Rotating positive displacement compressors rotors

Screw compressors are rotary positive displacement machines. Two helical rotors are rotated by a senes 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-lubri-cated 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. 

Positive displacement blowers Positive displacement blowers are probably the most commonly used type of compressor for dilute phase conveying systems. They provide an ideal match, in terms of pressure capability, with the conventional low pressure rotary valve, and are a typical working combination on many plants. They can be used as vacuum pumps, or exhausters, as well as blowers. The principle of operation is illustrated in Figure 4.26. Twin rotors are mounted on parallel shafts within a casing, and they rotate in opposite directions. As the rotors turn, air is drawn into the spaces between the rotors and the casing wall, and is transported from the inlet to the outlet without compression. As the outlet port is reached, compression takes place when the air in the delivery pressure pipe flows back and meets the trapped air. 

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