Compressor, centrifugal efficiency

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... 

Compressor section efficiencies at the design point are in the mid-80s per cent for an axial compressor and in the high 70s per cent for a centrifugal compressor. Using equation (17.64) in conjunction with equation (17.59), the isentropic efficiency of the section may be expressed in terms of enthalpy differences as... 

Centrifugal compressors Centrifugal compressors deliver actual volumetric flows (cubic decimeters per second and performance should not be expressed as mass, mols or standard volumetric flow). Assume compression ratios equal in all stages. Maximum number of stages that can be on one shaft or fit in the frame = 8 minus 1 stage for each side nozzle. Compression ratio 2.5-4. The pressure coefficient = 0.5-0.65 assume 0.55. The pressure differential increases with increase in suction gas density (increased molar mass, suction pressure or decrease inlet temperature, decrease in k). Power up to 7.5 kW/m /s. Efficiency of large centrifugal compressors 76-78%. 

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 %). 

Here are some approximate efficiencies to use for centrifugal, axial flow, and reciprocating compressors. 

Figure 1, Approximate polytropic efficiencies for centrifugal and axial flow compressors.

Axial compressors are high speed, large volume compressors but are smaller and somewhat more efficient than comparable centrifugal com-... 

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. 

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