RSES Compressor Systems 4 — Questions and Answers
Question 1: What is the purpose of a check valve installed in the compressor discharge line?
- To prevent reverse rotation during the off cycle (Correct answer)
- To regulate discharge pressure during high-load conditions
- To allow oil return from the condenser to the compressor
- To limit suction pressure during pull-down
Correct answer: To prevent reverse rotation during the off cycle
A discharge check valve prevents high-pressure refrigerant from flowing backward through the compressor during the off cycle, which could cause reverse rotation and compressor damage.
Question 2: A technician measures equal suction and discharge pressures on a running compressor. What is the most likely diagnosis?
- Normal operation at low ambient temperature
- Completely failed or broken compressor valves (Correct answer)
- Correct refrigerant charge with balanced pressures
- Refrigerant overcharge flooding the suction line
Correct answer: Completely failed or broken compressor valves
Equal suction and discharge pressures on a running compressor indicate the compressor is not pumping, almost always caused by totally failed discharge and/or suction valves.
Question 3: Which type of compressor is best suited for large-tonnage chiller applications requiring variable capacity with high efficiency?
- Hermetic reciprocating compressor
- Open-drive centrifugal compressor (Correct answer)
- Rotary vane compressor
- Diaphragm compressor
Correct answer: Open-drive centrifugal compressor
Open-drive centrifugal compressors are used in large chiller applications because they provide high efficiency over a wide capacity range and are easily adapted to variable loads with inlet guide vanes.
Question 4: What is the function of a crankcase heater on a compressor?
- To maintain oil viscosity during compressor operation
- To prevent refrigerant migration into the crankcase oil during the off cycle (Correct answer)
- To preheat suction gas before it enters the compressor
- To keep the motor windings warm in low-ambient conditions
Correct answer: To prevent refrigerant migration into the crankcase oil during the off cycle
Crankcase heaters keep the oil warm during the off cycle to prevent refrigerant from migrating into and diluting the crankcase oil, which could cause oil foaming and bearing damage on startup.
Question 5: What is 'liquid slugging' in a compressor, and what component failure can result?
- Vapor flashing in the suction line; causes icing on the suction valve
- Liquid refrigerant entering the compressor cylinder; can break valves and pistons (Correct answer)
- Oil pooling in the discharge line; causes bearing failure from starvation
- Non-condensable gas in the refrigerant; causes compressor overheating
Correct answer: Liquid refrigerant entering the compressor cylinder; can break valves and pistons
Liquid slugging occurs when liquid refrigerant enters the compressor cylinder; since liquid is incompressible, it can crack or shatter the valves, pistons, or connecting rods.
Question 6: What is the recommended corrective action when a compressor oil pressure safety control trips?
- Reset and restart the compressor immediately without inspection
- Check oil level, inspect for refrigerant dilution, and inspect the oil pump before restarting (Correct answer)
- Add refrigerant charge to restore normal operating pressures
- Replace the compressor motor capacitor and restart
Correct answer: Check oil level, inspect for refrigerant dilution, and inspect the oil pump before restarting
Before restarting after an oil pressure trip, technicians must verify the oil level, check for refrigerant dilution of the oil, and inspect the oil pump to identify the root cause.
Question 7: In a parallel compressor rack system, what is the primary advantage of staging compressors on and off?
- It allows each compressor to run at maximum capacity for best efficiency
- It eliminates the need for oil equalization between compressors
- It matches system capacity to varying load while keeping running compressors near full load (Correct answer)
- It reduces refrigerant charge requirements for the system
Correct answer: It matches system capacity to varying load while keeping running compressors near full load
Staging compressors allows the system to modulate total capacity while keeping each individual running compressor near its most efficient full-load operating point.
RSES Compressor Systems 4
What is the purpose of a check valve installed in the compressor discharge line?