September 2015 Tech Challenge – Answer


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A 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” (not there is only one problem intended) with the following measured conditions on this field service check sheet?

Field Service Check Sheet

Compressor Discharge Temp. 240 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. 89 deg. F.
Evaporator Press./Temp. 157 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

And the Answer is…
The problem appears to be non-condensables (air) in the system causing the unit to cycle on the high head pressure control. If air entered the system, or 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.

 


Posted In: ACCA Now, Tech Challenge

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