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May Tech Challenge ANSWER

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A commercial rooftop with R-410A refrigerant and a fixed orifice feed device has a customer complaint of insufficient cooling. The symptoms confirm a warm space and the compressor appears to be running continuously. What are the “possible causes” with the following measured conditions on this field service check sheet? (Note: There is only one problem intended.)

Field Service Check Sheet

Compressor Discharge Temp. 205 deg. F.
Condensing Press./Temp. 338 psig / 104 deg. F.
Condensing Outlet Temp. 90 deg. F.
Condenser Sub cooling 14 deg. F.
Condenser Split 16 deg. F.
Entering Feed Device Temp. 50 deg. F.
Evaporator Press./Temp. 55 psig / 32 deg. F.
Evaporator Outlet Temp. 68 deg. F.
Evaporator Superheat 36 deg. F.
Compressor Inlet Temp. 78 deg. F.
Total S. H. 46 Deg. F.
Ambient Temp. 88 deg. F.
Room/Box Temp. 80 deg. F.
Compressor Volts 240 V.
Compressor Amp. Draw Low


The problem is likely to be a partial restriction ahead of the feed device. The temperature entering the feed device is lower (50 deg. F.) than the ambient temperature of 88 deg. F., caused by the partial restriction boiling of refrigerant (evaporation). The likely problem is a partially blocked filter/drier. The low side of the system is starved and is not picking up heat in the evaporator, so the high side conditions are low also with low pressures and splits. The superheat is high, causing a high discharge temperature, and the condensing subcooling is within one degree of ambient and appears to be normal because of the low refrigerant flow. 


Dick Shaw

Posted In: Tech Challenge

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