Tech Challenge – September 2012
A frozen food case utilizing R-404A with a TXV metering device (and receiver) has a customer complaint of “warm case.” The symptoms at the store confirm that the case is 30 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.||225 deg. F.|
|Condensing Press./Temp.||205 psig / 90 deg. F.|
|Condensing Outlet Temp.||78 deg. F.|
|Condenser Sub cooling||12 deg. F.|
|Condenser Split||15 deg. F.|
|Entering Feed Device Temp.||77 deg. F.|
|Evaporator Press./Temp.||10 psig / -30 deg. F.|
|Evaporator Outlet Temp.||5 deg. F.|
|Evaporator Superheat||35 deg. F.|
|Compressor Inlet Temp.||60 deg. F.|
|Total S. H.||90 Deg. F.|
|Ambient Temp.||75 deg. F.|
|Room/Box Temp.||14 deg. F.|
|Compressor Volts||240 V.|
|Compressor Amp. Draw||Low|
Share this challenge with your technicians to find out if they can figure out the problem. You can also share your thoughts here by posting them in the comments section below. Check back after November 1 for the correct solution.
And the answer is…
The problem is an inefficient compressor. The pressures have tended to equalize and a very high discharge temperature is caused by the recompression of gases. The TXV is wide open, but there is very low refrigerant flow causing maximum subcooling and high superheat. The amps are low because the compressor is not—cannot— work very hard.
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