CARO Compressor Operations and Maintenance 5 — Questions and Answers
Question 1: A compressor's high-pressure cutout trips repeatedly. Which condition is the most likely cause?
- Low oil level
- Non-condensable gases or loss of condenser water/air flow (Correct answer)
- Clogged suction strainer
- Excessive suction superheat
Correct answer: Non-condensable gases or loss of condenser water/air flow
Non-condensables or poor condenser heat rejection raise head pressure until the high-pressure cutout opens.
Question 2: What is the function of the oil pressure safety (oil failure) switch on a compressor?
- Controls the crankcase heater
- Shuts down the compressor when discharge pressure is too high
- Stops the compressor if net oil pressure does not reach a safe level within a set time delay (Correct answer)
- Opens the hot-gas bypass during startup
Correct answer: Stops the compressor if net oil pressure does not reach a safe level within a set time delay
The oil failure switch shuts down the compressor when net oil pressure stays below its setpoint beyond the time delay.
Question 3: Why does an oil failure switch include a time delay, often around 90-120 seconds?
- To let refrigerant pressures equalize
- To cool the discharge gas
- To keep the motor running after shutdown
- To allow oil pressure to build during startup without nuisance trips (Correct answer)
Correct answer: To allow oil pressure to build during startup without nuisance trips
The delay gives the oil pump time to build pressure at startup so the compressor is not shut down needlessly.
Question 4: What happens if a compressor short-cycles frequently?
- Increased motor and contactor wear, poor oil return, and higher energy use (Correct answer)
- Oil return improves and wear decreases
- Capacity increases at every start
- Discharge temperatures drop permanently
Correct answer: Increased motor and contactor wear, poor oil return, and higher energy use
Frequent starts stress motors and contactors and may not allow enough run time to return oil properly.
Question 5: What is the purpose of the suction strainer installed at a compressor inlet?
- To separate oil from discharge gas
- To trap scale, dirt, and debris before they enter the compressor (Correct answer)
- To meter refrigerant into the evaporator
- To remove non-condensable gases
Correct answer: To trap scale, dirt, and debris before they enter the compressor
The suction strainer catches solid debris that could damage valves, pistons, or rotors.
Question 6: In a two-stage ammonia compression system, what is the role of the booster compressor?
- To compress gas from the intermediate pressure to the condensing pressure
- To purge non-condensables from the condenser
- To compress gas from the low-stage suction to the intermediate pressure (Correct answer)
- To pump liquid to the evaporators
Correct answer: To compress gas from the low-stage suction to the intermediate pressure
The booster raises low-temperature suction gas to the intermediate pressure, where the high-stage compressor takes it to condensing pressure.
Question 7: An operator observes that a compressor's discharge temperature is higher than normal while suction superheat is also very high. What is the likely cause?
- Excessive oil in the crankcase
- Overcharged condenser
- Liquid floodback to the compressor
- Starved evaporator or underfed system producing high-superheat suction gas (Correct answer)
Correct answer: Starved evaporator or underfed system producing high-superheat suction gas
High suction superheat means hot gas enters the compressor, which raises discharge temperature, often due to evaporator starvation.
CARO Compressor Operations and Maintenance 5
A compressor's high-pressure cutout trips repeatedly.
Which condition is the most likely cause?