Rotary Screw Compressor Selection

Executive Summary About Rotary Compressors By Scot Desiderio

The rotary screw belongs to the positive displacement compressor family. The two most commonly used compressors today are the rotary screw (helical rotor) and the reciprocating piston. The Rotary screw compressor is composed of two intermeshing helical rotors contained in a housing. In the case of an oil injected machine, the female rotor is driven by the male rotor through a thin film of oil. As the rotor diameter increases, so does the air pumps capacity; As the length of the rotor increases, so does the pumps final pressure.

As the rotor maintains past the inlet port the intermesh space continues to expand until the gas completely fills the interlobe space. When the male rotor enters the interlobe space it begins to convey and compress the gas towards the discharge port. Continued rotation causes the gas volume to reduce to the stated design pressure. Proper gas analysis is critical in oil selection as during initial start up, gas will dilute the viscosity of the oil.

Some rotary screw compressor designs employ a variable discharge valve that continuously seek maximum efficiency by opening and closing depending on system pressure conditions. The net effect is a shorter length rotor resulting in variable displacement operation allowing power requirements to drop.

The interlobe volume is a function of rotor profile, length and diameter.

How Do Rotary Screw Air Compressors Operate?

Executive Summary About Rotary Compressors By Mike Selvon

The most common type of compressors used in industrial settings are the rotary screw air compressors. There are many weird ideas surrounding these rotary screw compressors.

Some rotary screw air compressors have thermal or bypass valves in the oil cooling system. This helps protect the compressor parts during startup in cold weather.

Cold weather sometimes causes condensation to buildup in the air lines of the rotary screw air compressors. If not properly drained before the compressor is shut down, this can damage some of the compressor parts and expensive components.

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