Rotating positive displacement compressors

Rotary compressors are positive-displacement machines. Figure iO-7 shows a typical vane compressor. The operation is similar to that of a vane pump shown schematically in Figure 10-10 of Volume 1, 2nd Edition (Figure 10-9 in 1st Edition). A number of vanes, typically from 8 to 20, fit into slots in a rotating shaft. The vanes slide into and out of the slots as the shaft rotates and the volume contained between two adjacent vanes and the wall of the compressor cylinder decreases. Vanes can be cloth impregnated with a phenolic resin, bronze, or aluminum. The more vanes the compressor has, the smaller the pressure differential across the vanes. Thus, high-ratio vane compressors tend to have more vanes than low-ratio compressors. 

Installation requirements for rotary positive-displacement compressors are similar to any rotating machine. Review the installation requirements for centrifugal pumps and compressors for foundation, pressure-relief, and other requirements. As with centrifugal compressors, rotary positive-displacement compressors must be fitted with pressure-relief devices to limit the discharge or inter-stage pressures to a safe maximum for the equipment served. 

Within the class of rotating positive displacement particular interest has been recently addressed toward scroll and screw compressors. 

Using the example of pumps and, with regard to quaUty, also compressors. Figure 13.1 shows the typical throttle curves for the three main groups of machinery (turbomachines as well as reciprocating and rotating positive displacement pumps). 

As high-pressure compressors are based on the rotating positive displacement principle, preferably screw compressors are used. 

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. 

Performance of a rotary positive-displacement compressor can be evaluated using the same criteria as a positive-displacement pump. As constant-volume machines, performance is determined by rotation speed, internal slip, and total back-pressure on the compressor. 

Positive displacement compressors offer more flexible compression ratio at low fuel cell loads, moreover rotating devices offer the additional advantage of a not fluctuating or flow rate with respect to alternative machines. 

In the classification of compressors presented in Figure 4.24, five different types of positive displacement compressor are included. The constant speed operating characteristic of each of these is similar to that shown in Figure 4.25. A particular feature of most of these machines is that very fine operating clearances are maintained between rotating parts. As a result there is no possibility of the conveyed material being conveyed through the... 

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. 

This leakage air must be taken into account in the specification of the compressor since it is lost from the system and is not available for conveying. Manufacturers of rotary valves will generally provide this data. It would always be recommended that rotary valves should be vented, as shown, to prevent the leakage air from interfering with the smooth fiow of material into the pipeline. The rotary valve is a positive displacement type feeder and so for a given valve and speed of rotation the material feed rate will be almost directly proportional to the bulk density of the material. 

Positive displacement compressor uses mechanical components to trap gas and then compress it to hi er pressure. Positive displacement compressor can be further classified into reciprocating compressor and rotary compressor. Reciprocating compressor uses reciprocating motion of piston, plunger, or diaphragm in a cylinder or head to compress gas. Rotary compressor uses continuous rotating screws, lobes, vanes, or liquid ring to compress gas. 

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