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Thread: Strange TXV setup
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06-03-2010, 04:18 AM #1
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
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06-03-2010, 05:29 AM #2
Re: Strange TXV setup
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?
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06-03-2010, 05:35 AM #3
Re: Strange TXV setup
I'm thinking there must be more to the story, some means of controlling the temps of the warmer plates.
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06-03-2010, 06:00 AM #4
Re: Strange TXV setup
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
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06-03-2010, 06:35 AM #5
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06-03-2010, 06:46 AM #6
Re: Strange TXV setup
Internal equaliser - TF2 with #4
System diagram.GIF
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
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06-03-2010, 07:01 AM #7
Re: Strange TXV setup
Interesting. If we assume the charge has migrated, then the TXV should be at minimum flow.
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06-03-2010, 07:23 AM #8
Re: Strange TXV setup
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06-03-2010, 07:25 AM #9
Re: Strange TXV setup
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
Never heard of a TXV working like that before though?
Chef
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06-03-2010, 07:28 AM #10
Re: Strange TXV setup
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?
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06-03-2010, 07:28 AM #11
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06-03-2010, 07:35 AM #12
Re: Strange TXV setup
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
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06-03-2010, 07:41 AM #13
Re: Strange TXV setup
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06-03-2010, 07:45 AM #14
Re: Strange TXV setup
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!
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06-03-2010, 08:00 AM #15
Re: Strange TXV setup
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06-03-2010, 08:01 AM #16
Re: Strange TXV setup
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.
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06-03-2010, 08:05 AM #17
Re: Strange TXV setup
Apparently the various glycol solutions determine the different box temps.
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06-03-2010, 08:15 AM #18
Re: Strange TXV setup
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.
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06-03-2010, 08:16 AM #19
Re: Strange TXV setup
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
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06-03-2010, 08:24 AM #20
Re: Strange TXV setup
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.
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06-03-2010, 08:34 AM #21
Re: Strange TXV setup
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
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06-03-2010, 08:41 AM #22
Re: Strange TXV setup
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.
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06-03-2010, 08:44 AM #23
Re: Strange TXV setup
On the other hand, it seems to be working fairly well as a fixed orifice system.
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06-03-2010, 08:45 AM #24
Re: Strange TXV setup
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
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06-03-2010, 08:51 AM #25
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06-03-2010, 08:56 AM #26
Re: Strange TXV setup
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!
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06-03-2010, 09:06 AM #27
Re: Strange TXV setup
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!!!!
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06-03-2010, 09:14 AM #28
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06-03-2010, 09:27 AM #29
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06-03-2010, 03:53 PM #30
Re: Strange TXV setup
Last edited by Gary; 06-03-2010 at 04:39 PM.
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06-03-2010, 04:18 PM #31
Re: Strange TXV setup
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07-03-2010, 01:24 AM #32
Re: Strange TXV setup
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
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07-03-2010, 05:06 AM #33
Re: Strange TXV setup
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07-03-2010, 05:47 AM #34
Re: Strange TXV setup
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
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07-03-2010, 06:03 AM #35
Re: Strange TXV setup
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
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07-03-2010, 06:08 AM #36
Re: Strange TXV setup
Of the two boxes that are not intended to freeze things, the fridge is closest to doing so.
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.
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07-03-2010, 06:52 AM #37
Re: Strange TXV setup
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
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07-03-2010, 06:53 AM #38
Re: Strange TXV setup
What happens to the compressor, does it seize?
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07-03-2010, 07:04 AM #39
Re: Strange TXV setup
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
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07-03-2010, 07:25 AM #40
Re: Strange TXV setup
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07-03-2010, 07:46 AM #41
Re: Strange TXV setup
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
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07-03-2010, 08:09 AM #42
Re: Strange TXV setup
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
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07-03-2010, 08:12 AM #43
Re: Strange TXV setup
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07-03-2010, 08:30 AM #44
Re: Strange TXV setup
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
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07-03-2010, 08:36 AM #45
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07-03-2010, 08:41 AM #46
Re: Strange TXV setup
Moving the bulb to the freezer would solve the charge migration problem.
Then we would see if the other plates are indeed SH cooled.
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07-03-2010, 08:43 AM #47
Re: Strange TXV setup
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
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07-03-2010, 08:50 AM #48
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07-03-2010, 08:54 AM #49
Re: Strange TXV setup
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
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07-03-2010, 09:03 AM #50
Re: Strange TXV setup
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