you add a little gas it starts to freeze back to compressor, its not a controlled system its run by feel the and stop the timer thats why it has never got back to the compressor (yet).
Most compressor manufacturers call for a minimum of 8-11K SH at the compressor inlet. At 2psi, the saturation temp is about -27C. If we add 8-11K SH, that gives us a minimum suction line temp of 16-19C at the compressor inlet. At those temps we would expect frost all the way back to, and maybe even on the compressor.
In truth, the suction line temp could be even lower, down to about 5C, without causing any damage. Frost at the compressor inlet does not necessarily indicate flooding.
The bulb must be tightly strapped and heavily insulated.
Is the sightglass at the receiver outlet?
Its tight strapped but no insulation, even so that pipe never gets closer than say 6C to being as cold as the valve so it does not seem relevant at the moment.
Sight glass is after the filter drier, about 2 feet from receiver.
Chef
Its tight strapped but no insulation, even so that pipe never gets closer than say 6C to being as cold as the valve so it does not seem relevant at the moment.
Sight glass is after the filter drier, about 2 feet from receiver.
Chef
Hmmm... The sightglass shows solid liquid and the orifice is adequate, if not oversized. This should be easily capable of flooding the plates.
Yet the suction line at the final evap outlet does not get colder than the freezer air, which is 17K above SST.
This might lead us to believe there is a restriction between the sightglass and the orifice (undersized/kinked liquid line, restricted HX, partially plugged inlet screen, etc.).
On the other hand, just a little more refrigerant can frost the suction line after the HX.
There is something very wrong with this picture.
Its tight strapped but no insulation, even so that pipe never gets closer than say 6C to being as cold as the valve so it does not seem relevant at the moment.
Sight glass is after the filter drier, about 2 feet from receiver.
Chef
In your part of the world, insulating the bulb should be mandatory practice. The slightest disturbance on the sensing bulb eg. from a breeze, or cooling fan, can make the whole system incredibly sensitive. If it is an MOP bulb, then all the worse.
I saw this first-hand last week. Took a fair process of elimination to locate the main disturbance driver.
Water drips on the bulb can also be interesting disturbance drivers. :)
I tried to look back over the last the 155 posts to find answers to the following questions:
Why is the installation of heat exchanger being question?
What is causing the excessive high pressure on this system?
Who fabricated the holding plates?
What temperature are plate solutions set to freeze at? Plates Set to freeze at less than zero are difficult to freeze with 134a refrigerant.
What is the heat load of freezer box?
How much refrigerant is in this system?
When plates are frozen how long will plates stay frozen?
Refrigerant phase change at end of evaporator plates has happened, is there a chance test nitrogen may still be in system contaminating refrigerant.
How was it determined that there are no major restrictions to flow after TXV in low pressure side of system?
Chef, Someone suggested earlier to move sense bulb to line after freezer plates this may move refrigerant phase change back to freezer If not there could be a major blockage somewhere prior to refrigerator plates.
Having spent twenty years manufacturing more than hundred holding plates and selling engine drive boat refrigeration kits to amateur do it yourselfers I use a simple approach in trouble shoot plate system performance.
When servicing a new system add only one pound of refrigerant and start compressor then if suction pressure after start is less than 10 psi add refrigerant to achieve at least 10 psi. Because refrigerant sight glasses are not all the same I recommend that additional refrigerant to eliminate bubbles be avoided until frost is visible on exit line of last plate. When the small automotive receiver is used a full charge on most small systems is 2 to 3 pounds of refrigerant adding more than this only fluids seawater condenser.
The boating industry standards for one HP engine drive refrigerant piping was set by designs from Crosby, Grunert, Frigoboat and SeaFrost. All piping from TXV through plates was ½ in OD. Suction line piping from plates to compressor are either ½ inch OD or ½ inch ID. Crosby and Grunert both used heat exchangers between liquid and suction lines. Because seawater was used as a condenser cooling medium a manually operated seawater bypass was used to control super cooling if high side pressure was too low. Most condensers were oversized to allow for cruising in tropical waters. On system designed and sold in cool climates like Seafrost’s units or units from companies in the northwest refrigerant charges must be reduced or compressor needs to be operated at a lower rpm.
All of the above engine driven drive system seem to demonstrate the same pressures and temperatures as refrigerant is free to flow through normal phase changes. I was contracted to train six mechanics from three charter boat companies in the islands. In two weeks we boarded and tested around 40 boat engine drive refrigerators. Most systems were one holding plates some had two or three plates. Each units performance was measures by gauge pressures, plate freezing time and frost migration. On systems that performed well high pressure was between 135 and 120. Temperature of liquid line between filter/dryer and TXV was around 115 degree F. TXV temperature after 10 minutes running time was frosted and too cold to keep hand on. Light frost forming on line between plates on and two. Frost eventually was present on line leaving last plate. After plates were frozen compressor temperatures were cool. Suction pressure dropped to zero or 10 inches vacuum.
