CRST Troubleshooting & Repair Techniques 4 — Questions and Answers
Question 1: A technician suspects a leaking discharge valve on a reciprocating compressor. Which test can confirm this?
- Measure the subcooling
- Perform a pump-down test and observe suction pressure rise after shutdown (Correct answer)
- Check discharge superheat
- Measure motor winding resistance
Correct answer: Perform a pump-down test and observe suction pressure rise after shutdown
After pump-down, a rapid rise in suction pressure with the compressor off indicates high-pressure gas is leaking back through the discharge valve.
Question 2: What is the purpose of installing a suction line accumulator on a refrigeration system?
- To subcool liquid refrigerant before the metering device
- To prevent liquid refrigerant from entering the compressor (Correct answer)
- To filter moisture from the refrigerant
- To reduce head pressure at startup
Correct answer: To prevent liquid refrigerant from entering the compressor
A suction line accumulator traps liquid refrigerant and oil, allowing only vapor to enter the compressor, protecting it from liquid slugging.
Question 3: A scroll compressor has normal amperage but produces no cooling. What should the technician check FIRST?
- Refrigerant charge
- Phase rotation — scroll compressors must rotate in one direction (Correct answer)
- Crankcase heater operation
- Head pressure control valve
Correct answer: Phase rotation — scroll compressors must rotate in one direction
Scroll compressors operate in only one rotational direction; incorrect phase rotation causes the compressor to spin backwards, moving little or no refrigerant.
Question 4: What condition is indicated when both suction and discharge pressures are abnormally low?
- Condenser fouled
- Refrigerant overcharge
- Large refrigerant leak or severe undercharge (Correct answer)
- TXV hunting
Correct answer: Large refrigerant leak or severe undercharge
When both pressures are low together, the system has lost a significant amount of refrigerant, reducing the overall pressure throughout the circuit.
Question 5: A technician is troubleshooting an intermittent low-pressure lockout on a system. The MOST likely cause is:
- Oversized metering device
- Defrost cycle ending
- Refrigerant leak that worsens under load (Correct answer)
- Condenser fan cycling on high ambient
Correct answer: Refrigerant leak that worsens under load
A slow refrigerant leak may not trip the low-pressure control at startup but causes suction pressure to drop below cutout as the system runs under load.
Question 6: When a filter-drier becomes partially plugged, what is the MOST noticeable symptom?
- High suction pressure
- Temperature drop across the filter-drier and bubbles in the sight glass (Correct answer)
- Increased subcooling downstream
- Higher than normal discharge temperature
Correct answer: Temperature drop across the filter-drier and bubbles in the sight glass
A plugged filter-drier creates a pressure drop that causes refrigerant to flash, resulting in a temperature drop across the drier and bubbles in the downstream sight glass.
Question 7: Acid test results on refrigeration oil show high acid content. What is the CORRECT repair procedure?
- Add oil neutralizer and continue operating
- Replace only the filter-drier
- Replace compressor oil, install suction and liquid line filter-driers, and monitor acid levels (Correct answer)
- Flush the system with nitrogen only
Correct answer: Replace compressor oil, install suction and liquid line filter-driers, and monitor acid levels
High acid content indicates a burnout or severe contamination; the correct procedure is to replace the oil, install acid-rated filter-driers, and recheck acid levels after operation.
CRST Troubleshooting & Repair Techniques 4
A technician suspects a leaking discharge valve on a reciprocating compressor.
Which test can confirm this?