I suspect that an auto a/c compressor may not be able to pull not
much of a vacum. Oil contamination may be part of the problem also,
but to change it you will have to drain the old out, maybe by tipping
it up and pouring it out the suction line. If water / moisture gets
into the compressor it will hide under the oil and affect
performance. You should be able to find out the oil charge in the
compressor by using the model number off the compressor. Which type
of oil is also a factor, the new oils are hydrophilic (sp) (water
loving). So they will suck moisture out of the air very quickly. POE
and PAG oils especiallly. You should use one of the old mineral oils
instead.
A refrigeration compressor (reciprocating) would be a better choice
for a recycled compressor for this type of use. A/C compressors have
a lower volumetric efficency. There is more volume between the
exhaust valves and the piston head (etc) that re-expands after the
compression stroke. I think for the amount of time you’ll spend on
maintanance etc. You may find it would be better to just buy a vacum
pump than to make do with something else. Vacum pumps for HVACR
applications are readily available, I think Graingers even sells
them. Ritchie Yellow Jacket, J/B, Thermal are all common brand names.
Some compressors just may not be able to pull much vacum as that’s
not what they are designed for. There are warnings on the new scroll
type compressors against running them into a vacum, due to damage
concerns.
Dan Wellman
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