Tech Challenge – July 2012
A walk-in Freezer designed to operate at Zero 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 warm product of 15 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.||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 technicans 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 September 1 for the correct solution.
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 subcooled 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 subcooling levels.
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