Tech Challenge - November 2018 Answer - IE3: Business Tools for HVAC & Plumbing Contractors  

Tech Challenge – November 2018 Answer

A frozen food case utilizing R-507A 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 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.34 20 deg. F
Compressor Volts 240 V.
Compressor Amp. Draw Normal

Answer: 

The problem with TC-6 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.

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