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Extreme Phase Change Pump Cooling

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Chris

Tue Mar 14 2006, 01:03AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

For my chlorine condenser for my downs cell project, I need a heat pump capable of achieving -35C while transferring at least 100W of heat. To assure this, I have a design goal of 200W at -45C. I have two large compressors for the unit, model number AJ5513F (they are 277V/265V, not 230/208V, but definately stamped AJ5513F, not AJ5513E). I finally got around to testing the compressors and they run smoothly and quietly. The evaporator coil volume is about 18 cubic inches (295cc) and the condenser volume is several times larger such as it is. I was going to use a simple needle valve as an expansion valve, since the system will be operating continuously with little dynamic change. I can charge with R-134a or propane, as long as it's compatible with PAG oil (for automotive AC retrofit), which I'm not sure of. I can use either one or both compressors, but it would be better to use just one.

A table with some cooling data for the compressors with R-22 refidgerant is listed here

and extrapolating from that, it will be impossible to come even close to the design goal as they would reach 0W of heat transfer before even reaching -30C. To hit -50C, R-134a refridgerant has to be pumped down on the cold side to about -10.4psig or 4.3psia (but that's something that should not be too hard to do), so I'm not sure if the specs I need can be achieved simply by adjusting the expansion valve or fill pressure, or if I need something totally different like propane as a refridgerant. I'm not sure if propane is compatible with the compressor oil either and have not been able to find any information regarding that, since no one uses propane really. All I know is that propane operates at a higher pressure and will put more stress on the compressors, but I guess a more restricted expansion valve would as well. Finding any cooling data for it especially relating to these compressors is a lost cause, but perhaps someone here knows how to extrapolate such data.

In any case the above mentioned hardware is what I have, going to peltiers or something would be far too expensive, plus they are only a fraction as efficient as phase change pumps. I have considered running both compressors in parallel, or perhaps in series with propane to reduce stress. Another possible option would be to run one pump with R-134a with the cold side coupled to the hot side of a propane system, which would allow the latter to run at lower pressure and act in a fashion equivalent to stacked peltiers. This method would require an intermittant heat exchanger and twice as many valves and widgets, however, so is probably very much something to avoid.

Anyone with suggestions or anyone who knows what to do to achieve the necessary cooling, your replies would be greatly helpful.

Chris

dan

Tue Mar 14 2006, 05:52AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

I have charged a standard fridge with propane.. Also some other people have to

. As long as you keep the condeser cool enough liquifiying the propane should be no problem.. I'm sure the compressor can take the added stress. These things are built like tanks and have very rugged design. I have run stnadard 120v 1/2HP fridge compressors to 350psi at which piont the motor stalls. Propane at 45C only needs ~200 psi which the compressor is capable of doing.. Of corse the cooler you get the condenser the easier it will be for the compressor.

I have recharged both R-22 and R-134a systems with propane and they work very well and I never had the problem of the propane being incompatable with the oil.. The only time this happens is when the refidgerant is bottled with a defferent type of oil then what is in the compressor.

I do not suggest restricting the expantion valve too much since you need decent refridgerant flow for your cooling requirements. However too much is bad too since your compressor wont be able to keep the condenser pressure up.

I would just parallel the compressors and run propane refidgerant. With propane and the right expantion valve setting you should be able to reach your -45C goal with no problems.

Chris

Wed Mar 15 2006, 12:12AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

I'd kinda like to know if one compressor is capable of meeting the requirements, since they draw quite a bit of power, and having to run for so long it would be nice to make the system at least somewhat efficient. My variac is only rated 22A continuous at 0-290V, too, and I need to use that to get enough voltage to run the compressors, which are 277V single phase units. I don't know how much the compressors draw under full load though of course. I suppose it could be estimated from the BTU/hr ratings, but I have no idea how.

