July 2017 Tech Challenge - Answer - IE3: Business Tools for HVAC & Plumbing Contractors

July 2017 Tech Challenge – Answer

A freezer utilizing R-402A with a TXV metering device (and receiver) has a customer complaint of “warm case.” The symptoms at the store confirm that the case that normally operates about 0 degrees is 34 degrees and the condensing unit is running continuously. What are the “possible causes” (note: there is only one problem intended) with the following measured conditions on this field service check sheet?et?

Field Service Check Sheet

Compressor Discharge Temp. 240 deg. F.
Condensing Press./Temp. 204 psig / 90 deg. F.
Condensing Outlet 87 deg. F.
Condenser Sub cooling 3 deg. F.
Condenser Split 5 deg. F.
Entering Feed Device Temp. 86 deg. F.
Evaporator Press./Temp. 48 psig / 14 deg. F.
Evaporator Outlet Temp. 36 deg. F.
Evaporator Superheat 22 deg. F.
Compressor Inlet Temp. 54 deg. F.
Total S. H. 40 deg. F.
Ambient Temp. 85 deg. F.
Room/Box Temp.34  deg. F
Compressor Volts 240 V.
Compressor Amp. Draw Low

And the answer is…The possible cause appears to be an inefficient compressor. The head pressure is lower and the suction pressure is higher than normal with a very high discharge temperature caused by the recompression of gases. The TXV is wide open, but there is very low refrigerant flow causing maximum subcooling (to with-in 2 deg. of ambient) and high superheat. The amps are low because the compressor isn't working. A compressor efficiency check per manufacturers specifications needs to be accomplished. 

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