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Tech Challenge May 2019 – Answer

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A commercial rooftop utilizing R-410A with a TXV feed device(and receiver)has a manager complaint of “no cooling.”The symptoms confirm inadequate cooling and the condensing unit and blower appears to be running all of the time and the thermostat is not satisfied. 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. 228 deg. F
Condensing Press./Temp. 390 psig / 115 deg. F.
Condensing Outlet 98 deg. F.
Condenser Sub cooling 17 deg. F.
Condenser Split 27 deg. F.
Entering Feed Device Temp. 8 deg. F.
Evaporator Press./Temp. 130 psig / 45 deg. F.
Evaporator Outlet Temp. 56 deg. F.
Evaporator Superheat 11 deg. F.
Compressor Inlet Temp. 68 deg. F.
Total S. H. 23 deg. F.
Ambient Temp. 88 deg. F.
Room/Box Temp. 78 deg. F
Compressor Volts 240 V.
Compressor Amp. Draw High

And the Answer Is: The possible problem appears to be a system overcharge. The overcharge would cause high head pressure/temperature and condenser split because the condenser is nearly filled with liquid refrigerant thereby reducing the effective size (area of heat transmission) of the condenser.The condensing pressure/temperature,and condenser split are high to expel the heat gained in the evaporator, superheat and compressor. The high compression ratio is causing recompression, high discharge temperatures and high amperage. The subcooling and superheat are high to normal because the TXV is attempting to maintain a constant S.H. with a lower refrigerant flow caused by the higher compression ratio.

Posted In: ACCA Now, Tech Challenge

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