Getting Colder, need advice

[mvtop]

Hi,
I need some advice on how to proceed if I’m looking to improve my cooling.

keep in mind I’m not in the hvac field, and don't do this for a living, but have some basic knowledge on the subject.
But mainly I just happen to have some old soda machine parts and enjoy tinkering and learning.

Here's my current situation;
My system is setup to chill a container of moving water/salt solution, about 3gal.
Which is used to fast chill beverages

The system consists of an insulated container, such as a large stew pot, that is wrapped with copper tubing that acts as my evaporator, the compressor is a 1/3 hp, 4500 btu, its running with r134a. Not sure about the condenser it came from a soda machine using the same type compressor. I’m using suction accumulator, not sure if its needed or not, but I had it and wanted to be safe.

The system works and chills well down to 27deg F.
I was shooting for around 10deg. F.

I thought the evaporator coil wasn't big enough to cool adequately. So I tried different evaporator configurations, such as more wraps around container, larger tubing, smaller tubing, different length cap tubes, etc.
after everything is reconfigured, adjusted, and recharged, it still only gets it down to about 27deg.


Now I’m thinking R134a isn't going to get it cold enough with my setup. So I’m leaning towards swapping out the compressor for one that can run r507a/r404a in order to get colder.
I would like to get colder than needed and then tune it back a bit to the temp I’m looking for.

So, I need some advice from the folks that do this daily.

thanks mvtop

[Chef]

This sounds very experimental and can be most interesting.

What is the salt/water ratio and do you have water starting to freeze in the 'pot'. If you do this will hold the temperature at the freezing point dictated by the solution mix ratio and it will not get any colder till the ice forms and forces the eutectic solution to a lower temperature. This can take quite a long time.

A picture would be most useful to see how the pot and coil are configured?

If you have some pressures to add it would also help.

Chef

[Gary]

It's not about the refrigerant. R134A is easily capable of reaching 10F... or much lower.

For starters you need a solution whose freezing point is well below 10F.

Then you need to fine tune the cap tube. For this you need to learn about subcooling and superheat.

Why do you need to reach 10F?

[Yuri B.]

Hi. I think there is too small heat transfer between evap and the container it is "wrapped" around. Apply substance that transfers heat, or immerse evaporator tubing into the solution, but then, replace salt in it by something more friendly to copper.

[mvtop]

Thanks for the quick respones.

Yuri.b
i've tried the copper in the water didn't like the setup of it and it doesn't work with my finished idea.

Chef.
good point about the solution ratio i didn't think that could have that much of an effect. I did have some ice forming. not anymore. I have a 20% salt solution now. And my temp is down to 11 deg F.
Brrrr.
I've included some pics too.

Gary.
yes the solution seem to be holding me back a bit.
i've been playing with the cap tube a bit. Since i changed my coil size a few times i needed to increase the flow by shorting it. i'm at the shortest length they recomed for a .040. i forgot the exact length but i'm around 32". The coil of 3/8 tubing takes a lot.
I've read a lot of your posts here on this site and many many others. you get around the web.
Infact i'm using your approch for tuning the system.
this might not be quoted exact but I think it came from one of your posts.
"Bottom line: At design cutout temp, if the SC is 15F or less and the compressor inlet SH is 20-30F (preferably closer to 20F), the system will function properly, be it TXV or cap tube"
so with that, my SC is 14 and SH is 19.
low side is 7.1 and high is 128. the suction inlet is
about 20 deg.
i've been working the SC mostly as to measure the charge. So, i'm thinking any more refig. added could start flooding the compressor. am i'm charged correctly?

i'm thinking this is more of your work Gary.

"When considering any charging method, whether it is TEV or cap tube, define what you are trying to accomplish.

At design temperature/load:
Our first goal is to flood the coil. (I think i got this)
Our second goal is to cool the compressor. (This too)
Our third goal is to NOT flood the compressor. (watching this)
Our fourth goal is to NOT back liquid up into the condenser. (watching this too)
We need to keep in mind that the charge is most accurate AT DESIGN TEMP"

with my different coil configurations and cap tube lengths, i got very different low and high side reading. which i guess that would be right, if have more restriction in the cap tube you'll be adding head pressure.

why 10deg.? Just a good starting point. i would like to get to see if i could get it lower still.

i've added some pics of the non insulated version runing and not running so to show it better.

