Tech Challenge – November 2013
A Commercial Roof Top unit utilizing R-22 with a TXV metering device (and receiver) has a customer complaint of “insufficient cooling.” The roof top appears to be running 100% 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.||235 deg. F.|
|Condensing Press./Temp.||275 psig / 124 deg. F.|
|Condensing Outlet Temp.||112 deg. F.|
|Condenser Sub cooling||12 deg. F.|
|Condenser Split||39 deg. F.|
|Entering Feed Device Temp.||88 deg. F.|
|Evaporator Press./Temp.||73 psig / 43 deg. F.|
|Evaporator Outlet Temp.||52 deg. F.|
|Evaporator Superheat||11 deg. F.|
|Compressor Inlet Temp.||65 deg. F.|
|Total S. H.||24 Deg. F.|
|Ambient Temp.||85 deg. F.|
|Room/Box Temp.||80 deg. F.|
|Compressor Volts||240 V.|
|Compressor Amp. Draw||High|
Share this challenge with your technicians to find out if they can figure out the problem. You can also share your thoughts here by posting them in the comments section below. Check back after January 2, 2014, for the correct solution.
And the answer is…
The problem with tc-8 is a dirty or restricted air flow over the condenser caused from cotton wood restricting the condenser inlet. The condensing pressure/temp. and condenser split are high to expel the heat gained in the evaporator, S.H and compressor. The high compression ratio, is causing recompression, high discharge temperature and high amperage. The Subcooling and superheat are high to near normal because the TXV is attempting to maintain a constant S.H. with a lower refrigerant flow rate caused by the higher compression ratio.
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