RSES Compressor Systems 2 — Questions and Answers
Question 1: What condition is indicated when a reciprocating compressor's suction pressure is significantly lower than expected during operation?
- Overcharge of refrigerant
- Worn or leaking suction valves (Correct answer)
- High ambient temperature
- Oversized metering device
Correct answer: Worn or leaking suction valves
Worn or leaking suction valves allow refrigerant vapor to flow back into the suction side, reducing pumping efficiency and lowering suction pressure.
Question 2: Which type of compressor uses a fixed-volume ratio and cannot adjust to varying system pressure ratios?
- Centrifugal compressor
- Reciprocating compressor
- Screw compressor (Correct answer)
- Scroll compressor
Correct answer: Screw compressor
Screw compressors have a fixed built-in volume ratio determined by the rotor geometry, making them less efficient when operating far from their design pressure ratio.
Question 3: What is the primary purpose of an unloader on a reciprocating compressor?
- To increase discharge pressure during peak load
- To reduce compressor capacity by disabling cylinders (Correct answer)
- To cool the compressor oil during high-load operation
- To bypass refrigerant from discharge to suction
Correct answer: To reduce compressor capacity by disabling cylinders
Unloaders disable individual cylinders or hold suction valves open to reduce compressor capacity and match system cooling load.
Question 4: A compressor operates with high discharge temperature and low compression ratio. What is the most likely cause?
- Refrigerant overcharge
- High suction superheat entering the compressor (Correct answer)
- Flooded start condition
- Liquid slugging
Correct answer: High suction superheat entering the compressor
High suction superheat means the refrigerant vapor enters the compressor at an elevated temperature, resulting in high discharge temperatures even with a normal compression ratio.
Question 5: In a two-stage compression system, what is the function of the intercooler?
- To subcool liquid refrigerant before the expansion valve
- To cool discharge gas between the first and second stage (Correct answer)
- To superheat suction gas before first-stage compression
- To separate oil from refrigerant between stages
Correct answer: To cool discharge gas between the first and second stage
The intercooler removes heat from the first-stage discharge gas before it enters the second-stage compressor, improving overall efficiency.
Question 6: What causes a scroll compressor to produce a loud rattling noise shortly after startup?
- Refrigerant migrated to the compressor during the off cycle causing liquid slugging (Correct answer)
- Worn motor bearings from normal use
- Excessive superheat at the suction port
- High discharge pressure from a blocked condenser
Correct answer: Refrigerant migrated to the compressor during the off cycle causing liquid slugging
Refrigerant migration during the off cycle can cause liquid accumulation in the compressor shell, and upon startup, slugging of liquid through the scroll sets creates a loud rattling noise.
Question 7: Which compressor characteristic is described by the term 'clearance volume'?
- The total volume swept by the piston on one stroke
- The dead space remaining in the cylinder at the top of the compression stroke (Correct answer)
- The volume of oil retained in the crankcase
- The space between rotor tips in a screw compressor
Correct answer: The dead space remaining in the cylinder at the top of the compression stroke
Clearance volume is the small volume remaining between the piston and cylinder head at top dead center, and re-expansion of this gas reduces volumetric efficiency.
RSES Compressor Systems 2
What condition is indicated when a reciprocating compressor's suction pressure is significantly lower than expected during operation?