So at this point, feed back would be most helpful.
pointed towards:
how to cool better/faster, it does take about 40 min or more to get down to temp. which seems way to long.
what could be inproved on.
And please do point out things that just way off base.

Gary keep up the good work. your posting are of great help. here and out on other sites. i've referenced your posts when tinkering with phase cooling.

again, thank you for your help and suggestions.

Mvtop

[Gary]

The balance between SC and SH tells me the cap tube is just right, as is the charge. The high side pressure of 128psi converts to SCT of about 102F. If we can assume an ambient temp of about 72F, this gives us a TD of 30F over ambient, indicating a good strong load.

In short, your numbers seem to be near perfect, given the current load.

Congratulations.

[Chef]

mvtop - As Gary points out it all looks very good.

Meanwhile as you have the system right there in testing mode would it be possible to run it from ambient down to 10F and measure the discharge pressure, the suction pressure and the temperature of your solution say every 5 minutes. This information would be really helpful and may shed some light on any fine tuning to increase performance.

Chef

[Gary]

Performance can be improved in a couple ways:

The cap tube can be soft soldered to the suction line between the evaporator and the accumulator. This cools the liquid on it's way to the evaporator, reducing flashing at the evaporator inlet thus improving it's performance.

Adding insulation is a good thing. If it frosts, or even sweats, insulate it.

After making these changes you will probably need to remove a little refrigerant.

[mvtop]

Chef.
i'll post some more pressures and temps next week.
It was still in the process of warming up. 8hours and still hadn't reached ambient temp. its well insulated now.

Gary.
thanks for the tip on the cap tube. i'll need to rearange things a bit since the tube in short.

I do have a couple of questions on coil size and compressor size.
1. if i change the size of the coil tubing, say smaller, and but keep the same volume, will i gain higher pressures or is the pressures related to the restriction of the captube and its length? i plan to redo the coil yet again, but with a tighter wind and smaller tubing. so, i'm assuming that things would be similar to what we have now but maybe better performance, assuming we keep the volume the same on the coil with the same length captube.

2. if we were to keep the same setup, just change out the compresor to a slightly bigger with more btus/ smaller one with less btus, do you gain or lose performance as in speed in which it cools? or is perfomance more based on coil and captube setup.


i do appreciate the comments here, they are very helpful.

thanks
mvtop

[Peter_1]

Mvtop, some adds to the already valuable information you got here.

We fabricate often eutectic plates in SS, (see other threads)

The eutectic point of -10F isn't possible with normal NaCl salt
You can add till +/- 22% salt for the lowest temperately but the more you add, the mope you exponentially increase your eutectic point again.http://www.ec.gc.ca/nopp/roadsalt/reports/ParkingLot/EN/phasediagram.jpg
You have to use CaCl2 for this.
I should go for -20°C (-4°F) with NACl.

I see you're trying to cool beverages, why then that low?
Compressor cooling capacity decreases enormous with lowering your evaporating pressure. The higher you allow the LP, the more capacity it will have. For a brine at -20°C (-4°F), you need to evaporate at -30°C (-22°F)
For a brine at -2°C (28.5°F), you evaporate at -12°C(10°F)
But cooling capacity is almost triple at -12°C (10°F)
Be carefully the beverages doesn't free in this brine.

Normal brine coolers for beer and beverages freezes at 0°C(32°F)

Chef's remark is very important : it takes +/- 80 times more capacity to go through the eutectic point.
If we freeze down eutectic plates, it sometimes takes a whole night to pass through the eutectic point.

Sorry, reread my post you said 10°F which is -12°C.
Remarks remains anyhow valid, only there's a double capacity on compressor side then, not triple.
But for 10°F, yo have too much salt in your solution, it should be something around 14%.

Possible problem is that your cooling capacity doesn't match your heat load. So isolate it and retry your tests.
It also depends how fast you ant to freeze it all.

[Chef]

I do have a couple of questions on coil size and compressor size.
1. if i change the size of the coil tubing, say smaller, and but keep the same volume, will i gain higher pressures or is the pressures related to the restriction of the captube and its length? i plan to redo the coil yet again, but with a tighter wind and smaller tubing. so, i'm assuming that things would be similar to what we have now but maybe better performance, assuming we keep the volume the same on the coil with the same length captube.

