On Fri, 5 May 2017 01:49:33 -0000 (UTC), bob prohaska
wrote:

wrote:
So what are the disadvantages of using a
repurposed refrigeration compressor besides getting my hands on a good

The principal drawback is volumetric efficiency. Reefer compressors are
usually piston-type with finite compression ratio. Vacuum pumps are
usually rotary vane types with (ideally) infinite compression ratio.

Whether that matters depends entirely on the base pressure you want to
achieve. For food, the base pressure will be around the vapor pressure
of water. At room temperature that's about 20 torr. A piston compressor
needs a compression ratio of roughly 40 to get there when exhausting to
atmosphere, and its efficiency is zero in that limit.

40 seems like a rather high compression ratio, purely from a mechanical
point of view. It would be surprising if a reefer compressor is that good.

hth,

bob prohaska
Apparently the air conditioner compressors are usually the vane type
while refers use pistons. Here in the US that is.
Eric