Compressor Operation and Maintenance Flashcards

On a reciprocating ammonia compressor, how do cylinder unloaders typically reduce capacity?

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.

What is the main purpose of a crankcase heater on a reciprocating compressor?

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.

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?

Answer: Liquid refrigerant slugging into the compressor

Liquid floodback causes knocking because liquid is incompressible, and the frost shows cold liquid reaching the compressor.

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?

Answer: 6.0

Using absolute pressures, (165 + 14.7) ÷ (15 + 14.7) = 179.7 ÷ 29.7 ≈ 6.0.

On a reciprocating compressor, net oil pressure is calculated as:

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.

Leaking discharge valves on a reciprocating compressor will usually cause:

Answer: Higher discharge temperature and reduced capacity

Hot discharge gas leaks back into the cylinder and is recompressed, raising temperature and reducing pumping capacity.

Increasing the clearance volume in a reciprocating compressor cylinder will:

Answer: Reduce volumetric efficiency

Gas trapped in the clearance space re-expands on the suction stroke, so less fresh suction gas enters each cylinder.