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September 2014 Tech Challenge Answer

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A Reach-in Ice Cream Freezer designed to operate at -15 degrees and utilizing R-404A with a liquid receiver and TXV metering device has a customer complaint of “warm product.” The symptoms at the convenience store confirm that soft product of 0 deg. F. and the condensing unit and blower are running continuously. What are the “possible causes” with the following measured conditions on this field service check sheet?

Field Service Check Sheet

Compressor Discharge Temp. 230 deg. F.
Condensing Press./Temp. 198 psig / 88 deg. F.
Condensing Outlet Temp. 76 deg. F.
Condenser Sub cooling 12 deg. F.
Condenser Split 13 deg. F.
Entering Feed Device Temp. 77 deg. F. Read just ahead of going into the freezer
Evaporator Press./Temp. 7 psig / -35 deg. F.
Evaporator Outlet Temp. 5 deg. F.
Evaporator Superheat 40 deg. F.
Compressor Inlet Temp. 62 deg. F.
Total S. H. 97 Deg. F.
Ambient Temp. 75 deg. F.
Room/Box Temp. 0 deg. F.
Compressor Volts 240 V.
Compressor Amp. Draw Low

And the answer is…

The possible Problem is a partially restricted TXV Inlet creating a very low refrigerant flow rate and causing the evaporator, Compressor and Condenser to be starved of refrigerant. This results in low suction and condensing pressures, high superheats and low amperage draw. If the sub-cooled liquid entering the Freezer is above ambient temperature(which it is), there has been no flash off of liquid refrigerant prior to the TXV and the restriction is likely a plugged screen at the inlet of the TXV. Since the symptoms for an undercharged system are very similar, many may have listed this as the possible cause, and the main difference is that with an undercharge, there would be lower sub-cooling levels.

Posted In: ACCA Now, Tech Challenge

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