September 2016 Tech Challenge – Answer

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An Ice Cream freezer utilizing R-404A with a TXV metering device (and receiver) has a customer complaint of “soft ice cream.” The symptoms at the store confirm that the case is 20 degrees warmer than normal and the condensing unit appears to be cycling on the low pressure control. What are the “possible causes” (note: there is only one problem intended) with the following measured conditions on this field service check sheet?

Field Service Check Sheet

Compressor Discharge Temp. 92 deg. F.
Condensing Press./Temp. 205 psig / 90 deg. F.
Condensing Outlet Temp. 82 deg. F.
Condenser Sub cooling 8 deg. F.
Condenser Split 15 deg. F.
Entering Feed Device Temp. 77 deg. F.
Evaporator Press./Temp. 29 psig / -4 deg. F.
Evaporator Outlet Temp. -3 deg. F.
Evaporator Superheat 1 deg. F.
Compressor Inlet Temp. 0 deg. F.
Total S. H. 4 Deg. F.
Ambient Temp. 75 deg. F.
Room/Box Temp. 20 deg. F
Compressor Volts 240 V.
Compressor Amp. Draw Normal

Answer: The possible problem is restricted air flow over the evaporator coil, most likely a frosted evaporator caused by a failed defrost system. With the frost insulating the load (heat) from the refrigerant the pressures and splits are low as well as the S.C. and S.H. The refrigerant entering the compressor is barely above saturation and compressor flooding resulting in compressor failure could result. The normal amp draw is caused by the nearly saturated, high density refrigerant coming back to the compressor.

Posted In: ACCA Now, Tech Challenge

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