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Screw Compressor

General

Helical rotary (or screw) compressors are positive displacement machines. Two types are used - single-screw and twin-screw. A twin-screw compressor consists of accurately matched rotors (one male and one female) that mesh closely when rotating within a close tolerance common housing. One rotor is driven while the other turns in a counter-rotating motion.

A single-screw compressor uses a single main screw rotor meshing with two gate rotors with matching teeth. The main screw is driven by the prime mover, typically an electric motor. The gate rotors may be metal or a composite material. The screw-like grooves gather vapors from the intake port, trap them in the pockets between the grooves and compressor housing, and force them to the discharge port along the meshing point path. This action raises the trapped gas pressure to the discharge pressure. If the power input is adequate and pressure differential between outlet and inlet pressures is within the design range of the machine, the screw compressor delivers the appropriate refrigerant gas volume.

Notice that the refrigerant gas enters and exits the compressor through ports; not valves like reciprocating compressors. Compressors of this type are called ported compressors for this reason. The mating rotors are rotating at such close tolerances, they require cooling and lubrication. This may be provided by forcing oil into the compressor at strategic points. The oil also acts as a seal for rotor-to-rotor and rotor-to-housing clearances.

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The oil is entrained by the flowing refrigerant gas, leaves the compressor, and is recovered by an oil separator for reuse (after cooling and filtering). Since the oil sump is on the high pressure side of the system, a mechanical pump is not required for oil circulation. The compressive action of the screw itself provide the necessary pressure differential.

In other designs, subcooled liquid refrigerant injection (instead of oil) cools and seals the compressor. The use of liquid refrigerant eliminates oil management problems as there are no oil separators or oil recovery systems. The system is sealed, cooled and lubricated with liquid refrigerant which also attenuates the noise. Capacity is controlled with two slide valves.

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Since the screw compressor is most often driven by a constant speed electric motor and the screw compressor is a positive displacement machine, the natural tendency is to move a fixed volume of refrigerant gas. This would make refrigeration capacity control difficult. The design uses a slide valve that opens to vent some gas back to the suction port, reducing both the net gas flow and power input.

Several manufacturers offer packaged water chillers using helical rotary or "screw" compressors. Water-cooled units range in size from 50 tons to over 1200 tons. They normally use HCFC-22 and HFC134a as refrigerants in space cooling designs and ammonia in process refrigeration (particularly food processing). In the smaller sizes, they compete with reciprocating chillers. In larger sizes they compete with centrifugals. Screw compressors usually employ hermetic or semi-hermetic designs for higher efficiency, minimum leakage, ease of service, and volume production reasons.

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Air- and evaporatively-cooled models can be used from about 60 to 350 tons, and can use open-drives. Chillers using ammonia always use open type compressors, typically with direct-coupled electric motors. The selection of open or hermetic design depends on the application, refrigerant, and the manufacturer.

Technology types (resource)

Two types are used - single-screw and twin-screw. A twin-screw compressor consists of accurately matched rotors (one male and one female) that mesh closely when rotating within a close tolerance common housing. One rotor is driven while the other turns in a counter-rotating motion.

A single-screw compressor uses a single main screw rotor meshing with two gate rotors with matching teeth. The main screw is driven by the prime mover, typically an electric motor. The gate rotors may be metal or a composite material. The screw-like grooves gather vapors from the intake port, trap them in the pockets between the grooves and compressor housing, and force them to the discharge port along the meshing point path. This action raises the trapped gas pressure to the discharge pressure. If the power input is adequate and pressure differential between outlet and inlet pressures is within the design range of the machine, the screw compressor delivers the appropriate refrigerant gas volume.

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