When you consider that the coils inside a holding plates are the same as coils in evaporator the only difference is in the heat absorption dynamics of eutectics solution is much slower than finned plate evaporators. If standard evaporators are efficient because the heat absorbing phase change occurs shortly after interring evaporator then TXV superheat must be effective in first holding plate. Years ago I had a open top holding plate on test stand and using a super heat bulb test chamber I am sure these tools are no longer available refrigerant was vented to cool bulb, I could monitor affects of superheat change on coils ice formation. If refrigerant phase change do not occur in the first plate that plate will not freeze.
In your part of the world, insulating the bulb should be mandatory practice. The slightest disturbance on the sensing bulb eg. from a breeze, or cooling fan, can make the whole system incredibly sensitive. If it is an MOP bulb, then all the worse.
I saw this first-hand last week. Took a fair process of elimination to locate the main disturbance driver.
Water drips on the bulb can also be interesting disturbance drivers. :)
In every part of the world the bulb must always be insulated. Its purpose, its very reason for existence is to sense the suction line temperature and ONLY the suction line temperature. It cannot accurately do so if it is exposed to the surrounding air. The temperature sensed by that bulb controls the flow of refrigerant throughout the system.
This is one of those seemingly small details that can make a HUGE difference.
I tried to look back over the last the 155 posts to find answers to the following questions:
Why is the installation of heat exchanger being question?
Not sure which exchanger you mean but there is no real question about this, we are trying to find out why the system works with a migrated TXV, critical charge and some other indicators that suggest something is wrong but we cant yet seem to understand it.
What is causing the excessive high pressure on this system?
I dont think there is excessive pressure?
Who fabricated the holding plates?
Seafrost the 2 freezer plates, unknown fridge plate but quality maker from UK, chiller is by us.
What temperature are plate solutions set to freeze at?
-20C -6C and -4C respectively through the system
Plates Set to freeze at less than zero are difficult to freeze with 134a refrigerant.
If you mean 0F then I see no reason why it wont go lower -25C should be easy?
What is the heat load of freezer box?
Depends on how full it is.
How much refrigerant is in this system?
Not sure
When plates are frozen how long will plates stay frozen?
24 hours with minimal door opening and loading of stuff
Refrigerant phase change at end of evaporator plates has happened, is there a chance test nitrogen may still be in system contaminating refrigerant.
Not a chance - 2 stage vac down
How was it determined that there are no major restrictions to flow after TXV in low pressure side of system?
Each plate and whole system suction side tested with flow rate and measured pressure drop using a water manometer - there is no blockage.
Chef, Someone suggested earlier to move sense bulb to line after freezer plates this may move refrigerant phase change back to freezer If not there could be a major blockage somewhere prior to refrigerator plates.
I believe Mad Fridgie suggested this as a possibility but it was discussed and decided the superheat cooling was not enough to cool the other plates properly and so was removed as an option
On system designed and sold in cool climates like Seafrost’s units or units from companies in the northwest refrigerant charges must be reduced or compressor needs to be operated at a lower rpm.
As you mention they use 1/2" pipe and we have changed this to 5/8" to better match the compressor performance.
Hope all this helps clear the problem for you
Chef
Hmmm... The sightglass shows solid liquid and the orifice is adequate, if not oversized. This should be easily capable of flooding the plates.
Yet the suction line at the final evap outlet does not get colder than the freezer air, which is 17K above SST.
This might lead us to believe there is a restriction between the sightglass and the orifice (undersized/kinked liquid line, restricted HX, partially plugged inlet screen, etc.).
On the other hand, just a little more refrigerant can frost the suction line after the HX.
There is something very wrong with this picture.
Exactly - its a system that works (with dumb luck as you mentioned earlier) and there are little or no signs of why it works but wrongly. Everything looks right except the SH at the compressor and that is so delicate with charge.
There is no blockage as system was flushed with flushing liquid several times and tested for flow with a manometer system.
DesA points out that the bulb should be insulated (agreed) but it still wont make any difference as it never gets colder than the TXV and Mad Fridgie reckons the #4 could still provide enough flow at minimum turn down - could be onto something there.
Chef
Exactly - its a system that works (with dumb luck as you mentioned earlier) and there are little or no signs of why it works but wrongly. Everything looks right except the SH at the compressor and that is so delicate with charge.
There is no blockage as system was flushed with flushing liquid several times and tested for flow with a manometer system.
DesA points out that the bulb should be insulated (agreed) but it still wont make any difference as it never gets colder than the TXV and Mad Fridgie reckons the #4 could still provide enough flow at minimum turn down - could be onto something there.
Chef
Adding charge drops the SH (drops the suction line temp). So...
Insulate the bulb and add charge. The suction line temp will drop until the bulb temp is below the TXV temp and will thus regain its charge. If the SC is then excessive, there is a restriction.
A restriction could be an undersized or kinked liquid line, plugged TXV inlet screen, restrictive HX, etc..
Insulate the bulb and add charge. The suction line temp will drop until the bulb temp is below the TXV temp and will thus regain its charge.
Except if his Madness is right (usually is) the #4 is too big and works only with migration so if the bulb is allowed to regain control it might not be able to turn down enough.
So put in a smaller orifice and wont wont work properly till the bulb eventually gains control if it ever does.
