CARO Compressor Operations and Maintenance 2 — Questions and Answers
Question 1: An operator notices frost forming on the suction line all the way back to the compressor crankcase of an ammonia reciprocating unit. What is the most likely concern?
- Liquid floodback risking slugging of the compressor (Correct answer)
- A leaking discharge valve
- Excessive superheat causing high discharge temperature
- Low oil level in the crankcase
Correct answer: Liquid floodback risking slugging of the compressor
Frost at the crankcase indicates very low or zero superheat, meaning liquid refrigerant may be returning and can slug the compressor.
Question 2: What is the primary purpose of a crankcase heater on a reciprocating compressor?
- To prevent the motor windings from freezing
- To keep the oil warm and limit refrigerant migration into the crankcase during the off cycle (Correct answer)
- To raise discharge pressure during startup
- To preheat suction gas before compression
Correct answer: To keep the oil warm and limit refrigerant migration into the crankcase during the off cycle
Crankcase heaters keep oil warmer than the coldest point in the system so refrigerant does not condense and dilute the oil while the compressor is off.
Question 3: During startup of a reciprocating compressor after a long shutdown, the oil in the sight glass begins to foam heavily. What is the most likely cause?
- Overfilled oil separator
- Excessive discharge superheat
- Refrigerant dissolved in the oil boiling off as crankcase pressure drops (Correct answer)
- Oil pump gear wear
Correct answer: Refrigerant dissolved in the oil boiling off as crankcase pressure drops
Refrigerant that migrated into the oil flashes off when suction pressure drops at startup, causing foaming and possible oil loss.
Question 4: On a reciprocating compressor, what does a 'net oil pressure' reading represent?
- Crankcase pressure minus atmospheric pressure
- Oil pump discharge pressure minus condensing pressure
- Total oil pump discharge pressure alone
- Oil pump discharge pressure minus crankcase (suction) pressure (Correct answer)
Correct answer: Oil pump discharge pressure minus crankcase (suction) pressure
Net oil pressure is pump discharge pressure minus crankcase pressure, the true pressure available to lubricate bearings.
Question 5: Which condition most directly increases the compression ratio of a refrigeration compressor?
- Lowering suction pressure while condensing pressure stays the same (Correct answer)
- Lowering condensing pressure
- Raising suction pressure while condensing pressure stays the same
- Reducing suction superheat
Correct answer: Lowering suction pressure while condensing pressure stays the same
Compression ratio is absolute discharge pressure divided by absolute suction pressure, so lowering suction pressure raises the ratio.
Question 6: Why must compression ratios be calculated using absolute pressures (psia) rather than gauge pressures (psig)?
- Absolute pressures are required only for screw compressors
- Using gauge pressure can produce a false ratio, especially when suction is near or below atmospheric pressure (Correct answer)
- Gauge pressure ignores refrigerant temperature
- Gauge readings are always inaccurate on ammonia systems
Correct answer: Using gauge pressure can produce a false ratio, especially when suction is near or below atmospheric pressure
Gauge pressure excludes atmospheric pressure, so ratios at low suction pressures would be grossly wrong or undefined without converting to absolute.
Question 7: An operator hears a sharp knocking sound from a reciprocating compressor that worsens when the suction line ices up. What should be done first?
- Increase compressor speed to clear the liquid
- Open the discharge bypass to the high side
- Throttle or close the suction stop valve and reduce feed to the evaporator to stop liquid carryover (Correct answer)
- Add oil to the crankcase immediately
Correct answer: Throttle or close the suction stop valve and reduce feed to the evaporator to stop liquid carryover
Knocking with frosted suction indicates liquid slugging, so the operator should throttle suction and correct the liquid feed to protect the compressor.
CARO Compressor Operations and Maintenance 2
An operator notices frost forming on the suction line all the way back to the compressor crankcase of an ammonia reciprocating unit.
What is the most likely concern?