Vcore

Sat Mar 18 2006, 09:15PM

Registered Member #328 Joined: Sat Mar 18 2006, 08:48PM
Location:
Posts: 2

Hi there!
Hava a look at

About r134a reaching -50ÂºC and r22 not doing it, it is absolutelly the other way, r134a sucks, there is not much more to say about it, doing -40ÂºC on r143 is already a good feat, -30 is more reasonable, doing -40 on r22 is easy -50 is doable, -70 has been done in some insane setups.
About propane it is compatible and soluble with all the oils, its pressure/temperature chart follows closelly that of r22, so it is really good, its state change enthalpy is about 3x higher than that of r22 but on the other side its density is about 3 times lower so the performance of both gases is about the same, the performance of r134a is somewhat lower, also propane doesn't form acids in the presence of moisture so a homemade setup is more likelly to survive.
By all means use propane.
Also the compressor will definitelly handle the added stress as it has been said they are rough, I have put mine up to 36 bar.
With a good condenser you will be at about 8-15 bar depending on the ambient temp.
Nedle valves work quite well, I used one in my setup.
Compressors in parallel work fine, in series they don't there are level of oil issues to take care of in that setup, plus one of them will be undercooled due to being at a vacuum in the high and low pressure sides, also the only increasses in performance will be due to a reduction of the low side pressure but it will come with a reduction in cooling capacity, you can get about the same beneffit bit putting both compressors in parallel an thigthening the valve.
A single compressor will be enough IMHO.
The setup you described with one r134a system cooling a propane one is known as a cascade,
if you are going to go the trouble of building such a thing fill the 1Âºst stage with propane and the 2nd one with ethilene and you are ready to get under -100ÂºC, if you cant get ethilene you can still use propane on both systems and -60/-70 is doable.
But trust me cascades are not easy and they can even be quite dangerous.
As far as some other suggestions, don't pay too much atention to condensor and evaporator volumes, it all will work out fine by itself with the gas load and the valve settings, in the end you will get about the same flow from the compressor at a given pressure and a halfilled condensor.
What is important is to use gravity for condensing and evaporating, that both increases performance and prevents liquid findind a way back into the compressor.
Also if you want to maximice performance make sure everything is as non restrictive as possible but the expansion valve.
BTW did you drop the aplication forum? I couldn't find it or any refference to it in the rules.

Chris

Sun Mar 19 2006, 10:52PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

Again looking at the table here

for this compressor, the AJ5513F, it states with R22 a maximum thermal transfer of 4263W at 15Â°C, 3580W at 10Â°C, 3203W at 7.2Â°C, 2385W at 0Â°C, 1910W at -5Â°C, and 1460W at -10Â°C. Also remember I need to move heat continuously, actually remove it from the cold area at a rate of 200W, at -45Â°C, not just simply achieve that temperature. If we plot those points and extend it on down we would reach 0W at about -30Â°C, if not warmer. -50Â°C would be absolutely out of the question by that data. That's the specs for it with R22, and if propane is similar to that, the data should be similar for propane. R22 boils at -40.8Â°C at 1atm, propane is quite similar at -42Â°C.

What do I need to do to get more capability than that? It's not even close to the temperature and thermal transfer required, and seems to imply I would need a cascade. Is it really possible at all to reach the goal with just one of these compressors by itself? Not if the data on that table holds up under most any condition. These are quite large industrial compressors, several times more powerful than your typical fridge compressor, but they hit 0W at -30Â°C? I don't understand, does it depend that much on the expansion valve setting? The condenser temperature?

Vcore

Mon Mar 20 2006, 04:02AM

Registered Member #328 Joined: Sat Mar 18 2006, 08:48PM
Location:
Posts: 2

I don't have much time, i should alredy be sleeping, but a cascade doesn't have more capacity (they usually have less), if you want more capacity put compressors in parallel, maybe the compressors you have are not designed for big pressure diferentials, so the flow goes to almost 0 with ease.
If you want high capacity and extralow temperatures build an autocascade instead with both compressors in parallel.
I know you mean to cool 200W continuouslly, I knew it from begining, it is not too different from the requeriments of cooling processors.
The cooling power depends just on compressor flow, refrigerant state change enthalpy and refrigerant density.
The flow archieved by the compressor depends on the pressure diferential and on its design, the pressure diferential changes depending on how restrictive is the circuit, most of that resistance is on the expansion valve, it changes too with the condensing temperature.
In fact that chart doesn't tell the whole truth because it is at 55ÂºC condensing temp, that means about 17 bar of condensing pressure for propane, make it drop to 35Âºc and we are talking about 10bar, much more workable.
Also one quite typical problem in home fridges, I don't know about bigger better systems is having a too unrestrictive cap tube compensating that by underloading and waiting for the underloading to compensate itself with condensor overheating. (I did run my system like that with a damaged expansion valve and it sucked, I even had to mix some butane to get some capacity).
Really this things are much more complex than it seems at first, have a good look at phase-change forums and you will get it.
pt chart of propane:http://www.airliquide.com/en/business/ products/gases/gasdata/images% 5CVaporPressureGraph% 5CPropane_Vapor_Pressure.GIF
pt chart of r134a,r22 and others(pressured under ambient in red):http://www.chillers.com/PT%20charts%20for% 20refrigerants.htm

As you will see to get -40ÂºC on r134A you need quite a bit of vacuum==low capacity, you want the highest pressure refrigerant at the goal temp you can condense at a reasonable pressure for capacity, r-22/propane are pretty much the best options for you.Higher pressure refrigerants than propane are probably too dangerous if the temp goes a bit out of hand, and in fact if condensing temp goes a bit high capacity will dissapear.

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