Not sure I like that scheme of things
Chef
Except if his Madness is right (usually is) the #4 is too big and works only with migration so if the bulb is allowed to regain control it might not be able to turn down enough.
So put in a smaller orifice and wont wont work properly till the bulb eventually gains control if it ever does.
Not sure I like that scheme of things
Chef
Once the bulb is in control... if the TXV hunts, the orifice is oversized.
Adequate bulb insulation is a must. Do that & monitor the results.
Adjusting charge, or orifice, or a number of other things can come later. :)
Hunting can come about due to a number of reasons, not only charge, or orifice-related. Take a close look at the height between TXV actuating plate & bulb, orientation of filter-drier, liquid-line feed into TXV (important), size of external balance line. Once the pipework is correctly set up & the bulb adeqately insulated from environmental effects, this will be a good start.
Hmmm... two more plates and a HX were added, with presumably the same liquid line. An undersized liquid line would not be out of the question.
Hmmm... two more plates and a HX were added, with presumably the same liquid line. An undersized liquid line would not be out of the question.
Now you will have to explain this one, 3/8" line - same compressor, suction controlled flow rate - what can possibly get outside of the parameters acceptable for this line by adding plates.
Speculations about many things are maybe OK but it does not get us any closer to understanding how the system worked as it was.
I am still convinced it was a critically charged orifice system not under txv control and have yet to see any compelling evidence otherwise. No blockages, no kinks, no hunting, line sizes OK and orientation of filter drier OK - though that is also a scratch the head item - what has it really got to do with the issue at hand?
Chef
Do you have any details of the liquid line details prior to entry into the TXV? The route from condenser exit, through sightglass, flter-drier, into TXV are in mind here.
The piece of pipe immediately preceeding the txv is incredibly important in systems where txv operation is marginal, for whatever reason. This follows general basic common-sense design principles for control valves.
If you could post a picture of this set-up, I'm almost convinced we will find an explanation for part of the system behaviour.
(I had such a case-in-point a week, or so, ago. Was able to adjust the pipework in such a way that the system became rock-solid stable, off a system that was previously totally uncontrollable. It too had been acting as a fixed-orifice system until remedied).
Now you will have to explain this one, 3/8" line - same compressor, suction controlled flow rate - what can possibly get outside of the parameters acceptable for this line by adding plates.
Speculations about many things are maybe OK but it does not get us any closer to understanding how the system worked as it was.
I am still convinced it was a critically charged orifice system not under txv control and have yet to see any compelling evidence otherwise. No blockages, no kinks, no hunting, line sizes OK and orientation of filter drier OK - though that is also a scratch the head item - what has it really got to do with the issue at hand?
Chef
The TXV bulb charge is sensing freezer temp or it is sensing chiller temp. Either way it is not at its minimum flow position. It is running wide open with an orifice that is at least adequate if not oversized.
If there was solid liquid at the orifice the system would be flooded. Therefore, there is solid liquid at the sightglass but not at the orifice... ergo there is a restriction in between. Adding refrigerant drops the suction temp because it is overcoming the restriction.
The only times I have seen these conditions before was when a major restriction after first plates increased their pressure or when freezer plates have no eutectic solution in them. If the freezer plates are SeaFrost’s vertical cast aluminum plates the front cover sealant has failed before allowing solution to leak out. For a -20 C solution plate to freeze solid suction pressure on this type system needs to reach a vacuum of 9 inches. What is line temperature after each plate when compressor suction pressure is 4 psi or lower?
I would use a #1 orifice as it reduces fluid back problems although it extends first days pull down time. A #2 orifice will better match combined eutectic plates Btu capacity.
The only times I have seen these conditions before was when a major restriction after first plates increased their pressure or when freezer plates have no eutectic solution in them.
If the restriction were after the first plates, then the first plates would not be at -20C. The restriction must therefore be before the orifice.
If restriction were before TXV this restriction should cause a pressure drop and line cooling after restriction. SH
between TXV and sense bulb is very high but what is SH between each plate and sense bulb?
If restriction were before TXV this restriction should cause a pressure drop and line cooling after restriction.
I agree.
SH between TXV and sense bulb is very high but what is SH between each plate and sense bulb?
On my P/T chart -20C is about 4.5psi. In order to drop the plate temp to -20C, the pressure in the first plates must be somewhat less than 4.5psi. The pressure at the compressor inlet is 2psi, so the pressure drop from the first plates to the compressor can be no more than 4.5-2=2.5psi and in fact must be somewhat less than 2.5psi... probably more like 1-1.5psi.
mad fridgie
21-03-2010, 08:28 PM
Sorry gentlemen, I have been away for a week (sales show, i was like a duck out of water)
Chef has shown that this is a critical charge working on a fixed orifice, regardless if the system is right or wrong. If he added more refrigerant then his fresh products froze. This shows that "most" but not all of the refrigeration effect in the chillers was produced by sensible cooling, which leads to real increased superheat. The superheat entering the compressor is further increased due to the suction liquid HX.
