Figure 5-28. Campbell diagram of tested impeller
.ibliography
Alford，J.S.， ""Protecting Turbomachinery from Self-Excited RotorWhirl，'' Journal of Engineering forPower， ASMETransactions，October，1965， pp. 333-344.
Ehrich，F.F.， ""Identification and Avoidance of Instabilities and Self-ExcitedVibrations in RotatingMachinery，''Adopted from ASME Paper72-De-21，General ElectricCo.， Aircraft EngineGroup， Group EngineeringDivision，May11， 1972.
Gunter，E.J.，Jr.，""Rotor BearingStability，'' Proceedings of the 1st Turbo-machinerySymposium， Texas A&MUniversity，October，1972， pp. 119-141.
Lund，J.W.， ""Stability and Damped Critical Speeds of a Flexible Rotor in Fluid-FilmBearings，'' ASME No. 73-DET-103.
Newkirk，B.L.，""ShaftWhipping，''G巾n巾rGlEl巾JtriJR巾Vi巾w，Vol.27，(1924)，
p. 169.NicholasJohn，C.， and MollRandall，W.， ""Shifting Critical Speeds Out of the Operating Range by Changing From Tilting Pad to Sleeve Bearings'' Proceed-ings of the 22nd TurbomachinerySymposium， Texas A&M University. p.25， 1993.Prohl，M.A.， ""General Method of Calculating Critical Speeds of FlexibleRotors，''Trans.ASME， b. Appl. 巾Jh.，Vol.12， No.3， September1945， pp. A142-A148.Reiger，D.， ""The Whirling ofShafts，''Engin巾巾r，London， Vol.158，1934， pp. 216-228.
Thomson，W.T.， 巾JhGniJGlVibrGti n盧， 2ndedition，Prentice-Hall，Inc.， Engle-woodCliffs，N.J.， 1961.
Part 11
Major Components
CentrifugalCompressors
Centrifugal compressors are used in small gas turbines and are the driven units in most gas turbine compressor trains. They are an integral part of thepetrochemcial industry， finding extensive use because of their smooth opera-tion， large tolerance of process fluctuations， and their higher reliability compared to other types of compressors. Centrifugal compressors range in size from pressure ratios of 1:3 per stage to as high as 12:1 on experimentalmodels. Discussion here will be limted to pressure ratios below3.5:1， since this type is prevalent in the petrochemical industry. The proper selection ofa compressor is a complex and important decision. The successful operation of many plants depends on smooth and efficient compressor operations. To ensure the best selection and proper maintenance of a centrifugal compres-sor， the engineer must have a knowledge of many engineering disciplines.
In a typical centrifugal compressor the fluid is forced through the impeller by rapidly rotating impeller blades. The velocity of the fluid is convertedto pressure， partially in the impeller and partially in the stationary diffusers. Most of the velocity leaving the impeller is converted into pressure energy in the diffuser as shown in Figure 6-1. It is normal practice to design the compressor so that half the pressure rise takes place in the impeller andthe other half in the diffuser. The diffuser consists essentially ofvanes， which are tangential to the impeller. These vane passages diverge to convert the velocity head into pressure energy. The inner edges of the vanes are in line with the direction of the resultant airflow from the impeller as shown in Figure 6-2.
Centrifugal compressors in general are used for higher-pressure ratios and lower-flow rates compared to lower-stage pressure ratios and higher-flow rates in axial compressors. Figure 6-3 is a map for centrifugal compressors that shows the effect ofspecific speed (Ns) and specific diameter (Ds) on their
22. Gas Turbine Engineering Handbook
Figure 6-1. Pressure and velocity through a centrifugal compressor.
Figure 6-2. Flow entering a vaned diffuser.
efficiency. The most efficient region for centrifugal compressor operation is in a specific speed range between 60 < Ns > 1500. Specific speeds of more than 3000 usually require an axial-flow-type compressor. In a centrifugal compressor the angular momentum of the gas flowing through the impeller is increased partly because the impeller's outlet diameter is significantly
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