2. if we were to keep the same setup, just change out the compresor to a slightly bigger with more btus/ smaller one with less btus, do you gain or lose performance as in speed in which it cools? or is perfomance more based on coil and captube setup.
mvtop

Peter explained much more eloquently than I did about the need to get the right solution for your cooler. Nice diagram too, I will keep a copy of that for future reference.

As far as using a smaller pipe size for the evap coil - well you have to look at the velocities inside the coil as well as the pressure drop it will induce. At the moment your quite high with what seems to up to 11m/s at its maximum and if you drop to the next size down it may be beyond recomended values.

It seems you have a working system so to mess with it before you have nailed it down may be a retrograde step. Tune this one up and get all the parameters correct first, its already sat there on the shop floor.

If you put a bigger compressor in the system you may need to have a bigger condensor and a differant cap tube dia and length and larger evap coils. Sort of a complete redesign and rebuild from scratch.

That maybe what is required for your purpose and its your choice but dont dump the current system until you have it fully sorted out. It will surely help you with the next phase of the project.

Chef

[Chef]

You also suggested installing a smaller compressor.

This would probably require a complete redesign but this would reduce the cooling and your aim is to make it better. Maybe you could put this option on a back burner for now.!!!

Chef

[Gary]

i've tried the copper in the water didn't like the setup of it and it doesn't work with my finished idea.


Copper in the water would in fact enhance the heat transfer, but...

We have no clue as to what your finished idea is. If we don't know where you are going, how can we help you get there?

[mvtop]

Peter explained much more eloquently than I did about the need to get the right solution for your cooler. Nice diagram too, I will keep a copy of that for future reference.

As far as using a smaller pipe size for the evap coil - well you have to look at the velocities inside the coil as well as the pressure drop it will induce. At the moment your quite high with what seems to up to 11m/s at its maximum and if you drop to the next size down it may be beyond recomended values.

It seems you have a working system so to mess with it before you have nailed it down may be a retrograde step. Tune this one up and get all the parameters correct first, its already sat there on the shop floor.

If you put a bigger compressor in the system you may need to have a bigger condensor and a differant cap tube dia and length and larger evap coils. Sort of a complete redesign and rebuild from scratch.

That maybe what is required for your purpose and its your choice but dont dump the current system until you have it fully sorted out. It will surely help you with the next phase of the project.

Chef

Thanks chef.
I'm not planning on scrapping the project as it stands, but was asking in order to gain more info what drives what so when i'm building my next project i would be able to calculate things better than then working at a best guess approach as i'm doing now. I'm learning as i go. :D
I was planning on moving back to the 5/16 tubbing as i had in my first run at this, but might stay with what i have now in order to improve this setup rather than starting from scratch as you mentioned.

i'm not sure how to calculate the velocities and pressure drops, but i'll start looking that up.

i'll post some numbers next week to show how the system performs from ambient to temp.
i started it but never had time to finish. it was getting close at 1.5 hrs. but needs maybe 2+ hrs.


thanks.
Mvtop

[mvtop]

Thanks peter,
very helpful info, will keep as a ref. as well

as to why so cold, the chiller is to be used to quickly chill them, not to leave them floating for any period of time. so colder the better.

"Compressor cooling capacity decreases enormous with lowering your evaporating pressure. The higher you allow the LP, the more capacity it will have."
sorry not up on all the abrievations. LP liquid pressure, or low pressure.

I tried to quote your reply but can't because it has urls in it and i'm a new the forum.

thanks for the reply.
Mvtop

[nike123]

Thanks peter,
very helpful info, will keep as a ref. as well

as to why so cold, the chiller is to be used to quickly chill them, not to leave them floating for any period of time. so colder the better.

"Compressor cooling capacity decreases enormous with lowering your evaporating pressure. The higher you allow the LP, the more capacity it will have."
sorry not up on all the abrievations. LP liquid pressure, or low pressure.

I tried to quote your reply but can't because it has urls in it and i'm a new the forum.

thanks for the reply.
Mvtop

LP is low pressure.
In this case, compressor suction pressure.

[mvtop]

Copper in the water would in fact enhance the heat transfer, but...

