CARO Compressor Operation and Maintenance 2 — Questions and Answers
Question 1: On a reciprocating ammonia compressor, how do cylinder unloaders typically reduce capacity?
- By throttling the discharge stop valve
- By holding the suction valves open so the cylinder does not compress gas (Correct answer)
- By slowing the motor speed through the starter
- By bypassing oil from the crankcase to the suction
Correct answer: By holding the suction valves open so the cylinder does not compress gas
Unloaders lift the suction valve plates off their seats so gas moves in and out of the cylinder without being compressed.
Question 2: What is the main purpose of a crankcase heater on a reciprocating compressor?
- To raise discharge temperature during startup
- To thin the oil so the pump primes faster
- To prevent refrigerant from migrating into and condensing in the crankcase oil during the off cycle (Correct answer)
- To keep the motor windings dry
Correct answer: To prevent refrigerant from migrating into and condensing in the crankcase oil during the off cycle
Keeping the crankcase warm prevents refrigerant condensing in the oil, which would cause foaming and poor lubrication at startup.
Question 3: An operator hears a sharp knocking noise from a reciprocating compressor and sees heavy frost on the suction side and crankcase. What is the most likely cause?
- Liquid refrigerant slugging into the compressor (Correct answer)
- Low condenser water flow
- A loose drive coupling guard
- Excess non-condensable gases
Correct answer: Liquid refrigerant slugging into the compressor
Liquid floodback causes knocking because liquid is incompressible, and the frost shows cold liquid reaching the compressor.
Question 4: A compressor has a suction pressure of 15 psig and a discharge pressure of 165 psig at sea level. What is the approximate compression ratio?
- 11.0
- 8.2
- 4.7
- 6.0 (Correct answer)
Correct answer: 6.0
Using absolute pressures, (165 + 14.7) ÷ (15 + 14.7) = 179.7 ÷ 29.7 ≈ 6.0.
Question 5: On a reciprocating compressor, net oil pressure is calculated as:
- Oil pump discharge pressure minus crankcase (suction) pressure (Correct answer)
- Oil pump discharge pressure plus suction pressure
- Discharge pressure minus oil pump discharge pressure
- Oil filter outlet pressure minus atmospheric pressure
Correct answer: Oil pump discharge pressure minus crankcase (suction) pressure
Net oil pressure is the pump discharge pressure above crankcase pressure, which is what actually forces oil to the bearings.
Question 6: Leaking discharge valves on a reciprocating compressor will usually cause:
- Lower discharge temperature and higher capacity
- Higher discharge temperature and reduced capacity (Correct answer)
- Lower oil pressure only
- Higher condenser water flow
Correct answer: Higher discharge temperature and reduced capacity
Hot discharge gas leaks back into the cylinder and is recompressed, raising temperature and reducing pumping capacity.
Question 7: Increasing the clearance volume in a reciprocating compressor cylinder will:
- Increase volumetric efficiency
- Have no effect on capacity
- Reduce discharge pressure
- Reduce volumetric efficiency (Correct answer)
Correct answer: Reduce volumetric efficiency
Gas trapped in the clearance space re-expands on the suction stroke, so less fresh suction gas enters each cylinder.
CARO Compressor Operation and Maintenance 2
On a reciprocating ammonia compressor, how do cylinder unloaders typically reduce capacity?