What seem to be upsetting the applecart is the full sight glass, If we assume that the sightglass is an aid to charging and not a definite method, then we could ignore the sightglass, then the system as charged is acting as one would expect. Critical charged system.
Adding more refrigerant will bring the TXV into play, either correct control if orifice is the correct size or some level of hunting if oversized, either way at this point we do not have a low enough superheat to close the valve.
There are no mystery pressure drops!
What is required is some form of reduction of heat transfer between the plates and the chiller boxes (to ensure product does not freeze)
A full compliment of refrigerant then can be added. At this point can you see if the valve seletion is correct.
I would add a suction accumulator, just as a measure of protection.
Insulating the bulb always a good idea, but at this stage has no real relevence.
I am with RLK use a number 2 orifice.
The number one goal of this system is to keep the products frozen and chilled, the system works as a critical charge.
A full sightglass with high SH proves that there is indeed a restriction. We can view the full sightglass as a guide and compensate by adding refrigerant... however this backs up liquid upstream from the sightglass and raises high side pressure, thus raising energy usage.
Sorry gentlemen, I have been away for a week (sales show, i was like a duck out of water)
Chef has shown that this is a critical charge working on a fixed orifice, regardless if the system is right or wrong. If he added more refrigerant then his fresh products froze.
Apparently I missed the post where Chef said adding refrigerant froze the products?
mad fridgie
21-03-2010, 09:48 PM
A full sightglass with high SH proves that there is indeed a restriction. We can view the full sightglass as a guide and compensate by adding refrigerant... however this backs up liquid upstream from the sightglass and raises high side pressure, thus raising energy usage.
It could be as simple as a bit of flash gas forming in the liquid line (after the sight glass) this would reduce mass flow through the valve, increasing the charge would increase head pressure, a increase actual mass flow through the valve, which in turns increase the nett refrigeration effect, which in turn increases the total heat of rejection. At present the condensor is only running about a 5/6C split, therefore only a small amount of liquid sub-cooling is possible.
The main problem is increased refrigeration effect freezes the fresh product, hence reducing the refrigerant charge resolves the problem, (less latent energy being transfered in the chiller plates) at the expense of the compressor (very high superheat)
mad fridgie
21-03-2010, 09:59 PM
Apparently I missed the post where Chef said adding refrigerant froze the products?
Page 1 , 37
Frozen lettice:D
It could be as simple as a bit of flash gas forming in the liquid line (after the sight glass) this would reduce mass flow through the valve, increasing the charge would increase head pressure, a increase actual mass flow through the valve, which in turns increase the nett refrigeration effect, which in turn increases the total heat of rejection. At present the condensor is only running about a 5/6C split, therefore only a small amount of liquid sub-cooling is possible.
The main problem is increased refrigeration effect freezes the fresh product, hence reducing the refrigerant charge resolves the problem, (less latent energy being transfered in the chiller plates) at the expense of the compressor (very high superheat)
The restriction is probably a relatively minor point for this particular system, assuming the high side pressure does not rise substantially with increased charge.
Insulating the bulb and increasing the charge should activate the bulb and put the TXV in control. The orifice should also be downsized if it causes the valve to hunt. And the plates should be shielded if the products freeze.
mad fridgie
21-03-2010, 10:13 PM
The restriction is probably a relatively minor point for this particular system, assuming the high side pressure does not rise substantially with increased charge.
Insulating the bulb and increasing the charge should activate the bulb and put the TXV in control. The orifice should also be downsized if it causes the valve to hunt. And the plates should be shielded if the products freeze.
100%,
only other point to worry about is the minimum closing of the valve (unknown), we do not want liquid flood back either.
I would add a suction acc (just for protection, if large orifice is kept in use)
A liquid solenoid to be used in the off cycle to stop liquid migration to the plates. Again reducing the chance of flood back, and helps slightly with stored load
Page 1 , 37
Frozen lettice:D
He didn't say his lettuce froze when he added refrigerant. He said if all his plates got to -20C, then his lettuce would freeze.
Well... yeah
He just needs to shut the system down when the fridge/chiller is down to temp.
I see no evidence that the insulated shields are needed... but it wouldn't hurt. :)
mad fridgie
21-03-2010, 11:09 PM
He didn't say his lettuce froze when he added refrigerant. He said if all his plates got to -20C, then his lettuce would freeze.
Well... yeah
He just needs to shut the system down when the fridge/chiller is down to temp.
I see no evidence that the insulated shields are needed... but it wouldn't hurt. :)
My iterpretation, chef can answer this one.
You can not simply shut down on temp, which of the 3 boxes are you going to use, also remember that they are eutectic plates in series, so you require prelonged run time at setpoint (to freeze the solution) Again he has managed to control the system (as far as the product goes) succefully for many years at the expense of the compressor.
By sheilding the plates, he can reduce the suction superheat (allowing the plates to drop in temperature) and increasing the life of the compressor. It also may give the advantage that the time between recharges can be increased (suspect freezer dependant.)
oldesky
22-03-2010, 12:48 AM
Chef, I would suggest an electronic tx valve. Say a sporlan with a Kelvin controller. But what ever brand you prefer. This will eliminate any TX valve charge migration issues.An Accumulator and suction line solenoid fitted in the suction line to the compressor may help. If the compressors are failing regularly it may be due to oil dilution. The accumulator and sucton solenoid should help to prevent this.