We have no clue as to what your finished idea is. If we don't know where you are going, how can we help you get there?

good point Gary. :D
I didn't like the copper in the water because it harder to work around the tubing with a lid and a way to circulate the water.

the finshed product will be an insulated chilled container that has a lid. the lid will have several round holes the size of a beer can. the cans will only be able to go down into the holes about 4.5 inches.
the solution below will be circulatiing around the cans
cooling them. there will be a timer on the lid, so as not to freeze the beer.
I expect this portion of the project to be the longest and most time demanding.
i'll have someone hold my keys while i'm in the lab testing. ;)

So as you can see, colder/faster is better. for this project anyway.

if your interested, i can post some pics as i'm getting closer.

thanks
Mvtop

[Gary]

It occurs to me that your container needs to be 4.5 inches deep.

[mvtop]

It occurs to me that your container needs to be 4.5 inches deep.

the container that i'll be using will be about 10" deep.
to allow for some displacement issuse and to allow adequate solution.

the container in the pic is not the one for the final version, it was just something i had.

Mvtop

[Gary]

I would approach this in an entirely different way, but then where is the fun in building somebody else's design? And who knows, your design may end up performing better than mine anyway.

[mvtop]

I would approach this in an entirely different way, but then where is the fun in building somebody else's design? And who knows, your design may end up performing better than mine anyway.


exactly.
don't throw to many differant ways of doing this project at me.
i can easily go off course trying improve everything at once and end up getting nothing done and scrap the whole project. :D

thanks
Mvtop

[icecube51]

i have seen a couple of years back something very simelar, a beercooler but for cans, and they used for container a cind of plate welded polyester and the tubing was inox. workt very wel, it could hold 24 cans.
sorry don't have pics, but the polyester container whit inox tubing in the container was the tric.....i guess

Ice

[Gary]

i'll post some numbers next week to show how the system performs from ambient to temp.
i started it but never had time to finish. it was getting close at 1.5 hrs. but needs maybe 2+ hrs.


In particular, I would be curious to see the highest values measured, presumably shortly after startup... as well as the temperature of the air entering the condenser at the time.

[mvtop]

In particular, I would be curious to see the highest values measured, presumably shortly after startup... as well as the temperature of the air entering the condenser at the time.

I have this info at the shop. i'll get them next week.
i'm guessing at the exact numbers.
I remember thinking they where high compared to the values as it gets closer to temp.
low side was close to 29
high side maybe 168,
SC was around 30.
Dont remember SH, and didn't get temp of air going in to condenser.

at is got down to temp, we got back to low side of about 7 and high side of 128, SC was at 13 and SH was at 3 or 4. these numbers are different that we had before. i did insulate things though.
could that have and effect on the numbers alot? is there a small leak, with my SC dropping 2 or 3 points?


Mvtop

[Gary]

We need real numbers, but for the sake of illustration, let's assume that your numbers are correct and let's further assume that the temp of the air entering the condenser was 72F.

For R134a, 168psi converts to about 117F SCT. 117 - 72 = 45F TD. 45F TD is a very heavy load, but apparently your compressor was able to handle it since it kept running. You would not want to subject it to this punishment too often and when the condenser gets dirty it may in fact overload.

A more reasonable TD with clean condenser would be about 35F over ambient. The max TD could be lowered by making the cap tube more restrictive (lengthening it), but this would limit the load through the entire pulldown cycle, making for a longer run time.

Also, as we improve the heat transfer in the evaporator, the max TD will be higher, endangering the compressor.

Which brings us to a gadget that can limit the max TD thus safeguarding the compressor, without lengthening the run time: A crankcase pressure regulating valve (CPR). A CPR limits the pressure entering the compressor to a maximum value, and thus limits the load placed upon the compressor.

[Gary]

When your condenser TD was 45F, your suction pressure was 29psi. If you check through your numbers, you will find a point where the suction pressure resulted in a TD of 35F. Let's assume this was 20psi. If this is the case then we might set the CPR for a maximum of 20psi, thus maximizing the TD at 35F.

When the temp of the working fluid drops and the suction pressure coming from the evaporator drops below this set value, the CPR has no further effect upon the operation. It is strictly for setting the max pressure at the compressor inlet.

