RSES Compressor Systems 5 — Questions and Answers
Question 1: Which parameter is used to express the overall efficiency of a compressor by comparing actual work input to ideal isentropic compression work?
- Coefficient of Performance (COP)
- Volumetric efficiency
- Isentropic (adiabatic) efficiency (Correct answer)
- Mechanical efficiency
Correct answer: Isentropic (adiabatic) efficiency
Isentropic efficiency compares the actual work input to the theoretical minimum work of reversible adiabatic compression, quantifying how well the compressor converts power into refrigerant compression.
Question 2: A scroll compressor will not start and the motor draws locked rotor amperage. After ruling out electrical faults, what mechanical condition should be investigated?
- Low refrigerant charge preventing the scroll sets from mating
- Liquid refrigerant locked in the compression chamber preventing rotation (Correct answer)
- Worn scroll tip seals reducing discharge pressure
- Failed start capacitor on the condenser fan motor
Correct answer: Liquid refrigerant locked in the compression chamber preventing rotation
Liquid refrigerant trapped in the scroll compression chambers is incompressible and can hydraulically lock the compressor, preventing the motor from turning and drawing locked rotor current.
Question 3: What does the term 'compression ratio' mean for a refrigeration compressor?
- The ratio of compressor displacement to condenser volume
- The ratio of absolute discharge pressure to absolute suction pressure (Correct answer)
- The ratio of actual capacity to rated capacity
- The ratio of oil pressure to suction pressure
Correct answer: The ratio of absolute discharge pressure to absolute suction pressure
Compression ratio is the absolute discharge pressure divided by the absolute suction pressure, and it directly affects compressor efficiency, discharge temperature, and valve reliability.
Question 4: What causes 'oil pumping' in a reciprocating compressor, and what symptom would a technician observe?
- Worn piston rings allowing oil past the piston; indicated by oil carryover into the system (Correct answer)
- Low oil viscosity thinning the oil film; indicated by high oil pressure
- Excess refrigerant charge forcing oil to the evaporator; indicated by flooding
- High suction superheat vaporizing oil; indicated by increased discharge temperature
Correct answer: Worn piston rings allowing oil past the piston; indicated by oil carryover into the system
Worn piston rings allow crankcase oil to bypass into the compression chamber and be discharged into the system, resulting in oil carryover and oil-fouled heat exchangers.
Question 5: Why is it important to equalize oil levels between compressors in a parallel rack system?
- To ensure each compressor has adequate lubrication and prevent bearing failure from oil starvation (Correct answer)
- To increase overall system efficiency by reducing compressor load
- To equalize refrigerant charge between compressors
- To prevent pressure imbalances in the discharge manifold
Correct answer: To ensure each compressor has adequate lubrication and prevent bearing failure from oil starvation
Without oil equalization, one compressor may accumulate excess oil while another runs low, leading to bearing failure in the oil-starved compressor.
Question 6: Which condition would cause a compressor to short-cycle (rapid on/off cycling)?
- Oversized expansion valve maintaining stable suction pressure
- A refrigerant overcharge raising head pressure above the high-pressure cutout setpoint (Correct answer)
- Low ambient temperature reducing the cooling load below system capacity
- Correct refrigerant charge with a properly sized condenser
Correct answer: A refrigerant overcharge raising head pressure above the high-pressure cutout setpoint
Refrigerant overcharge raises condensing pressure, which can repeatedly trip the high-pressure cutout, causing the compressor to cycle off and then reset and restart rapidly.
Question 7: What is the purpose of the oil separator in a refrigeration system with a semi-hermetic compressor?
- To filter moisture and acid from the refrigerant before it enters the compressor
- To separate oil from discharge refrigerant gas and return it to the compressor crankcase (Correct answer)
- To separate liquid refrigerant from vapor at the evaporator outlet
- To remove non-condensable gases from the refrigerant charge
Correct answer: To separate oil from discharge refrigerant gas and return it to the compressor crankcase
An oil separator captures oil entrained in the high-pressure discharge gas and returns it directly to the compressor crankcase, maintaining adequate oil level and reducing oil circulation throughout the system.
RSES Compressor Systems 5
Which parameter is used to express the overall efficiency of a compressor by comparing actual work input to ideal isentropic compression work?