Compressors performance characteristics

Figure 3-9. Typical centrifugal compressor performance characteristics.

Figure 1-17. Performance characteristics of different types of compressors.

This chapter examines the overaii performance characteristics of compressors and turbines. This materiai is presented here to famiiiarize the reader with the behavior of these machines, ciassified under the broad term tur-bomaciiinery. Pumps and compressors are used to produce pressure turbines produce power. These machines have some common characteristics. The main eiement is a rotor with biades or vanes, and the path of the fluid in the rotor may be axiai, radiai, or a combination of both. 

Rodgers, C., Influence of Impeller and Diffuser Characteristic and Matching on Radial Compressor Performance, SAE Preprint 268B, January 1961. 

Figure 16-16 shows the performance characteristic of a split-shaft turbine where the only power output limitation is the maximum allowable temperature at the inlet of the turbine section. In actual practice a torque limit, increased exhaust temperature, loss of turbine efficiency, aud/or a lubrication problem on the driven equipment usually preclude operating at very low power turbine speeds. The useful characteristic of the split-shaft engine is its ability to supply a more or less constant horsepower output over a wide range of power turbine speeds. The air compressor essentially sets a power level and the output shaft attains a speed to pnivide the required torque balance. Compressors, pumps, and various mechanical tinvc systems make very good applications for split-shaft designs. 

Figure 12-112. Performance characteristics for a typical single-stage rotary helical rotor compressor, using matching helical rotors. (Used by permission Fairbanks, Morse, Co. for earlier editions. [Company no longer exists producing compressors, 1998, per research information.])...

Size the discharge surge drum and associated piping for a single-acting, single-cylinder, motor-driven compressor with the following performance characteristics ... 

Similarly, vacuums can be created when a blower, fan, compressor, or jet ejector removes gases from equipment. The magnitude of the vacuum attainable will be governed by the performance characteristics of the device. Other mechanisms for generating a vacuum, which have been demonstrated by industry experience, include the following. 

The performance characteristics of a single stage of the three main types of compressors are given in Table 10-16. The pressure ratios of the axial and centrifugal compressors have been classified into three groups industrial, aerospace, and research. 

Controlling two or more compressors operating in parallel and having identical characteristics would be relatively simple. It is very difficult, if not impossible, to find two compressors having identical performance characteristics. Slight variations in flow can fully load one compressor and cause the other to operate with wasteful recycling. The control scheme shown in Figure 2.111 alleviates that problem. 

Real compression processes operate between adiabatic and isothermal compression. Actual compression processes are polytropic processes. This is because the gas being compressed is not at constant entropy as in the adiabatic process, or at constant temperature as in the isothermal processes. Generally, compressors have performance characteristics that are analogous to those of pumps. Their performance curves relate flow capacity to head. The head developed by a fluid between states 1 and 2 can be derived from the general thermodynamic equation. 

The experimental evaluation of other effects associated with the operative temperatures and stoichiometric ratio is performed in a range of pressure around 130 kPa, using the side channel air compressor whose characteristics are described in Table 7.1. The effect of the stack temperature on the voltage measured at the stack terminals is shown in Fig. 7.8. In this case, the temperamre is controlled by varying the water flow rate in the heat exchanger shell while the air flow rate and humidification conditions are the same as those of Fig. 7.7. It can be observed that a decrease in temperamre from 346 to 305 K determines a voltage reduction <10% up to 70% of load (200 A), while a satisfactory stack behavior at 313 K is detected in almost all load conditions (see Sect. 3.3). Then, the effect of the stoichiometric ratio is verified for three different loads at the stack and humidification temperature of 313 K. From Fig. 7.9, it can be noted that values of R higher than 2 are... 

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