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Compressors in coolant systems

  1. Apr 20, 2015 #1
    I'm told the delay time (the time i turn down the thermostat and the time the AC kicks in) is a safety measure to let the coolant pressure equalize. I don't really understand why.
  2. jcsd
  3. Apr 20, 2015 #2


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  4. Apr 20, 2015 #3
  5. Apr 20, 2015 #4
    more about the delay i'm talking about: https://nest.com/support/article/Why-is-cooling-or-heating-sometimes-delayed

    all i'm told so far is " the delay is for the coolant to equalize" which somehow prevents damage to the compressor.

    what i don't get is, the compressor creates a pressure differential. why does it matter whether or not there is an initial state of pressure difference?
  6. Apr 21, 2015 #5
    I think this must be about shock loading, if the pressure across the compressor is zero as the compressor starts to turn it will take several revolutions till the full pressure differential, and load build up. If the full DP is there already the mechanical load will go from zero to full in a fraction of a second. Think about driving, you don't wind up the revs to max and then drop the clutch in you increase them slightly and ease it in.
  7. Apr 22, 2015 #6
    The coldest portions of the a/c system are the evaporator and the suction line. It is possible for the refrigerant to condense in these areas at shut-down, especially near the TXV or orifice tube (that area is partially liquid already). A quick restart could cause liquid to be pulled into the compressor....which is a recipe for compressor damage. A pause allows the refrigerant to vaporize, the evaporator to warm up, and the high side pressure to drop, as Jobrag stated.
  8. Apr 23, 2015 #7

    jim hardy

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    The compressor should not be asked to start from standstill when there is pressure across it, ie high and low sides not almost equalized.
    It should start with low differential across itself so that it has momentum to carry the piston past its compression stroke by the time pressure builds up.
    (actually it's probably a rotary compressor, rolling ring style)

    see fig 5c here, page 13 of pdf

    you've felt this effect if you have ever pull started a big outboard motor or a chainsaw.
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