Compressors characteristic curves

Figure 12-61. Manufacturer s typical centrifugal compressor characteristic curve.

Figure 12-86 shows the effect of changing speed on the compressor characteristic curve. The speed can be adjusted to meet a desired point on the system curve. This is the most popular form of control. 

FIGURE 3.9 Compressor characteristic curve of turbocharger GT35 and surge control line... 

In this part we have investigated the effect of this mechanism on performance of system. For this purpose we compared pressure ratio versus modified flow rate curve of a compressor with compressor characteristic curve for totally open wastage in 1,800 2,200 and 3,000 rpm. The comparisons have been illustrated in Figures 3.11 and 3.12. According to Figure 3.11, the performance of compressor in rotation of 3,200 rpm is in safe region and faraway of surge and... 

If turndown is a consideration, again the reciprocating compressor, with its many unloading options, does meet the challenge. This flexibility is difficult to achieve reliably in the dynamic machine and may require the compressor to operate at an unfavorable point on its characteristic curve. 

For each compressor unit with its associated individual cylinders, a fixed horsepower characteristic curve exists. The curve rises, peaks, and falls as the range of pressure ratio requirement varies. See Figure 12-17B. 

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. 

Characteristic curves—pressure against flowrate—of centrifugal and axial compressors usually have a peak. Figure 3.22 is an example. In order to avoid surging, the flow through the com-... 

Figure 3.22. Characteristic curves of a centrifugal compressor at different speeds, showing surge limits.

Figure 5.12 Characteristic curve for a centrifugal compressor. From Ref. 19 with permission.

Centrifugal Compressor Constants and Characteristic Curves—Quantity Constant.—The quantity of gas delivered by a centrifugal compressor is proportional to UaDaha, or, quantity constant = UaDaha- Compressor Constant.— compressor model can be used with practically the same efficiency for various combinations of quantity and pressure such that K = Q/y/p where K is the compressor constant, Q and pe are the desired quantity and mean effective pressure, respectively. The compressor constant can be more conveniently written aaK = QN /pi. ... 

Positive-displacement fans, blowers, and eompressors are similar in action to positive-displacement pumps, and include reciprocating compressors, two- or three-lobe blowers, and screw compressors. However, with gases, the almost vertical characteristic curves, bend to the left more than for liquids because of the greater tendency for slip. 

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