Centrifugal compressors example

Synchronous speeds are calculated by Eq. (29-10). Speeds above the limits given are obtained through step-up gears large high-speed centrifugal compressors are examples. Two-pole (3600 r/min at 60 Hz) synchronous motors can be built but are uneconomical in comparison with geared drives. 

Figure 10-16. Example process flow diagram of centrifugal compressor.

Expansion turbines are related in many design features to the centrifugal compressor. The key exception being that the turbine receives a high pressure gas for expansion and power recovery to a lower pressure and is usually accompanied by the recovery of the energy from the expansion. For example, applications can be (1) air separation plants (2) natural gas expansion and liquefaction (for gas let-down in pipeline transmission to replace throttle valves where no... 

The mode of operation of a specific application should be the first consideration. The inherent design of each type of compressor defines the acceptable operating envelope or mode of operation that it can perform with reasonable reliability and life cycle costs. For example, a bullgear-type centrifugal compressor is not suitable for load-following applications but will prove exceptional service in constant-load and volume applications. 

For high demand compression duties, the overall compression can be broken down into stages and different types of compressor used in the different casings. For example, a large flowrate with a large pressure differential might use an axial compressor followed by a centrifugal compressor. 

Example 13.5 A recycle gas stream containing 88% hydrogen and 12% methane is to be increased in pressure from 81 bar to 98 bar. The inlet temperature is 40°C and the flowrate is 170,000 Nm3 h 1 (Nm3 = normal m3). Estimate the power requirements for a centrifugal compressor for this duty. 

A little bit of entrainment is not always bad. For example, entraining a clean naphtha mist into the suction of a wet-gas centrifugal compressor is fine. The mist helps keep the rotor parts clean. If the steam from a boiler is to be used to reboil towers, entrainment is of no consequence. 

Polytropic efficiencies are obtained from measurements of power consumption of test equipment. They are essentially independent of the nature of the gas. As the data of Figure 7.27 indicate, however, they are somewhat dependent on the suction volumetric rate, particularly at low values, and on the compression ratio. Polytropic efficiencies of some large centrifugal compressors are listed in Table 7.6. These data are used in Example 7.9 in the selection of a machine for a specified duty. 

In centrifugal compressors with all stages in the same shell, the allowable head rise per stage is stated in Table 7.6 or correlated in Figure 7.26. Example 7.9 utilizes these data. 

A fluid may be made to rotate as a solid body without relative motion theoretically between particles, either by the rotation of a containing vessel or by stirring the contained fluid so as to force it to rotate. Thus an external torque is applied. A common example is the rotation of fluid within the impeller of a centrifugal pump or the rotation of a gas within the impeller of a centrifugal fan or centrifugal compressor. 

Experience with reciprocating machines for other applications (sweet gas for example) indicates that the scaling factor is about 0.9. Thus, these machine do not quite scale directly with the required power. Table 12.1 lists an example centrifugal compressor. This is an estimate from one of the projects in my files. Using the scaling factor and this cost estimate gives ... 

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... 

Example 5.3 Sizing a Steam-Turbine Drive for a Centrifugal Compressor... 

---