March 2017 Tech Challenge - Answer - IE3: Indoor Environment & Energy Efficiency

March 2017 Tech Challenge – Answer

An R-410A rooftop system with a fixed orifice has a customer complaint of insufficient cooling. The symptoms confirm a warm building and the blower and compressor appears to be running all of the time. 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. 243 deg. F.
Condensing Press./Temp. 431 psig / 122 deg. F.
Condensing Outlet  100 deg. F.
Condenser Sub cooling 22 deg. F.
Condenser Split 42 deg. F.
Entering Feed Device Temp. 85 deg. F.
Evaporator Press./Temp. 143 psig / 50 deg. F.
Evaporator Outlet Temp. 59 deg. F.
Evaporator Superheat 9 deg. F.
Compressor Inlet Temp. 65 deg. F.
Total S. H. 15 deg. F.
Ambient Temp. 80 deg. F.
Room/Box Temp. 82 deg. F
Compressor Volts 240 V.
Compressor Amp. Draw  High

And the answer is: The possible cause appears to be an overcharged system. The high discharge temperature is caused by a high compression ratio and the high condensing pressure/temperature is caused by the bottom of the condenser being flooded with subcooled refrigerant reducing the effective size so that all of the heat absorbed in the evaporator, suction line, motor heat, high heat of compression from the high compression ratio has to be rejected in a smaller portion of the condenser.

The low side is following the high side pressures/temperatures preventing adequate cooling and the near normal superheats are the result of the higher discharge pressure forcing more subcooled liquid into the evaporator. If more refrigerant was present in the system the superheats would decrease and could cause liquid flood back to the compressor. The Amperage draw is somewhat higher because of the higher evaporator pressure.

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