Rotary characteristic curves

Rotary Compressors Rotary compressors are machines of the positive-displacement type. Such units are essentially constant-volume machines with variable discharge pressure. The volume can be varied only by changing the speed or by bypassing or wasting some of the capacity of the machine. The discharge pressure will vary with the resistance on the discharge side of the system. A characteristic curve typical of the form produced by these rotary units is shown in Fig. 10-93. 

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

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.

An example of the application of a hnear characteristic drying curve is given in the section on rotary dryers. 

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