Centrifugal compressors

Multistage centrifugal blowers when capable of handling gases against pressures greater than 35 psig are generally classed as compressors. They resemble multistage centrifugal pumps, and many of the problems encountered in their design are similar to those encountered in pump design. The impellers of a complete centrifugal compressor unit are of the single-suction type, and passages lead the air or gas from the discharge of one impeller to the suction side of the next impeller.

Because of an increase in temperature of the gas or air as the pressure is increased, cooling is generally necessary. If the pressures are not high, cooling water circulated in labyrinths between impellers may be sufficient. When high pressures are encountered, the gas may be cooled in interstage coolers. The reason for maintaining the gas at a low temperature is to permit an increase in the mass rate of flow with a corresponding reduction in size and horsepower.

Axial-flow compressors are designed on the principles of the airfoil section, and the blade shapes will be similar to the axial-flow fan.

These compressors are an essential part of the gas-turbine cycle. The gas is not cooled between stage, because a portion of the additional work necessary to compress the gas adiabatically over the work necessary to compress it isothermally will be recovered in the gas turbine. The advantages of centrifugal and axial-flow blowers and compressors are: 1) nonpulsating discharge of the gas, 2) no possibility of building up excessive discharge pressures, 3) a minimum of parts subject to mechanical wear, 4) no valves necessary, 5) a minimum of vibration and noise, 6) high speed, low cost, and small size or high capacity.

CHAPTER VI