September Tech Challenge — Answer
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A medium temperature beer cooler utilizing R-134a with a TXV metering device and liquid receiver, has a customer complaint of warm beer. The symptoms at the convenience store confirm inadequate cooling and the compressor appears to be short cycling on the high- or low-pressure control. What are the “possible causes,” (Note: There is only one problem intended) with the following measured/calculated conditions on this field service check sheet?
Field Service Check Sheet
- Compressor Discharge Temp. 225 deg. F.
- Condensing Press. /Temp. 255 psig / 88 deg. F.
- Condensing Outlet Temp. 88 deg. F.
- Condenser Sub cooling 30 deg. F.
- Condenser Split 43 deg. F.
- Entering Feed Device Temp. 85 deg. F.
- Evaporator Press. /Temp. 22 psig / 25 deg. F.
- Evaporator Outlet Temp. 40 deg. F.
- Evaporator Superheat 15 deg. F.
- Compressor Inlet Temp. 68 deg. F.
- Total S. H. 43 deg. F.
- Ambient Temp. 75 deg. F.
- Room/Box Temp. 50 deg. F.
- Compressor Volts 240 V.
- Compressor Amp. Draw Normal/High
Answer: The possible cause of the warm beer appears to be a dirty, restricted air flow, recirculated air, condenser fan or fan blade issue. The condensing pres/temp. is high to reject the heat picked up in the evaporator, S.H. (evaporator and suction line) and compressor. The high compression ratio of 7.3 (absolute discharge pressure of 269.7PSIA, divided by the absolute suction pressure of 36.7PSIA) is causing recompression and a very high discharge temperature. The subcooling and superheat are high to near normal because the TXV is attempting to maintain a constant S.H. with a lower refrigerant flow rate caused by the higher compression ratio.
Posted In: ACCA Now, Tech Challenge
