Rotary pump characteristic

For the most part, positive displacement pumps can be classified either as rotary pumps or as reciprocating pumps. However, pumps do exist which exhibit some of the characteristics of both types. 

A Throttling characteristics of centrifugal (c) -and positive- displacement (e, reciprocating d, rotary) pumps. B Control characteristics of positive displacement pumps a,b, Systems characteristics (b, after speed control) VFluid flow Ap, Differential pressure n, Speed h, Stroke length A, B, B, Operating points. 

Chapter 3 summarised initially the various types of vacuum pump available and the pressure ranges in which they normally operate. Subsequent sections dealt specifically with types of pump and, in some cases, to support calculations, reviewed the operating principles and characteristics. For example, aspects of oil-sealed rotary pump operation were discussed (Examples 3.1-3.5) and Roots pumps, widely used in applications where large gas loads at pressures in the rough-medium range have to be handled, were examined (Examples 3.7-3.9). 

Fig. 2.4-6 Characteristic curve of a rotary pump (pumping height as a function of volume flow curve parameter d = diameter of the bladed wheel, tj = efficiency).

The total delivery height is a function of the volume rate and starts at the lifting height for Vi = 0, and then increases almost proportionally to and v, respectively (Figure 3.4.7, see also Example 3.4.2). Thus, the operating point of a rotary pump is given by the intersection of this curve of operation with the respective characteristic curve of the pump that depends on the rotational frequency n. For a fixed value of n, the volume rate can be adjusted by using a choke valve, which increases the pressure drop and reduces the rate. For a piston pump, the characteristic curves are almost vertical lines, and the volume rate can only be adjusted by the rotational frequency. 

Figure 3.4.7 Characteristic curves of a rotary pump and characteristic curves of operation for a lifting height, ii2 i> of3 m [see also Eq. (3.4.34) dashed line example of the change of the characteristic curve of operation if a choke valve is used Ppump is the efficiency of the pump).

Figure 4.11 Characteristic curves (pump head, effective capacity and efficiency subject to the discharge rate) of a rotary pump from KSB.

Rota.ry Lobe Pumps. Rotary lobe pumps are similar to gear pumps in principle. These pumps have an added advantage of noncontacting metal parts by use of external gears, which reduces the wear, but adds complexity. Low wear and improved shear characteristics make these pumps apphcable to the food industry, where cleanliness and absence of contamination are required. These pumps produce a strong pulsing flow that must be addressed. 

The reciprocating and rotary positive displacement pumps primary characteristic is that they have a nearly direct relationship between the motion of the pumping... 

Condensation of vapours must be avoided since it causes a deterioration of the oil water vapour condense at 234 torr at 70°C, which is the typical working temperature of the pump. To avoid condensation, most pumps have a ballast valve in the high-pressure stage. After the gas has been closed off from the inlet, some gas is bled in through the ballast valve. This causes the discharge valve to open sooner, with a reduction of the compression ratio. Rotary piston pumps have similar characteristics. 

A system of cascaded booster pumps has the characteristics of a medium-size rotary vane pump but does not present the drawback of the back-streaming. For this reason, it finds application in cryogenics, for example, for the circulation of the He mixture in dilution refrigerators. 

Figure 5.7 shows that positive-displacement compressors, like vacuum pumps, are divided into two main classes reciprocating and rotary. Table 5.4 lists characteristics of these compressors. Ludwig [14] discusses compression equipment and calculation methods in detail. 

Figure 5.26 Characteristic curve for a rotary positive-displacement pump.

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