A commercial rooftop utilizing R-410A with a TXV feed device has a customer complaint of insufficient cooling. The symptoms confirm inadequate cooling and the compressor appears to be short cycling. 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. 248 deg. F.
Condensing Press./Temp. 444 psig / 125 deg. F.
Condensing Outlet Temp. 90 deg. F.
Condenser Sub cooling 35 deg. F.
Condenser Split 37 deg. F.
Entering Feed Device Temp. 90 deg. F.
Evaporator Press./Temp. 154 psig / 55 deg. F.
Evaporator Outlet Temp. 68 deg. F.
Evaporator Superheat 13 deg. F.
Compressor Inlet Temp. 73 deg. F.
Total S. H. 18 Deg. F.
Ambient Temp. 88 deg. F.
Room/Box Temp. 78 deg. F.
Compressor Volts 240 V.
Compressor Amp. Draw High
Answer: The condenser air flow and coil cleanliness appear normal. The problem appears to have non-condensable’s (air) in the system causing the unit to cycle on the high head pressure control. If air/moisture entered the system, and was not evacuated it will accumulate in the top of the condenser reducing the effective size of the condenser causing high head pressures and high discharge temperatures caused from high compression ratios. Higher condensing subcooling and split will result because of the elevated condensing temperatures. With a TXV the evaporator side (pressures and superheats) will be normal to a little higher unless the pressures exceed the range of the valve. The amps are high because of the higher compression ratios causing the compressor to work harder. The refrigerant needs to be recovered (and recycled), filter drier replaced and the system evacuated with deep evacuation and critically charged to mfg. specifications.
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