[Gary]

at is got down to temp, we got back to low side of about 7 and high side of 128, SC was at 13 and SH was at 3 or 4. these numbers are different that we had before. i did insulate things though.
could that have and effect on the numbers alot? is there a small leak, with my SC dropping 2 or 3 points?


Adding insulation changes things (for the better). Don't worry about the drop in SC, but do worry about the drop in SH. We don't want to flood the compressor, so you may need to remove a little refrigerant.

[Chef]

Mvtop - so its a beer chiller - well that is serious and needs a concerted effort by all till this thing is humming properly.

One small thought is that the saline solution will have a specific gravity higher than a tin of beer and so it may float a little in the solution. Maybe it will need some sort of weight on top of it to get the maximum amount of immersion and hence cooling. :D

It does appear from the data you have already posted that there is quite a lot of room to improve the efficiency and especially when the system is down at 10F.

It seems the tube may be too short and so an increase in the length will add cooling effect when running at the design condition.It will of course increase your start up head pressure from the 168psi you posted earlier to maybe 180 or 190psi (depending on how much extra length is added)

A startup head of 168 is pretty low really and most LBP systems get close to 200psi for the first 10 or 15 minutes. It is here you will need the compressor specifications to check what is the maximum current draw allowed. Armed with that you can then decide how much higher to push the compressor.

As you increase the tube length you will get an increase in performance of up to a maximum of about 30% at operating conditions but you will suffer from higher head pressures on the startup. The more efficiency the more the head pressure.

Maybe Gary's idea of a suction throttle valve to limit the suction pressure at startup would work here, you get a protected compressor and higher effeciency. Lets wait till you have the pulldown data to see just what can be done.

When it is finally completed then the most difficult part is upon you - testing how cold the beers are and as you said - someone will hold your keys.

Chef

[Goober]

MVTOP, I'm seriously impressed with what you're doing. For someone who only "tinkers" you're going great guns. Also, you've got digital gauges! I don't have a set and I'm yet to come across anyone who has, but then I'm not on the road much these days!! And keep up the good advice guys!

[Gary]

Ideally, in perfecting an experimental evaporator we would want to control the variables of refrigerant flow while safeguarding the compressor. This accomplished, we can focus on improving the evaporator itself. Thus the ideal would be to use a TXV and CPR... but then we would also need a receiver.

The CPR could indeed be set to where the manufacturers amperage limit is not exceeded, but then a rise in ambient temp and/or a little dirt on the condenser could result in compressor overload.

I would tend to set the CPR to where the compressor draws amperage a little under the maximum.

[Gary]

Let's keep in mind that cap tube systems are not designed for pulldown speed. We plug them in, they pull down to the desired temp and remain very near that temp for the remainder of their useful lives. If we are designing for pulldown, we need a TXV.

[Gary]

We have already seen that adding insulation lowers the heat load which drops the superheat at the compressor. In order to compensate, we need to reduce the refrigerant charge, which will in turn lower the subcooling. Similarly, if we were to lower the desired temp of the working fluid, we could also expect a lowering of the heat transfer rate and the compressor inlet superheat... and again we would need to reduce the refrigerant charge. At some point we would also need to lengthen the cap tube in order to ensure a minimal amount of liquid (subcooling) at its entrance.

On the other hand, if we find ways to more efficiently transfer heat from the working fluid to the refrigerant, we are increasing heat transfer, in which case we would need to increase the refrigerant charge and perhaps shorten the cap tube.

So you see, as we perfect the evaporator we must also screw around with the refrigerant charge and flow characteristics every step of the way in order to compensate for the changes in heat transfer.

A TXV system would automatically compensate for these changes, allowing us to focus our attention entirely upon improving the evaporator.

[mvtop]

Goober.
Thanks. the gauges are nice. necessary no. but i'm a tool nut.

Chef,
yes the cans would float, but i was planning something like a 3 wired cage that the cans would slide down into with enough friction to hold them in place.
Here is a link to the compressor specs.
www.panasonic.com/industrial/appliance/pdf/qa125k29cau6.pdf (http://www.panasonic.com/industrial/appliance/pdf/qa125k29cau6.pdf)

yes the toughest part will be the testing. i'll be sure to have a friend near by for support.



