Control centrifugal compressors

The turboprop engine has a power turbine instead of the nozzle as seen in Figure 4-2. The power turbine drives the propeller. The unit shown schematically is a two-shaft unit, this enables the speed of the propeller to be better controlled, as the gasifier turbine can then operate at a nearly constant speed. Similar engines are used in helicopter drive applications and many have axial flow compressors with a last stage as a centrifugal compressor as shown in Figure 1-14. 

Lubricating-oil governor and seal system cleaned Instrumentation and speed control checked Auxiliary heat recovery system Centrifugal Compressors... 

Probably the most widely used capacity control for the centrifugal compressor is speed control. The capacity curve when used with speed control covers a wide range. While electric variable speed motors offer a continuation to the speed control practice, there are some other alternatives available. Suction throttling has been widely used and offers a r sonable control range for a relatively low cost. 

Boyce, Meherwan P., et al. Practical Aspects of Centrifugal Compressor Surge, and Surge Control, Proceedings of the 12th Turbomachinery S ni-posium, Purdue University, West Lafayette, IN, 1983, pp. 147-173. 

The centrifugal compressor is well established for the compression of gases and vapors. It has proven its economy and uniqueness in many applications, particularly in which large volumes are handled at medium pressures. This compressor is particularly adaptable to steam turbine or other continuous speed change drives, as the two principles of operation and control are quite compatible. It is also adaptable to the electric motor, gas engine, and gas turbine with each installation being specific to a particular problem or process. Installation as well as operating costs can be quite reasonable. 

Figure 12-61D. Centrifugal compressor surge control schematic diagram shows instrumentation required when primary flow-measuring device is located in centrifugal compressor discharge line. Symbols T = temperature P = pressure A = differential across compressor outlet to inlet. See Reference 89 for a detailed discussion. (Used by permission White, M. H. Chemical Engineering, p. 54, Dec. 25,1972. McGraw-Hill, Inc. All rights reserved.)...

Figure 12-86. Typical effects of capacity control on horsepower for centrifugal compressors. (Used by permission Stepanoff, A. J. Turbo-blowers, 1955. John Wiley Sons Inc. All rights reserved.)...

Halloran, J., Take a Closer Look at Control of Centrifugal Compressors, Power, Dec. (1986). 

Oscarson, G. L., Application of A-C Motors and Controls to Centrifugal Compressors, Bui. 200-TEC-1120, Electric Machinery Co., div. Dresser-Rand (1955). 

Large centrifugal compressors are usually so expensive that it is generally not economically feasible to specify spares.29 To put two of these in parallel can result in surging, due to their flat operating characteristics, unless a control scheme such as that given in reference 35 is specified. 

Figure 3.23. Control of centrifugal compressors with turbine or motor drives, (a) Pressure control with turbine or motor drives, (b) Flow control with turbine or motor drives. SC is a servomechanism that adjusts the guide vanes in the suction of the compressor, (c) Surge and pressure control with either turbine or motor drive. The bypass valve opens only when the flow reaches the minimum calculated for surge protection.

Centrifugal compressors can go unstable if the flowrate of gas through the compressor is reduced too far. To avoid this, an antisurge controller is used that prevents the flowrate through the compressor from dropping below some minimum. Recycling from compressor discharge to compressor suction is used to keep the load on the compressor (see Fig. 7.3). 

---