After all this, why bother with a TXV at all? If the bulb is not going to be properly insulated, or the correct in/out piping arrangement set up, then it is operating in an uncontrolled state. Throw it overboard.
Go fixed orifice & be done with it. Keep it simple. :)
It seems Mad Fridgie has the exact same position on the operation as I do.
The lettuce does freeze if we increase the charge as the chiller plate then gets subcooled below its normal freezing point. Hence just enough charge to see all the plates nicely frozen but at the expense of the compressor
RLK - the seafrost plate has been modified so it cannot leak again and we have installed a check plug so we can see how the fluid properties are.
A #2 orifice as suggested by you and MF is useful input at this stage.
desA I do not believe the inlet and outlet piping are wrong as it is just a simple run and there is enough SC to see liquid at the TXV (or whatever it is pretending to be) and is about 5 or 6C.
Not too sure about chucking it overboard as most of it can be used with some small modifications to get it right.
oldesky - See you are from Brisbane and I was there all last week, shame. The idea of the accumulator is on the new build agenda and I will have to look into an electronic valve but I prefer the simpler TXV.
So this is whats on the new build schedule so far.
Move the TXV body into the chiller to stop migration.
Fit a #2 orifice
Increase the size of the receiver to about 2 or 3 times the auto unit fitted.
either leave the bulb before the HX or move after it - thats an easy mod at any time.
INSULATE the bulb - got that one.
Place PU plates in front of the last 2 plates to avoid freezer burn and keep Grande Fromage happy - key ingredient here.
Install an accumulator in suction line.
Install stubs so its easy to go to a 2 TXV system if it looks like it may be necessary as a last resort.
Fully insulate the suction line to the compressor - the difficult part.
Flush system, new drier, charge, cooldown and then drink cold beers.
Any more additions for the wish list?
Chef
mad fridgie
22-03-2010, 06:10 AM
"then" drink more beers?
should read "whilst" drinking more beers.
I would fit a valve in the liquid line to stop migration during the off period.
(hand or solenoid)
I do not believe the inlet and outlet piping are wrong as it is just a simple run and there is enough SC to see liquid at the TXV (or whatever it is pretending to be) and is about 5 or 6C.
That's not possible... but there is no sense in arguing the point. :)
Ah your correction is duly noted, drink more beer to ensure a proper installation, best advice yet.
With a suction accumulator do you really think the solenoid valve is needed? It assumes the compressor valves are perfect! Never had cause to check if they are and if they can hold the charge for up to 24 hours.
Chef
That's not possible... but there is no sense in arguing the point. :)
Whats not possible - which part - explain?
If there is something needed to to be done about the piping then give it a go and I will include it in the new build specs - best time is right now.
Chef
Whats not possible - which part - explain?
If there is something needed to to be done about the piping then give it a go and I will include it in the new build specs - best time is right now.
Chef
It is not possible to have 5-6C SC at the TXV inlet.
It is not possible to have 5-6C SC at the TXV inlet.
OK this is the figure at the exit of the condenser so pressure drops will lose some SC along the way but it has the HX and this could add some SC but dont know how much. By hand feel only it is just colder than room temp so maybe 1 or 2 degrees down - a guess.
Never measured it at the TXV but the liquid line has a permanent thermometer on it so we can see what temp it is at during all stages of the operation and it is normally a couple of degrees above the water temp.
Chef
mad fridgie
22-03-2010, 08:09 AM
Ah your correction is duly noted, drink more beer to ensure a proper installation, best advice yet.
With a suction accumulator do you really think the solenoid valve is needed? It assumes the compressor valves are perfect! Never had cause to check if they are and if they can hold the charge for up to 24 hours.
Chef
Got to do something while waiting for vacuum to be completed "drink beer"
Extra valve an ideal, acc will protect.
I except the point about compressor valves.
Solution to all technical problems - drink beer.
The bigger the problem, the more beer required... :D
Chef, Holding plates are not efficient evaporators they are for storing heat absorbing energy. There are two things to remember about holding plates they are difficult to freeze solid in a short period of time without very low temperatures do to surface area of evaporator coil inside and slow to cool box do to plate’s limited surface area. If you resealed their covers you see how SeaFrost attempts to increase coils surface area by placing a bent flat sheet of copper in with the evaporator coil. With a -20 degree C solution and 134a refrigerant it would be difficult to freeze these plates solid in a reasonable amount of time. Eutectic solution phase change first starts on evaporator coil’s surface, it begins to reduce heat conductive of coil as the ice builds. Thermo heat conductive between coil and plate’s exterior skin can be as much as a delta T of 10 degrees F. Before the phase change liquid temperatures changes rapidly assisted by thermo tumbling of liquid at a rate of one btu per degree F per pound of solution. For the actual latent heat phase change to be completed quickly with 134a refrigerant a low suction pressure at the end of day’s compressor run is necessary, this is why I suggested an ending gauge vacuum of 9.8. All of the maybe one hundred freezer holding plates I built had eutectic solution set at -12 to -13 C.
mad fridgie
22-03-2010, 08:15 PM
Hi Chef, sorry did not pick up the part about difficulty in insulating the suction line, I think you also brought up the problem with water droplets being a concern. If this is a real problem insulating, then here is a solution
Fit TXV bulb on to outlet of 2 box (there is sufficient energy in vapur to cool final plate)
Leave Suction/liquid heat exchanger in.
