From: Neon John <johngd@bellsouth.net> Newsgroups: sci.engr.heat-vent-ac Subject: Re: Auto vs. Home HVAC Test Temps? Date: Tue, 04 Dec 2001 15:56:06 -0500 Message-ID: <3C0D3866.1BE5E3A5@bellsouth.net> Inspector wrote: > > Can someone in this group please explain to me why it is believed in the > home inspection business that a residential A/C compressor can be damaged by > operation at too low an outside temperature, but yet an automobile A/C > operates in conjuction with the defroster at any temperature? Like all rules of thumb, this one isn't universal. The potentially damaging condition can arise with split units that use long refrigerant tube runs. The refrigerant will condense in the coldest spot in the system. When outside is colder than inside, this spot will be the condensing unit (compressor and condenser.) A split unit with long refrigerant tube runs may contain enough refrigerant to fill the compressor can and/or condenser. Normally the compressor can will fill first because the cold compressor can will reduce the pressure in the suction side and cause refrigerant gas to flow from other parts of the system where it condenses on the can. Many times a compressor will damage its valves or other structures if it attempts to compress liquid. If the compressor is (partially) filled with liquid refrigerant/oil mix, it is likely that the compressor will try to compress liquid when it starts. The normal solution to this problem is for the compressor to be equipped with some sort of crankcase (can) heat so that the compressor is no longer the coldest spot in the system. This may be in the form of a wrap-around strap heater, a cartridge heater inserted in a welded socket in the can or even a capacitor that bypasses the contactor and meters some power to the compressor when it is "off". The coil heating does the trick. Even the cheapest units manufactured today seem to have crankcase heaters so this isn't much of a problem anymore. And this problem normally never manifests itself with package units or window units simply because the system does not contain enough refrigerant to fill the compressor plus most of the system is in the cold space so the refrigerant is spread out. A car AC doesn't have this problem because a) the refrigerant charge is relatively low, b) most of the system is in the cold space so the refrigerant spreads out, c) the compressor, being on top of the engine, will normally remain warm long after the condenser and plumbing has cooled to ambient, d) the compressor is normally turning fairly slowly at startup, when the problem would manifest itself and e) the automotive compressors are fairly resistant to damage from liquid pumping, particularly at low speed. BTW, you are correct - many AC systems run the compressor on defrost regardless of the temperature. Older systems from all the Big Three did that. They used some sort of back pressure regulating valve (POA in the case of GM) to keep the evaporator from freezing. Newer stuff (80s up) uses either an evaporator thermostat or low side head switch to cycle the compressor. John -- John De Armond johngdDONTYOUDARE@bellsouth.net http://personal.bellsouth.net/~johngd Cleveland, occupied TN |