Rotating positive displacement machines

It has to be mentioned, however, that the impellers in such machines are comparatively small and the hydraulic flow behavior in the impellers cannot be adjusted optimally. That is why these machines scarcely exceed an efficiency of 40% and often reach an efficiency far less than this. Above all, there is a high sensibility toward particles in the fluid, which with high speeds can turn into bullets. 

The flows in high pressures are usually small. Hence, turbomachines play a role in high-pressure use only as circulating machines. There are only a few exceptions (e.g., in long-distance transport of gas). 

The group of rotating positive displacement pumps comprises a lot of different versions. When setting a borderline for high-pressure usabUity at 10 MPa, only the gear pumps (70 MPa), the screw pumps (32 MPa), and the progressing cavity pumps (15 MPa) remain. The basic conveying characteristics of these pumps can be 

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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. 

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. 

Rotary, positive displacement machines in which two intermeshing rotors, each in helical configuration displace and compress the air available in lubricated and non-lubricated construction the discharge air is normally free from pulsation high rotation speed. 

In general, the positive displacement machines - there are rotating and reciprocating features - show their main application range for high-pressure and lower-capacity conditions. The turbo- (centrifugal) machines, however, are best suited for high capacity but lower pressure conditions. The selection should be based on an analysis of the life-cycle costs (or costs of ownership) [1]. 

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. 

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. 

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. 

Because the capacity limitations of the helium turbine do not permit unlimited miniaturization on that conceptual basis, development work on positive displacement expanders is being carried on. This type of machine shows promise for rather small capacities below 10 w at less than 10°K. From that effort the part incorporated in a rotating expander is shown in Fig. 10. This expander is not miniaturized as yet and is still in the course of preliminary trials in a program for obtaining information in using materials with no lubrication. 

This popular type of rotating impeller positive displacement vacuum pump is often used in vacuum enclosed cleaning machines. 

Single screw extruders [9,10,34] are pumps that convey the material by a combination of pressure and drag flows. The operation of a single screw extruder relies on the frictional forces between the polymer and the barrel to push the polymer forward. When the ratio of polymer/ barrel friction vs. polymer/screw friction is low, then the polymer will adhere to the screw and simply rotate with it, resulting in no forward motion. If the ratio is high enough, then forward motion will occur. The lack of positive displacement characteristics means that the machines are far from self wiping. 

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