-45 Chiller Head

We are trying to make a cooler for a sensor head to get down to appx -45 Deg C. We have an SC18CL (R404A) with matched condenser, an AKV10-1 (I gather people don't seem to like these) running on a 4 second cycle at 15% full load and an EVR 2-22 on a timer to give a bypass flow to the compressor every couple of minutes to prevent overheating where cooling is not required on the sensor. The Evaporator is our problem. We currently have a brass block appx 46mm square with the AKV mounted on the inlet(the inlet tube to the evaporator is larger than the outlet) and 3/8" tube on the return. The block is a single machined chamber 12.5mm dia and 33mm deep. With the unit running we can achieve an on load temperature of -35 to -38 deg C (with the block etc fully insulated) but are unable to drop the extra 5 to 10 deg C that we would like. As you may have gathered, we are not refrigeration engineers! so can anyone give us any pointers as to what we can do to improve cooling? either by modifying/redesigning the block or by some "Cool" pipe trickery!!

Hi AMSYSTEMS :)

You will be very lucky to get -45 out of your machine,the whole design seems a bit odd.Was it supplied from a manufacture with a AKV fittted.

Kind regards.Mark

You will be lucky to get much more out of R404a, however the outlet (suction line) is usually larger than the inlet, the idea being to create a pressure drop and the desired temperature drop. You may be able to get a few more degrees by increasing the suction line diameter, however be aware :eek: larger pipe sizes decrease velositys and this is what carries the oil back to the compressor. Otherwise have you looked into cascade systems?

There doesn't excist such a thing as a 'cool pipe trickery'
What are your pressures and temperatures, LP, HP, discharge Pressure, subcooling, overheating,...
Without this, we can't do any magic for you. .

Hi Mark,

The system "design" came from a local refrigeration company who also gassed the system for us. The AKV is used so that we can control the cooling via a controller unit enabling us to set the cooling temperature automatically from the unit being cooled (usually set for either -40 or 20Deg C). The user also has the option of manual control to select any temperature within it's range. This is a prototype unit that we hope to build into our instruments - hence size is an issue!

Hi Craig,

We will try to increase the suction line diameter slightly and see what effect we have.

Hi Peter,

I will try to contact the engineer for the information you have asked for.

Best regards,

Mike

AMSYSTEMS :)

The AKV in my opinion is a bad choice of expansion device for your particular application.
Lose the AKV and fit a suitably sized capillary and modify the controls.
AKVs are best left in the enviroment they were designed for. A supermarket.
Best of luck with your "prototype".
Hope this helps.Mark :)

Agree Mark, it's a bad choice the AKV.

Hi Peter,

I have spoken to the engineer, the pressures on the system are appx 200 psi on the high pressure side, with appx 3 to 4 psi on the return. We do not have any temperatures except to say that the compressor is running with a slight chill at the return input, rest of the compressor is slightly uncomfortable to touch. The temperature is low on the high pressure side as we have a very efficient condenser.

We are going to try a capiilary tube on the next test unit we build and use a standard solenoid valve for control. We may even try a CPU Cooling system! we certainly do not generate any more heat then a Pentium 4 :D

Many thanks for the replies so far, we are now slightly less green than before :rolleyes:

Please seek advice from a refrigeration engineer first.It might save you money.