Gary,
it sounds like the txv system would simplify the system by removing several varialbles.
what's all needed to for this?
I am limited on space with the platform that the compressor sits on. So adding several items would be tough space wise.

I'll post some numbers tomorrow and we can see where we are with things.


I really appreciate all the feed back with this project. It makes it fun.


Mvtop

[Chef]

Mvtop - the compressor seems to be a 639 watt unit which is larger than the 1/3HP you mentioned earlier and it changes things somewhat.

Also it is a HBP unit and quoted as having an evaporater temperature down to 23F so your design point of 10F is a little beyond the specs, not sure how that will pan out in the longer term.

It also states 10mm (3/8") piping for the suction work which means your in the correct area with your coil.

But it does give the trip amps and so you could measure the amps during pulldown and see how close you are getting!

Chef

[mvtop]

Here are the numbers as it draws down to temp.
The readings were taken about every 10 min.

At startup:
Air temp is 69 for entire cool down.

I've attached a pdf with the info.


These are far diffent than when i first charged the system.

it's holding temperature well. kicks off at about 9deg.
and holds for about 50 min. then runs again for 15 min or so.

I'm not sure where i got the 1/3 hp from though.
it might of been from the old frame that it was on. The data plate was from some other compressor.

thanks
Mvtop

[Gary]

At what point on the suction line are you measuring SH?

[mvtop]

At what point on the suction line are you measuring SH?

about 4 to 5 inches from compressor

[Gary]

We seem to be getting strange SH readings. I'm thinking the accumulator is filling with liquid during the off cycle and the SH readings are low during the on cycle until the accumulator empties.

Liquid will form in the evaporator while the system is off. If the suction line goes from the bottom of the evaporator down to the accumulator, the liquid in the evap is transported by gravity to the accumulator.

This is easily remedied. Where the suction line leaves the evap, run the suction line up to the top of the evap and then back down to the accumulator. This traps the liquid in the evap.

[frank]

I've had a little play with the data

[Gary]

Note that the compressor inlet superheat is very low until near the end of the cycle, when it rises. This is the exact opposite of what we would expect to see on a cap tube system.

[Peter_1]

...
Which brings us to a gadget that can limit the max TD thus safeguarding the compressor, without lengthening the run time: A crankcase pressure regulating valve (CPR). A CPR limits the pressure entering the compressor to a maximum value, and thus limits the load placed upon the compressor.
Or a MOP TEV which act for 2 purposes: protecting the compressor for a too high load and once on stable conditions, feeding more precisely the evaporator.
And it's less expensive.
Installing a much bigger condenser which can handle the big load at startup is another option.

[Peter_1]

Goober.
it sounds like the txv system would simplify the system by removing several varialbles.
what's all needed to for this?
I am limited on space with the platform that the compressor sits on. So adding several items would be tough space wise.
And more specific a MOP TEV.All you need additionally is a small receiver which can be some length of 3/4 tube in a spiral form.

[Peter_1]

Keep also in mind that during the off cycles with a cap tube all the refrigerant will flow to the evaporator very fast.
This will provoke liquid slugging at startup.
We always install a SV with a TEV to prevent this.
Or let it pump down each time.
So a small liquid separator - which you find in most domestic refrigerators - will be needed to protect your compressor.
You can make this yourself.

[mvtop]

We seem to be getting strange SH readings. I'm thinking the accumulator is filling with liquid during the off cycle and the SH readings are low during the on cycle until the accumulator empties.

Liquid will form in the evaporator while the system is off. If the suction line goes from the bottom of the evaporator down to the accumulator, the liquid in the evap is transported by gravity to the accumulator.

This is easily remedied. Where the suction line leaves the evap, run the suction line up to the top of the evap and then back down to the accumulator. This traps the liquid in the evap.

I think i do have the suction at the bottom of the coil.
I'll move things around to incorporate a loop so any refig. in the coil stays in coil until pulled out.
I"ll also solder the cap tube to the suction line.

any other thoughts while i have it torn down?

i'll post some new numbers when i get this done so we can see what changes it makes.

as for charging, i'll be using this "At design cutout temp, SC is 15F or less and the compressor inlet SH is 20-30F" as my target.



as allways, thank you for your feed back

Mvtop

[mvtop]

And more specific a MOP TEV.All you need additionally is a small receiver which can be some length of 3/4 tube in a spiral form.

thanks for your thoughts peter.
how would you spec. MOP TEV for this system?


