Centrifugal compressors compressor efficiency

Gas turbine driven centrifugal compressors are very efficient under the right operating conditions but require careful selection and demand higher levels of maintenance than reciprocating compressors. Compression facilities are generally the most expensive item in an upstream gas process facility. 

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. 

Thermocompression Evaporators Thermocompression-evap-orator calculations [Pridgeon, Chem. Metall. Eng., 28, 1109 (1923) Peter, Chimin Switzerland), 3, II4 (1949) Petzold, Chem. Ing. Tech., 22, 147 (1950) and Weimer, Dolf, and Austin, Chem. Eng. Prog., 76(11), 78 (1980)] are much the same as single-effect calculations with the added comphcation that the heat suppied to the evaporator from compressed vapor and other sources must exactly balance the heat requirements. Some knowledge of compressor efficiency is also required. Large axial-flow machines on the order of 236-mVs (500,000-ftVmin) capacity may have efficiencies of 80 to 85 percent. Efficiency drops to about 75 percent for a I4-mVs (30,000-ftVmin) centrifugal compressor. Steam-jet compressors have thermodynamic efficiencies on the order of only 25 to 30 percent. 

Centrifugal compressors can have problems at the inducer and blade tips. These parts of the blade can be excited by aerodynamic forces. Blade discs can have stresses at the rotor tips, which lead to cracks. To solve this problem, the offending part is removed and a scalloped disc results as seen in Figure 21-20. This type of disc experiences some efficiency loss (about 2 %). 

While ricn common, engine-driven centrifugal compressors are used. This combination is desirable for low-ratio applications and in fuel cost situations where the high engine efficiency is attractive. The difference in rotating speeds (engine 300-600 rpm, compressor 3,000-5,000 rpm -t-) requires the use of a speed increaser. 

Medium range compressor efficiency (higher than centrifugal lower than low-speed). 

The approximate number of stages for a centrifugal compressor, column (I), is a function of the adiabatic (and actually the polytropic) head and varies with the efficiency and physical properties of the gas. 

Figure 12-66. Comparative efficiencies of a 1,550 bhp centrifugal compressor based on 80% polytropic efficiency. (Used by permission Woodhouse, H. Petroleum Engineer, Oct. 1953. Hart Publications, Inc. All rights reserved.)...

Centrifugal compressors usually have specific speeds of 1,500-3,000 rpm at the high efficiency point. The axial flow fans and blowers are high specific-speed wheels mixed flow units are lower and the centrifugal compressor wheels are the lowest in specific speed range because they have narrow impellers. 

Range 900-1,800 rpm. These speeds are used for pumps above 3,000 hp and for centrifugal compressors using speed increasers. The efficiency, power-factor correction, and other factors may favor motors below 3,000 hp. 

A two-stage centrifugal compressor will be used for both duties. Take the polytropic efficiency for both compressors as 70 per cent. The hydrogen supply pressure is 120 kN/m2 and the temperature 25 °C. The hydrogen chloride is cooled to 50 °C after leaving the burner. Assume that the compressor intercooler cools the gas to 50 °C, for both duties. 

The polytropic efficiency is a function of the physical properties of the gas and the machine design. Generally, the higher the feed flowrate, the higher the polytropic efficiency of the compressor. A first estimate of the polytropic efficiency of a centrifugal compressor can be obtained from ... 

Axial flow compressors have polytropic efficiencies up to 10% higher than centrifugal compressors for the same pressure ratio. A reasonable estimate is for the efficiency to be 5% higher than that of the corresponding centrifugal machine (i.e. multiply the estimate from Equation 13.18 by 1.05). 

For a centrifugal compressor, the polytropic efficiency can be estimated from Equation 13.18 ... 

Whereas reciprocating compressors are normally designed on the basis of adiabatic work (together with an isentropic efficiency), centrifugal compressors are usually designed on the basis of polytropic work. By analogy with Equation B.15, the work required for a poly tropic compression is given by ... 

The maximum efficiency of fans is 82-90%.34 Figure 8-8 gives the efficiency of reciprocating compressors. Centrifugal compressors are generally from 5-20% less efficient than reciprocating ones.30... 

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