Screw compressors are rotary positive displacement machines. Two helical rotors are rotated by a series of timing gears as shown in Figure 10-8 so that gas trapped in the space between them is transported from the suction to the discharge piping. In low-pressure air service, non-lubricated
screw compressors can deliver a clean, oil-free air. In hydrocarbon service most screw compressors require that liquid be injected to help provide a seal. After-coolers and separators are required to separate the seal oil and recirculate it to suction.
Screw compressors can handle moderate amounts of liquid. They can also handle dirty gases because there is no metallic contact within the casing.
It tends to be limited to 250 psig discharge pressures and a maximum of 400 hp in hydrocarbon service, although machines up to 6,000 hp are available in other service. Screw compressors are not as good as vane compressors in developing a vacuum, although they are used in vacuum service.
Non-lubricated screw compressors have very close clearances and thus they are designed for limited ranges of discharge temperature, temperature rise, compression ratio, etc., all of which can cause changes in these clearances. Lubricated compressors have a somewhat broader tolerance to changes in operating conditions, but they are still more limited than reciprocating compressors.
The major characteristics of screw compressors are:
Size
• Up to 6,000 hp in air service, but more common below 800 hp,
• Up to 400 hp in hydrocarbon service,
• Discharge pressures to 250 psig.
• Single- or two-stage in tandern on same shaft.
Advantages
• Available as non-lubricated especially for air service.
• Can handle dirty gas.
• Can handle moderate amounts of liquids, but no slugs.
• No pulsating flow.
• At low discharge pressure (<50 psig) can be more efficient than reciprocating.
Disadvantages
• In hydrocarbon service needs seal oil with after-cooler and separator to recycle oil.
• At discharge pressure over 50 psig takes 10 to 20% more horsepower than reciprocating.
• Low tolerance to change in operating conditions of temperature, pressure, and ratio.
