November Tech Challenge – Answer


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A  Rooftop with R-438A retrofitted refrigerant and a fixed orifice-metering device has a customer complaint of “no cooling.” The symptoms at the business confirm that the thermostat is not satisfied and the blower is 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. 242 deg. F.
Condensing Press./Temp. 287 psig / 122 deg. F.
Condensing Outlet Temp. 102 deg. F.
Condenser Sub cooling 20 deg. F.
Condenser Split 42 deg. F.
Entering Feed Device Temp. 90 deg. F.
Evaporator Press./Temp. 100 psig / 55 deg. F.
Evaporator Outlet Temp. 60 deg. F.
Evaporator Superheat 5 deg. F.
Compressor Inlet Temp. 70 deg. F.
Total S. H. 15 Deg. F.
Ambient Temp. 80 deg. F.
Room/Box Temp. 82 deg. F.
Compressor Volts 240 V.
Compressor Amp. Draw High

And The Answer Is…

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). 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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