This should increase actual temp and reduce droplet formation (not completly)
We have very high superheat!
Add a small TXV or capillary and inject into Accumulator inlet which is adjent to the compressor, the will then reduce SH to an acceptable level at the compressor inlet.
If i have mis understood, then disregard.
MF - now thats an interesting idea, and a cap tube into the accumulator, good idea and thats probably the one thing I will right. It can have a higher flow on startup to keep the SH down and less as the system cools down during the mid phase. Then when the system starts to pull very low suction the tube could be sized to be sonic at the outlet which would limit its flow to a set point say at 2PSI - even it it falls further the tube flow would stay the same. A very cool idea indeed. Probably also need a solonoid valve in the line to cut off the tube once the system is stopped?
The fun bit will be calculating all the flow rates and changes in SH. Maybe even a dynamic model of it - to see how the Sh changes with pulldown.
Thanks
Chef
Chef, At this point you should approach this system’s poor performance by directing your efforts to nondestructive changes. If you follow all the advice given you will end up with a Rube Goldberg system. Rube Goldberg was educated as an engineer, but made a living as a comic strip illustrator. His best-known creations were crazy machines designed to show a complicated way to perform a simple task.
Adding a capillary tube in suction line or adding a solenoid in suction line is not simple it is over kill. Oversize TXV orifice is not going to cause enough pressure hunting on a holding plate system to affect short term performance. Relocating TXV is not going to make much of a freezer plate correction but moving its temperature sense bulb will.
Frost on return line back to compressor inlet fitting is normal at times during plate temperature pull down. If suction line condensation is a problem then insulate it.
You have proven that conventional evaporator superheat methods can not be used on series holding plates. Set the TXV TE-2 valve back to Danfoss setting of 3 turns open, then move temperature sense bulb to line exiting second freezer plate. This will now allow freezer plates to freeze and possibly plates 3 and 4 if not 3 and 4 reduce valve superheat setting to increase flow to plates 3 and 4. You must monitor frost return at compressor in earlier stages of a warm plate pull down with the #4 orifice. Later change to #2 orifice should solve the liquid return. On systems with one plate or multible TXVs most system designers would use a suction line accumulator.
In a series holding plate system compressor run time to freeze freezer plates solid will temporarily control refrigerator plate temperature. Ounce refrigerator plate’s solution is frozen very little energy is stored at temperatures below the eutectic freeze point of solution. So when compressor stops refrigerator plates temperatures will quickly rise to frozen solution temperature. To prevent food from freezing in refrigerator while compressor is running kept it away from plate or cover lower portion of plate with insulated.
mad fridgie
23-03-2010, 09:13 PM
RLK, what we do not know is how difficult it is to insulate the suction line, only that he stated it was difficult, and we also do not know how important the water droplets are.
We do know that the inlet superheat needs to be dropped. If the suction line practically can not be insulated, and water droplets are an issue, then we have superheat the vapour, this can be achieved by placing the TXV bulb on the outlet of plate 3, adjust TXV as stated, there will be sufficient mass flow to freeze the last high temp eutectic plate, then the suction/liquid HX will increase superheat even further. We then need to reduce superheat at the compressor, it is common practice to install liquid injection (OK this normally directly into the compressor via a propriety device)
If he can insulate, then no need for liquid injection
Mad fridgie, What you do not understand is this system is not conventional balanced refrigeration system, it is a one ton compressor freezing maybe 70 pounds of ice in 4 small containers in less than one hour. In this type of one TXV four plate system heat is absorbed first in freezer plates than overflow saturated liquid vapor moves to the next warmer plate’s low pressure coil. Do to the mass refrigerant flow and poor thermo conductivity evaporator coils in plates there is always the possibility of vapor heat exchange in suction line. I only pointed out that the #4 orifice will at the beginning of one hour run increase moisture on return line. Whether suction line is insulated or not will have little effect on plates freezing process. Compressor temperature is expected to be very warm for several minutes as warm plate solution the first day is cooled down. If compressor remains hot till the end of compressor run SC in condenser is inadequate or system has too much refrigerant.
mad fridgie
24-03-2010, 12:32 AM
Mad fridgie, What you do not understand is this system is not conventional balanced refrigeration system, it is a one ton compressor freezing maybe 70 pounds of ice in 4 small containers in less than one hour. In this type of one TXV four plate system heat is absorbed first in freezer plates than overflow saturated liquid vapor moves to the next warmer plate’s low pressure coil. Do to the mass refrigerant flow and poor thermo conductivity evaporator coils in plates there is always the possibility of vapor heat exchange in suction line. I only pointed out that the #4 orifice will at the beginning of one hour run increase moisture on return line. Whether suction line is insulated or not will have little effect on plates freezing process. Compressor temperature is expected to be very warm for several minutes as warm plate solution the first day is cooled down. If compressor remains hot till the end of compressor run SC in condenser is inadequate or system has too much refrigerant.
