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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