View Full Version : A Search For A Tiny CPXV

22-04-2014, 04:07 AM
Hello all,

I am searching for a constant pressure expansion valve that is rated for some very tiny capacities, specifically in the range of 250 BTU/Hr to 850 BTU/Hr.

The reason that the capacity is variable is because the compressor RPM is variable, the compressor in question is here; http://www.aspencompressor.com/products/

I will be using the aspen Q4 series, data sheets are available on their site under downloads.

I am shooting for a evaporator pressure of 35 psi using R134a, The system will have a liquid receiver. The evaporator will be refrigerant to water heat exchange with the return water being in the range of 75 to 90 deg F. The maximum ambient operating temperature will be 130 deg F.
The condenser outlet will be 1/4"OD and evap inlet will be 3/8"OD, External equalization will not be necessary.

I was looking at a valve made by parker model a7-aa rated for 1/8th to 1/2 ton, but I fear that that valve will hunt badly on the low end of the compressor RPM. I have already contacted Parker about this and they say that it will most likely hunt and that they do not manufacture a valve of such small sizes.

So what I am asking of you all is if anyone may know of a valve that may fit my needs, I would prefer a valve with ODF connections but I am willing to use other types of connections.

If no such valve exists, then what is your opinion on using a manual needle valve as the expansion device and setting the valve then forgetting it. I would like to avoid using capillary tube due to the cost but a standard orifice would be OK if I had to.

I appreciate any input I am almost to the end of my rope on this plunder.

22-04-2014, 08:21 PM
Will you be varying the compressor speed during normal operation?

Or the other way of asking, do you have varying loads?

22-04-2014, 11:57 PM
The compressor speed will indeed be varied during operation, that is why I want to use a constant pressure ex valve so that it does not vary the suction pressure too.

The load too will vary slightly as well but not very wildly.

Perhaps I should specify that this is a project to build a personal cooling system, so the compressor speed will be adjusted to match the btu output of one's body at different times, hopefully with very little load from the environment.
I know the military manufactures them but there are no schematics or anything that I could find.

23-04-2014, 10:56 PM
I'm not sure why you want to maintain a fixed suction pressure when you state that the load will vary and with a variation on compressor speed.

You could perhaps look into using an evaporator pressure regulator to restrict it.

Read up on document 90- 20 here....

24-04-2014, 02:54 AM
Hmm would it be possible to have only a txv and then automatically control the compressor speed to maintain the suction pressure at a set psi?

So when the heat load comes up the txv will open in turn raising the suction pressure which should then cause the compressor to speed up to bring the suction pressure down thus increasing capicity. Does this sound about right to you?

24-04-2014, 10:25 PM
Yes, entirely possible. It is a common form of control.

27-04-2014, 10:18 PM
I am still having a hard time finding a thermal ex valve or a constant pressure ex valve that is small enough.

The smallest I could find is parker EF series and it's rated for 1/8th ton, the company also does not think that valve will work either.

Why is it exactly that it won't work, I understand that it will hunt and I know what it means to hunt, but couldn't I stop it from hunting by insulating the evaporator and valve bulb. Or could I even go as far as turning a new valve orifice and needle on a lathe?

27-04-2014, 10:46 PM
Can you use an electronic expansion valve?

Danfoss do an AKVO 10 which rated on R134a at 0.9kW at 5C and can be regulated down to 10% of duty.

http://www.danfoss.com/DanfossCom/Templates/Literature/LiteratureSearch.aspx?NRMODE=Published&NRNODEGUID=%7B92375A16-760D-4F98-93E0-FC9DC9F73F4F%7D&NRORIGINALURL=/North_America/BusinessAreas/Refrigeration%20and%20Air%20Conditioning/Literature/RA_Documentation.htm?CatalogKey=RA-6aa56192-f7e6-4793-b7f9-4c2fa908cc23-en-US:RA-6aa56192-f7e6-4793-b7f9-4c2fa908cc23-en-US&DocumentType=All&LiteratureNo=DKRCC.PD.VA1&NRCACHEHINT=Guest&CatalogKey=RA-6aa56192-f7e6-4793-b7f9-4c2fa908cc23-en-US:RA-6aa56192-f7e6-4793-b7f9-4c2fa908cc23-en-US&language=en (http://www.danfoss.com/DanfossCom/Templates/Literature/LiteratureSearch.aspx?NRMODE=Published&NRNODEGUID=%7B92375A16-760D-4F98-93E0-FC9DC9F73F4F%7D&NRORIGINALURL=/North_America/BusinessAreas/Refrigeration%20and%20Air%20Conditioning/Literature/RA_Documentation.htm?CatalogKey=RA-6aa56192-f7e6-4793-b7f9-4c2fa908cc23-en-US:RA-6aa56192-f7e6-4793-b7f9-4c2fa908cc23-en-US&DocumentType=All&LiteratureNo=DKRCC.PD.VA1&NRCACHEHINT=Guest&CatalogKey=RA-6aa56192-f7e6-4793-b7f9-4c2fa908cc23-en-US:RA-6aa56192-f7e6-4793-b7f9-4c2fa908cc23-en-US&language=en)

28-04-2014, 02:36 AM
Ive struck another idea; Would it work to mount the bulb of a txv and insulate it so that the temperature fluctuates very little and in turn make the txv into a constant pressure valve? Or I could even remove the charge from the bulb and fill it with compressed argon or some such just so that pressure is maintained on that side of the diaphragm.

The reason I want to use a constant pressure valve is so the compressor speed can be user controlled so the output temperature can be adjusted to what ever temperature the user prefers.

Looking into using an electronic valve leads me to believe it would be too costly and it would consume too much energy, and my system runs off of 12vdc and their valves are rated at 24vac at the lowest.

I have appreciated all your advice so far, thank you.

28-04-2014, 09:27 PM
How about a change of thought?

Can you control the water flow at all to allow for the change of load?

29-04-2014, 12:44 AM
yes, my water pump is PWM.