Mvtop

[Gary]

You might want to consider a TXV/w MOP and matching condensing unit/w receiver... or the modifications to your present system that Peter_1 has suggested. Or we can save some money and continue making improvements while adjusting referigerant flow and charge along the way. The decision is yours.

But then perhaps we need to know how the finished system will be used. If you are going to frequently subject it to a full pulldown from ambient, plugging it in on the weekends just before the party, then you may be better off with a TXV system anyway.

[mvtop]

Gary,
since this is the first run at this, and i started off the project planning on putting some used parts back in service, lets stay the course and adjust what we have and see how it turns out.

If it turns out good, I can always come back to the drawing board armed with better knowlege from my first run. As well as how it would be used.


So how to improve what I/we (group project) have now,

i'm thinking that i need to add some length to the cap tube to slow things down a bit. I think i'm running a 32" tube now.
I would expect to get better performance with the heat exchange once i rework the coil again. This will be done with the final version of the container. I will stay with the same size tubing, and as close to the same volume for the coil as i currently have.
Reduce refrig. charge

anything else?

thanks
Mvtop

[Gary]

I would leave the cap tube as is for now.

[Chef]

Very interesting data, the plot below shows the pulldown pressures versus mass flow rate, it is this plot that can show you where the tube is in perfect conditions and that is at the Knee of the discharge pressure. To the left of the knee i.e. at lower mass flow rates the tube will have liquid and gas entering it and your quality seems to be around x=0.1 which is fairly good and if you try to make the tube longer to gain that extra 10% of liquid and have an x=0 at the tube entrance you will certainly get higher start up pressures. 215psi is quite high but the amps seem ok so not much to be done.

The one thing is that at the exit to the tube the flow is sonic, this tends to restict an increase in flowrate when the evaporator falls in pressure. It is possible to opt for a larger tube say .050 with a length of about 9 feet - this would be about the same as your 32" of .040 but would not be sonic at the output and would be much better on a warmer day.

http://www.refrigeration-engineer.com/forums/picture.php?albumid=101&pictureid=879

The only caveat about this plot is the lack of data during the critical phase from intial startup to the knee, to really nail this down it would be handy to have at least 5 (or more) points. About one reading every 1 or 2 mins for the first 10 mins.

Otherwise if you think its not worth the messing about - box it up and get cold beer.

Chef

[Chef]

So how to improve what I/we (group project) have now,

i'm thinking that i need to add some length to the cap tube to slow things down a bit. I think i'm running a 32" tube now.
I would expect to get better performance with the heat exchange once i rework the coil again. Reduce refrig. charge

anything else?

thanks
Mvtop

If you lengthen the tube then the following may happen:
The controlling element is the compressor and not the tube, it is the compressors suction volume and suction conditions that dictate the systems mass flow rate. The other components just adjust themselves to fit to the compressor. So if you increase the length of the tube then:-
Before the knee - i.e. at flows hight than at the knee - the condenser will begin to back up liquid and reduce its effective cooling till the pressure is high enough to force the mass flow through the tube.
The consequences are a higher head pressure, a starved evaporator and more power consumption.

After the knee - i.e. at mass flows less than at the knee - the increase in the tube will make the quality move towards zero and the pressures will remain almost the same as now. You will see an increase in cooling effect, there will be more liquid in the evaporator because the the charge will need to be a little bit more. The corrollary is that more liquid is then in the system and during startup even more is built up in the condensor and the discharge pressures are higher.

Its a sort of win at one end and loose at the other.

If you are thinking about redoing the evap coil have you considered putting the coil inlet at the bottom and outlet at the top, it seems more common to do it that way - in all the systems I have seen anyway.

Chef

[mvtop]

Hi,
I've got the latest version up and running. It seems to pull it done to temp fairly fast.
Although, when it gets about 85 deg. in the shop it seems to struggle / runs longer to keep it at temp.
Would this be a condensor issue, as not large enough or needs more air flow?

i got my new/final container for the coil that i'm working on, i'll pulling the system apart again to get it installed.
hopefully the last time.


thanks for your help with all the feed back.

i'll post more as it gets closer to finished.


thanks.
Mvtop

[Chef]

Mvtop - we did'nt really get down to the cause of the very low SX on your last set of data i.e. 1psi instead of 7 psi as noted earlier - maybe a gas leak? Also what mods are you making with the coil etc.

