RSES Refrigeration Fundamentals & System Components 4 — Questions and Answers
Question 1: A reciprocating compressor with leaking discharge valves will most likely exhibit:
- Higher than normal discharge pressure
- Lower than normal discharge pressure and reduced capacity (Correct answer)
- Higher than normal suction pressure only
- Increased oil pressure
Correct answer: Lower than normal discharge pressure and reduced capacity
Leaking discharge valves allow compressed gas to flow back into the cylinder, reducing compression efficiency and lowering discharge pressure and system capacity.
Question 2: The term 'flooding back' in a refrigeration system describes:
- Excessive subcooling in the condenser
- Liquid refrigerant returning to the compressor through the suction line (Correct answer)
- High oil level in the evaporator
- Refrigerant migration to the crankcase during off-cycle
Correct answer: Liquid refrigerant returning to the compressor through the suction line
Flooding back occurs when liquid refrigerant travels through the suction line to the compressor, risking hydraulic damage and oil dilution.
Question 3: What is the purpose of a crankcase heater on a refrigeration compressor?
- Increase oil viscosity during operation
- Prevent refrigerant migration into the oil during off-cycle (Correct answer)
- Heat the suction gas before compression
- Maintain discharge temperature limits
Correct answer: Prevent refrigerant migration into the oil during off-cycle
Crankcase heaters keep the oil warm during off-cycles to prevent refrigerant from migrating into and dissolving in the crankcase oil.
Question 4: In an evaporative condenser, heat is primarily rejected by:
- Sensible heat transfer to dry air only
- Latent heat of evaporation of water sprayed over the coil (Correct answer)
- Conduction through the condenser wall to surrounding soil
- Radiation from the hot refrigerant coils
Correct answer: Latent heat of evaporation of water sprayed over the coil
Evaporative condensers reject heat mainly through the latent heat of water evaporation, making them more efficient than dry air-cooled condensers.
Question 5: Which of the following best describes 'approach temperature' for a water-cooled condenser?
- Temperature difference between entering and leaving water
- Temperature difference between condensing refrigerant and leaving water (Correct answer)
- Temperature rise of refrigerant across the condenser
- Saturation temperature at the condenser inlet
Correct answer: Temperature difference between condensing refrigerant and leaving water
Approach temperature is the difference between the refrigerant condensing temperature and the leaving condenser water temperature, indicating heat exchanger efficiency.
Question 6: A scroll compressor differs from a reciprocating compressor in that it:
- Uses pistons and connecting rods to compress refrigerant
- Uses two interleaving spiral scrolls without intake and discharge valves (Correct answer)
- Requires external lubrication through a separate oil pump
- Can only operate in a single speed mode
Correct answer: Uses two interleaving spiral scrolls without intake and discharge valves
Scroll compressors use two meshing spiral scrolls to trap and compress refrigerant, eliminating the need for suction and discharge valves.
Question 7: Non-condensable gases (such as air) trapped in a refrigeration system will cause:
- Lower than normal discharge pressure
- Higher than normal discharge pressure and reduced efficiency (Correct answer)
- Lower evaporator temperature
- Increased refrigerant subcooling
Correct answer: Higher than normal discharge pressure and reduced efficiency
Non-condensables occupy condenser volume and add to total discharge pressure without contributing to refrigerating effect, reducing system efficiency.
RSES Refrigeration Fundamentals & System Components 4
A reciprocating compressor with leaking discharge valves will most likely exhibit: