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cold temps

Can someone explain to me how the cold weather affects super heat and subcooling? Thanks Paul S
ASM Mechanical Company
Located in Staten Island NY
Servicing all 5 boroughs of NYC.
[email protected]


  • paul_79
    paul_79 Member Posts: 91
    cold temps

    looks like no one wants to take this one on.
  • Techman
    Techman Member Posts: 2,144
    Passing thru

    Cold temps affects the subcooling because each cubic foot of cond  air is  colder than the minimum ambient temp the system was designed for. A c.f. of 40* air pulls away too much heat from  the cond unit than say a minimum allowable c.f. of 60* ambient air . So the subcooling increases, above and beyond what is "normal" , THE HEAD PRESS DROPS  too low causing metering (TXV)  problems.The SuperHeat does not change. Lets say the AC TXV has 12* of SH at 40* EvapTemp ,giving 52* suction line temp at the thermal bulb . Then put that same TXV in a medium temp walk-in box that has an ET of 20*f and the suction line at the TXV thermal bulb will be 32*f. A freezer w/ a ET of -10*f will have a suction line temp of +02*F.In real life the AC TXV is a little different than a REF TXV and slightly different than a Freezer TXV , but I hope you get my point.
  • Spence
    Spence Member Posts: 316
    edited June 2013
    Cold Temps

    Assuming you're talking about residential units, low ambients have a greater impact with a fixed metering device. Since there is usually very little load, a machine with a FMD is in danger of not having a high enough superheat to avoid liquid refrigerant coming straight from the evaporator into the compressor. TXVs are more forgiving, as their job is to ensure a constant superheat at the compressor. Now to the other side. If the SH is too low, the compressor has to work very hard to mash liquid (instead of vapor) enough to raise the refrigerant temperature high enough above ambient so we can de-superheat it in the condenser. Since we can't change liquid into a liquid, you can imagine what your high and low side pressures will be, as well as a practically non-existing SC. If the compressor doesn't give up chewing on all of that liquid, it certainly will running when it doesn't have any work to do.
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