July/August 2024 ACCA Now Tech Challenge
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A frozen food case utilizing HFC R-404A with a TXV metering device (and receiver) has a customer complaint of “warm product.” The symptoms at the convenience 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. 203 psig / 88 deg. F.
- Condensing Outlet Temp. 81 deg. F.
- Condenser Sub cooling 7 deg. F.
- Condenser Split 13 deg. F.
- Entering Feed Device Temp. 77 deg. F.
- Evaporator Press./Temp. 29 psig / -6 deg. F.
- Evaporator Outlet Temp. -3 deg. F.
- Evaporator Superheat 3 deg. F.
- Compressor Inlet Temp. 2 deg. F.
- Total S. H. 8 Deg. F.
- Ambient Temp. 75 deg. F.
- Room/Box Temp. 20 deg. F.
- Compressor Volts 240 V.
- Compressor Amp. Draw Normal
Answer: The problem with the freezer centers on 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