What you are trying to explain is a load profile, (same as ice builder, batch blast freezer) high at the start reducing at the end, this is fully understood, the system works well as far as cooling in the cabinets go, that has never been in question.
The suction line insulation and condersation is a practical issue, not related to the boxes, if he can not insulate and does not want condensation, then he needs to superheat the vapour, but he also wants his cabinets to work for he desired period. We have already proved vapour only out of the freezer plates will not freeze both chiller plates, some liquid is required, if liquid is passing through the first 3 plates then there is sufficient vapour at the right temperature to freeze the 4th plate, (superheating the vapour) the bulb on the outlet of plate 3 will control the valve.
If we look at his problems, if add more refrigerant it freezes his product (all agreed need some insulation on chiller plates)
Very high suction superheat, causing premature compressor failure, would be resolved just by adding refrigerant (and a few tweeks)
Then he has condensation problems, but has difficulty insulating the suction line, "catch 22" solution as stated install liquid injection at the compressor (i said accumulator inlet, to allow for a small level of play in sizing) if I suggested at the comp inlet and cap selection was incorrect he could end flooding the compressor and were are back to the start again.
We have very high superheat!
Add a small TXV or capillary and inject into Accumulator inlet which is adjent to the compressor, the will then reduce SH to an acceptable level at the compressor inlet.
How low would you allow the SH to go, in this case?
Where would you place the existing txv bulb?
Could the evap/accumulator be allowed to run wet, with TXV bulb placed at outlet of accumulator?
mad fridgie
24-03-2010, 02:28 AM
How low would you allow the SH to go, in this case?
Where would you place the existing txv bulb?
Could the evap/accumulator be allowed to run wet, with TXV bulb placed at outlet of accumulator?
This was only needed if insulating the suction line was not practical. (answers based upon this assumbtion)
1; Superheat range at the compressor 5-15C
2; Bulb position of existing TXV outlet of plate 3
3; With a TXV fitted to acc, bulb would be at acc outlet, not really wet (refrigeration wise) I would partially insulate the acc. (I hear arguments coming, the acc is really a knock out pot, even with a bit of insulation liquid will boil off during long off periods)
This was only needed if insulating the suction line was not practical. (answers based upon this assumbtion)
1; Superheat range at the compressor 5-15C
2; Bulb position of existing TXV outlet of plate 3
3; With a TXV fitted to acc, bulb would be at acc outlet, not really wet (refrigeration wise) I would partially insulate the acc. (I hear arguments coming, the acc is really a knock out pot, even with a bit of insulation liquid will boil off during long off periods)
Many thanks, MF.
Would there ever be a situation where SH, as measured at accumulator outlet (wet acc), where SH would hover ~ 0-1K?
mad fridgie
24-03-2010, 02:42 AM
Many thanks, MF.
Would there ever be a situation where SH, as measured at accumulator outlet (wet acc), where SH would hover ~ 0-1K?
Very common, "flooded systems" by design, practically many refrig systems, due to a number of reasons, incorrect valve selection, low loads, fixed orifice types.
Very common, "flooded systems" by design, practically many refrig systems, due to a number of reasons, incorrect valve selection, low loads, fixed orifice types.
Thanks again.
Would this be a bad thing, for extended periods, or just part of the way things are?
Should this be taken care of? If so, how? (In other words, is it difficult to 'rip' the SH off the 0-1K position?)
mad fridgie
24-03-2010, 03:01 AM
Thanks again.
Would this be a bad thing, for extended periods, or just part of the way things are?
Should this be taken care of? If so, how? (In other words, is it difficult to 'rip' the SH off the 0-1K position?)
This really is down to 2 things, absorbing refrigerant into the oil, loosing lubricating properties and the chance of forming liquid in the compression chamber.
It is "deemed" as a bad thing, if refrigerant is direct in to compression chamber, less effect on the oil and if the compressor is in a high ambient (above SST) then less chance of liquid condensing in the compression chamber.
getting close to my limits of knowledge on specifics, at this point on rely on component manufactures recomendations. it maybe worth starting a new thread, I am sure there are those out there who know more about the specifics
mad fridgie
24-03-2010, 03:04 AM
Missed the how bit,
A simple pressure drop or what i was taught a cooling off coil, which actually meant a warming coil,
Suction/liquid heat exchanger
Thanks again, MF. Was just wanting to cover the bases on dry/wet accumulators & consequent high/low SH - with possible effects on compressor.
I'd expect that the determining SH would really have to be at final value taken at compressor suction & a judgement call made.
I see Sporlan have an injection valve for this purpose - TREV Y-1037.