From your earlier data the condenser may be a little small for a hot day but that will probably be on a startup issue only - have to wait to see the latest data to see what you have done.

Keep going.

Chef

[mvtop]

Mvtop - we did'nt really get down to the cause of the very low SX on your last set of data i.e. 1psi instead of 7 psi as noted earlier - maybe a gas leak? Also what mods are you making with the coil etc.

From your earlier data the condenser may be a little small for a hot day but that will probably be on a startup issue only - have to wait to see the latest data to see what you have done.

Keep going.

Chef

Hey Chef,
I do think there could of been a small leak, since the numbers were so different from where they started.
And I had a few flare fittings installed to make dismanteling easier. They can be tricky to keep the leaks away.

as to what mods, i did add the loop to the coil so gravitiy wouldn't drain any ref. into the compressor during off times, as Gary mentioned. And moved the return to the top of the coil and the supply to the bottom as recomended. Also, i did soft solder and wrap the cap. tube to the coil return and insulated well.

for the mods of the new coil, it will be made of stainless steel, not as good as the aluminum i have now, but it's work better overall for my design.
I plan to make a tighter wrap this time and remove all gaps between container and coil, solder/braze the coil to the container to help with heat transfer.
It will hold and cool 2 gallons instead of 3 gallons. I"m not sure how much smaller the coil will be in terms of volume compared to what i have now. i'll know in a day or so as i get closer to finishing the coil. i was going to try to stay fairly close to the volume i have now so we can stick with the same length cap tube. I have a 39" .040 cap tube. I remeasured it when i had it apart again. I think I said I was using a 32".

Is there any way to estimate the length of cap tube I might need if the volume of the coil is reduced by x%?
I was using trail and error before. I don't think i can lengthen the cap. tube by x% the same as what i reduced the size of the coil.
I really won't know what the volume of the new coil will be until i'm done with it. It might be close enough to what we have now. that's what i'm shooting for.

I'll have some pics as I progress with this.

Again, thank you all for your comments / feed back.

Mvtop

[Chef]

Hey Chef,
Is there any way to estimate the length of cap tube I might need if the volume of the coil is reduced by x%?
I was using trail and error before. I don't think i can lengthen the cap. tube by x% the same as what i reduced the size of the coil.
Mvtop

The length of the cap tube is related to the compressor mass flow rate and the condensing and evaporating temperatures (and hence pressures) - so as these have not been altered (much) then the tube length will stay the same.

However if you change the coil size by a lot then evap temps and pressures will change a little. In your case it is not so critical as you are running your cap tube with a sonic output and so changes in the evap pressure wont affect the flow rate in the tube.

It will affect the suction conditions though and as you have a smaller coil size the pressure will drop a little so the mass flow will drop a little too and you will have less charge in the system.

It all depends on how much you reduce the coil size, the smaller the coil the smaller the heat transefer area and so to get the same heat flow the temperature differance will need to greater, this in turn forces the suction pressure down. As you are using a HBP compressure you dont really want to push it down too far?

Chef

[mvtop]

Hey,
Happy Holidays.
It's been awhile since I’ve posted on this project, mainly becasue it's working fairly well and i've been off on other tangents.

I thought about putting this as a separate post, but i think there’s some history here that's relevant to the story line and instead of covering it all again, I’m just adding to the already long post.

Gary,
I would like to improve the performance of the cool down time. And as you mentioned, cap tube systems are designed for pulldown speed.


Let's keep in mind that cap tube systems are not designed for pulldown speed. We plug them in, they pull down to the desired temp and remain very near that temp for the remainder of their useful lives. If we are designing for pulldown, we need a TXV.

So I’m taking your suggestion Gary about using a TXV in my system. As you know, this is not your typical out of the box project. I'm ready to put this project back on the workbench again.

So my questions relate to how to go about sizing / set up a TXV for my setup and what would be needed in addition to what i have. Which is basically a compressor, accumulator, condenser, and homemade evap.


Again, thank you all for your posts and suggestions.
mvtop