September 2023 Tech Challenge Answer

Share this challenge with your staff and see who can figure out the diagnosis first.

A frozen food case utilizing R-404A with a TXV metering device (and receiver) has a customer complaint of “warm case.” The symptoms at the store confirm that the case is 20 degrees warmer than normal, and the condensing unit is running continuously. What are the “possible causes” (note: there is only one problem intended) with the following measured conditions on this field service check sheet? 

Note: There is only one problem intended.

Field Service Check Sheet 

• Compressor Discharge Temp. 225 deg. F.
• Condensing Press./Temp. 187 psig / 84 deg. F. 
• Condensing Outlet Temp. 81 deg. F. 
• Condenser Subcooling 3 deg. F. 
• Condenser Split 14 deg. F. 
• Entering Feed Device Temp. 78 deg. F. 
• Evaporator Press./Temp. 15 psig / -22 deg. F. 
• Evaporator Outlet Temp. 12 deg. F. 
• Evaporator Superheat 32 deg. F. 
• Compressor Inlet Temp. 48 deg. F. 
• Total S. H. 54 deg. F. 
• Ambient Temp. 70 deg. F. 
• Room/Box Temp. 20 deg. F. 

Answer: The problem appears to be an undercharged system. The symptoms include low pressures, subcooling, and amps with high superheats. With the undercharge, the TXV may be feeding the evaporator with liquid as well as vapor refrigerant starving, the evaporator. Between the high superheats and high compression ratio caused by a starved evaporator, the discharge temperature is high. The subcooling is low because there is insufficient liquid refrigerant, and the condenser split is low because of less heat being picked up in the evaporator caused by the undercharge.