Centrifugal compressor operating speed

Figure 19-4 Effect of changing gat conditions on centrifugal compressor operating at constant speed.

Basically, the performance of the centrifugal compressor, at speeds other than design, are such that the capacity will vary directly as the speed, the head developed as the square of the speed, and the required power as the cube of the speed. By varying speed, the centrifugal compressor will meet any load and pressure condition demanded by the process within the operating limits of the compressor and the driver. It normally accomplishes this as efficiently as possible, since only the heed required by the process is developed by thecompressor. This compares to the essentially constant head developed by the constant speed Compressor. 

The performance of a centrifugal compressor (discharge to suction-pressure ratio vs. the flow rate) for different speeds is shown in Fig. 11-87. Lines of constant efficiencies show the maximum efficiency. Unstable operation sequence, called surging, occurs when compressors fails to operate in the range left of the surge envelope. It is characterized by noise and wide fluctuations of load on the compressor and the motor. The period of the cycle is usually 2 to 5 s, depending upon the size of the installation. 

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. 

Historically, the most popular driver for the centrifugal compressor has been the steam turbine. Steam turbines can readily be speed matched to the compressor. Prior to the upsurge in energy costs, reliability, simplicity, and operational convenience were the primary factors in driver selection. The steam turbine, with its ability to operate over a relatively wide speed range, was ideal for the centrifugal compressor, which could be matched to the process load by speed modulation. 

The dynamic compressors, axial and centrifugal, exhibit one other non-steady force, which is often questioned by foundation designers. This is the torque reaction force at the casing feet and, thus, the foundation due to surge. As a rule of thumb, a value of A the maximum steady-state torque reaction at the casing feet can be used. The frequency is the surge frequency, which is less than operating speed but not readily predictable, because it is heavily dependent on the connected system. 

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 12-85. System operation of variable-speed centrifugal compressor.

Many process machines operate at speeds different from the one of their drivers. Typical of cases where the machine rotates slower than the driver are reciprocating compressors typical examples of machines rotating 5f than the drivers are centrifugal compressors driven by electric motors. In either case, gears are used to match the two speeds. Gears can also be designed to accommodate shafts that... 

Both axial and centrifugal compressors are subject to an unstable region of low flow operation called surge. This region is defined by the pressure-flow relationship which has a peak determined by the operating speed. For various compressor speeds, these peaks may be connected to determine a surge line, which defines a region of unstable operation at lower flows. 

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