USCG Exam PracticeRefrigeration cycle and components
A freezer room evaporator coil is operating normally but cooling capacity has been steadily declining over several days. Head pressure and suction pressure are within normal range, and the sight glass shows a solid liquid column. What is the MOST likely cause?
- A. Refrigerant undercharge due to a slow leak
- ✓ Frost and ice buildup on the evaporator coil insulating the surface and blocking airflowCorrect
- C. Non-condensable gas accumulating in the condenser shell
- D. The TXV sensing bulb has lost its charge, starving the evaporator
Why B is correct
When both operating pressures and the sight glass are normal, the loss of capacity points to an airflow problem at the evaporator rather than a refrigerant-side fault. Freezer coils operating below freezing accumulate frost and ice that insulate the coil and block airflow, steadily cutting capacity; the remedy is scheduled defrost. Non-condensable gas and undercharge would both affect operating pressures.