March 2015 Tech Challenge – Answer

Posted on:

April 30, 2015

A frozen food case utilizing R-404A with a TXV metering device (and receiver) has a customer complaint of a “warm case.” The symptoms at the store confirm that the case is 25 degrees warmer than normal 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?

Field Service Check Sheet

Compressor Discharge Temp.	230 deg. F.
Condensing Press./Temp.	181 psig / 82 deg. F.
Condensing Outlet Temp.	80 deg. F.
Condenser Sub cooling	2 deg. F.
Condenser Split	12 deg. F.
Entering Feed Device Temp.	78 deg. F.
Evaporator Press./Temp.	15 psig / -22 deg. F.
Evaporator Outlet Temp.	12 deg. F.
Evaporator Superheat	32 deg. F.
Compressor Inlet Temp.	48 deg. F.
Total S. H.	54 Deg. F.
Ambient Temp.	70 deg. F.
Room/Box Temp.	20 deg. F.
Compressor Volts	240 V.
Compressor Amp. Draw	Low

And the answer is…

The problem appears to be an undercharged system. The symptoms include low pressures, sub cooling and amps with high superheats. With the undercharge, the TXV may be feeding the evaporator with liquid as well as vapor refrigerant starving the evaporator. Between the high superheats and high compression ratio caused by a starved evaporator the discharge temperature is high. The sub cooling is low because there is insufficient liquid refrigerant, and the condenser split is low because of less heat being picked up in the evaporator caused by the undercharge.