Hi,
If you don't need much lift, look into the technology used by Labconco, Titan, and other companies in their single-stage freeze driers. I _think_ the new ones are rated as low as -80 single stage, at 32C ambient. I'm looking into this myself, so let me know if you find anything. My older units use a Danfoss FR7.5B, which is nominally a general purpose R12 compressor, though I'm not certian R12 is used. Oddly enough, it's marked HBP: perhaps this designation deosn't so much define operating range, but compression ratio. I _think_ the capillary is intentionally restricted a bit more than usual so the evaporator "starves", perhaps falling to below atmospheric pressure, even: however, it may be that the low load leads to such low evaporator temperatures that the system "starves" even with a standard size capillary. I think this also makes the compressor run a bit hot. I think as you look for components, you must keep in mind that you'll probably be "off all the maps" in terms of manufacturers' specs. With luck you'll be able to find compression ratio specs, and by looking at the vapor pressure vs. temperature curves for various refrigerants, find a suitable operating point. I have yet to see a low temp spec that was not also accompanied by a power spec @ that temp: these designs flirt with the absolute limits of the media and components near 0 power lift.
Of course, if your lift is _really_ low you might consider thermoelectrics. They are notoriously inefficient, especially at low temperatures. Whereas a temperature differential of 55C is fairly practical for a single stage to maintain with a hot side temp of 25C, with a hot-side temp of -20 I don't think a single stage design will cut it. This might be a $100-$200 solution vs. $500-$1000.
Eh, I shouldn't have started a new thread (which see), my problem is too closely related to yours.
-Jeff
