Vacuum pump from refrigeration compressor questions

snipped-for-privacy@whidbey.com wrote:


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

Apparently the air conditioner compressors are usually the vane type while refers use pistons. Here in the US that is. Eric
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snipped-for-privacy@whidbey.com wrote:

That's a little surprising, but if correct then a vane type AC compressor ought to work fairly well as a single stage vacuum pump. The only thing to watch out for is the vapor pressure of the lubricant, but it's unlikely to be a problem.
hth,
bob prohaska
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On 05/04/2017 08:48 AM, snipped-for-privacy@whidbey.com wrote:

Mechanical vacuum pumps tend to backstream pump oil when the pressure starts getting down. You commonly use a cold trap (LN2) in applications that won't tolerate pump oil in the vac chamber. Might add a strange taste to the food!
BobH
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This is a way bigger problem than you might expect. Proper vacuum pumps emit noxious oil mists too especially when pumping down stuff that's wet and won't allow a hard vacuum anyways. There are good quality clear oils that don't smell as as bad as others, but it's still gross.
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