Missed the how bit,
A simple pressure drop or what i was taught a cooling off coil, which actually meant a warming coil,
Suction/liquid heat exchanger
Thanks for that. :)
Mad Fridgie - a first look at the cap tube idea shows we might need between 1/8 of system flow for a 10K drop in SH and 1/4 of the system flow to get 20K drop in SH, both quite high relatively and will drop performance of quite a bit.
The suction line is built inside a wood former and water dripping of it would eventually cause damage - not good and then it passes by some sensitive equipment - although this could be shielded its again not ideal.
So I feel it has to be best to just cut it off and run a new suction line where it can be insulated. Solves several problems in one go. I am pretty sure there is a way to run it if we change some steel supports to get access.
Shame as the challenge of the injection system was of interest.
RLK - I fully get the plate freezing process and ice generation from plate 1 thro plate 4 as the system cools down but putting the bulb on the outlet to the freezer is out as the SH will not be enough to cool the last 2 plates.
Anyway - small changes are not possible as the system is now laying on the floor in pieces - so it has to redesigned and built in one go and all the correct changes made in one giant leap.
Chef
A thought, Chef.
Sketch your proposed new circuit layout. Take pics of all the parts. Paste them up on the board. You'll be able to settle your design fairly quickly I'd think.
mad fridgie
24-03-2010, 05:11 AM
Good that you are replacing the suction and insulating, does save a number of hassles.
Just a point for your own interest, liquid injection would have very little effect you net refrigeration, The refrigerant would be more dense entering the compressor, thus more mass flow.
I would leave leave bulb on the outlet of the fourth plate (then play which side of the hx, keeping SH at reasonable level)
You have to keep her indoors or should i say onboard happy!
Chef, it is always difficult to convince refrigeration service techs that on series holdover plates there are other ways to control superheat and flow. There is also no comparison between bulk ice production load profiling and freezing a -20 degree eutectic solution with 134a refrigerant in a system. All this is a mute point since system is apart and can now be redesigned. Over the years I have designed more than 100 engine drive boat refrigeration systems. There are over 60 different in service boat holding plate designs in my fourth DIY boat refrigeration book. Although I have built many systems with three plates in series, I recommend separate TXVs for each box to reduce daily engine run time. The valve orifices would both be #01 on your dual valve system. After the suction line T connecting all plates return lines back together I would use at least 7 ft of 5/8 tubing to compressor’s vibration flex tube or install a suction accumulator in this line.
Peter_1
24-03-2010, 07:14 PM
I've not written DIY books and have not installed hundreds of these systems - well, at at least 30 and we made most of the plates ourselves - but why make it all that difficult.
Move TEV to the hottest place, eventually in the compressor compartment, leave the plates in line and select the orifice slightly a little too large and install a suction accumulator. Did it this way tens of times without any problem. This is not a high tech install, isn't it? And cover the chill plates while charging them. Once charged and compressor disconnected, they will rise within 15 minutes to their eutectic point.
If you're concerned about high SH, you still could install a TEV with SV injecting its liquid at the inlet of the suction accumulator to control SH. Posted in other posts on RE pictures of this (capacity controlled compressor)
Peter_1
24-03-2010, 07:17 PM
Forgot, do a pump down or install an SV in the liquid in line with the compressor, otherwise the liquid will migrate to the cold plates. (Perhaps this was already mentioned in earlier posts but I will not read all 200 posts)
mad fridgie
24-03-2010, 07:56 PM
I agree with Peter, this is not a complex system, if we had been on the boat, all would of been sorted over a beer or two, it is somewhat easier to resolve problems, when you can actually see. For example replacing the suction line, I do hope those who have been watching on the sidelines have found it informative.
Peter_1
24-03-2010, 08:04 PM
These are very learn-fully posts (is this the correct expression learn-fully?) for everybody.
These are very learn-fully posts (is this the correct expression learn-fully?) for everybody.
I believe the word you are looking for is "informative".
Forgot, do a pump down or install an SV in the liquid in line with the compressor, otherwise the liquid will migrate to the cold plates. (Perhaps this was already mentioned in earlier posts but I will not read all 200 posts)
the good news is that the charge will migrate to the freezer plates (coldest plates). Since it then has to go through the warmer fridge plates, there should be no problem with floodback on startup. So, a pumpdown is not needed.
IMSTIEN
28-03-2010, 05:21 PM
Gary I like your design with two txv's and the Epr valve.I have done some modifications of this sort in the past It works like a charm.
sterl
29-05-2010, 06:10 PM
This thing would be a monster to design: Need to know the volume each palte, the volume each well of glycol and on and on...
Simple to find out what changes with the TXV diaphragm near liquid temp: Heat the power head, temporarily. The actual temperature not a detail concern as long as it is above the anticipated max bulb temperature during operation.
Almost certainly the system running critical charge, and TXV is at near-fixed postion once suction pressure has dropped during the restart; and once the coldest section has frozen out: the area of refrigerant evaporation moves on to the next-warmest and so on. Also means the coldest, now frozen plate partially full of liquid refrig; thus circuit will act short charged and receiver volume would be of benefit.
But other than establishing the point: Why bother? I assume the compressor that fails every year orr so is a Wobble plate? Acting at 2-4 Psi Suction? Yes, could be an issue....
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