USCG Exam PracticeRefrigeration cycle and components
A refrigeration compressor's suction pressure will not pull down to normal operating level and the head pressure will not build to normal, yet the compressor is running continuously. The sight glass shows a solid liquid column and there is no frost at the expansion valve or filter-drier. Which component fault BEST accounts for all of these observations?
- A. Plugged filter-drier restricting liquid flow to the expansion valve
- B. Expansion valve sensing bulb has lost its charge, holding the valve wide open
- ✓ Failed compressor with broken suction or discharge valvesCorrect
- D. Non-condensable gas trapped in the condenser raising head pressure
Why C is correct
A compressor with broken valves cannot develop differential pressure: suction pressure will not pull down and head pressure will not build, even though the compressor runs. The solid sight glass and absence of frost at the valve or drier rule out a liquid-line restriction or a failed TXV bulb. Non-condensable gas raises head pressure rather than preventing it from building. This symptom set — inability to develop pressure differential with the compressor running — is the classic indicator of failed compressor valves.