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November 2016 Tech Challenge Answer

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A rooftop system utilizing R-22 with a fixed orifice metering device has a manager’s complaint of “no cooling.” The symptoms at the store confirm that the thermostat is not satisfied and the condensing unit and blower are running continuously. What are the “possible causes” (there is only one problem intended) with the following measured conditions on this field service check sheet?

Field Service Check Sheet

Compressor Discharge Temp. 230 deg. F.
Condensing Press./Temp. 390 psig / 115 deg. F.
Condensing Outlet Temp. 88 deg. F.
Condenser Sub cooling 27 deg. F.
Condenser Split 35 deg. F.
Entering Feed Device Temp. 82 deg. F.
Evaporator Press./Temp. 129 psig / 45 deg. F.
Evaporator Outlet Temp. 50 deg. F.
Evaporator Superheat 5 deg. F.
Compressor Inlet Temp. 66 deg. F.
Total S. H. 25 Deg. F.
Ambient Temp. 80 deg. F.
Room/Box Temp. 84 deg. F
Compressor Volts 240 V.
Compressor Amp. Draw High

Answer: The Possible Cause for these conditions center on a system overcharge. The overcharge would cause high head pressure/temperature and split because the condenser is largely filled with liquid refrigerant reducing its effective size (area of heat transmission) of the condenser. The low side is following the high side to higher pressures/temperatures thus preventing adequate cooling. The high discharge temperature is caused by the high superheat. The unit may also be cycling on the high head pressure control or compressor overload if the outdoor ambient were to get warmer.

Posted In: ACCA Now, Tech Challenge

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