[Jewelry making - Article 00640] - Re: [Orchid] A/C Compressor for vacuum casting

[dan . wellman]

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    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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