Compressor Systems Flashcards

Which parameter is used to express the overall efficiency of a compressor by comparing actual work input to ideal isentropic compression work?

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.

A scroll compressor will not start and the motor draws locked rotor amperage. After ruling out electrical faults, what mechanical condition should be investigated?

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.

What does the term 'compression ratio' mean for a refrigeration compressor?

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.

What causes 'oil pumping' in a reciprocating compressor, and what symptom would a technician observe?

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.

Why is it important to equalize oil levels between compressors in a parallel rack system?

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.

Which condition would cause a compressor to short-cycle (rapid on/off cycling)?

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.

What is the purpose of the oil separator in a refrigeration system with a semi-hermetic compressor?

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.