Figure 6-..Centrifugal compressor map.(Balje，O.E.， ""A Study of ReynoldsNumber Effects inTurbomachinery，"" Journal of Engíneeríng for Power， ASMETrans.， Vol.86， SeriesA， p. 227.)
greater than its inlet diameter. The major difference between axial and centrifugal compressors is the variance in the diameters of the inlet and the outlet. The flow leaving the centrifugal compressor is usually perpendicular to the axis of rotation.
CentrifugalCompressor Components
The terminology used to define the components of a centrifugal compres-sor is shown in Figure 6-4. A centrifugal compressor is composed of inletguidevanes， aninducer， an impeller， adiffuser， and a scroll. The inlet guide vanes (IGVs) are used in only a high-pressure ratio transonic compressor. Centrifugal compressor impellers are either shrouded or unshrouded as seen in Figures 6-5 and 6-6.
The fluid comes into the compressor through an intake duct and is given prewhirl by the IGVs. It then flows into an inducer without any incidenceangle， and its flow direction is changed from axial to radial. The fluid is given energy at this stage by the rotor as it goes through the impeller whilecompressing. It is then discharged into a diffuser， where the kinetic energy is
Figure 6-5.Closed impeller. (Courtesy ElliottCompany，Jeannette， PA.)
converted into static pressure. The flow enters the scroll from which the compressor discharge is taken. Figure 6-1 shows the variations in pressure and velocity through a compressor.
There are two kinds of energy inducer systems: a single-entry inducer and a double-entry inducer as shown in Figure 6-7.
A double-entry inducer system halves the inlet flow so that a smallerinducer-tip diameter can beused， reducing the inducer-tip Mach number;however， the design is difficult to integrate into many configurations.
There are three impeller vanetypes， as shown in Figure 6-8. These are defined according to the exit blade angles. Impellers with exit blade angle
Figure 6-.. Head flow-rate characteristics for various outlet blade angles.
.2二 900 are radial vanes. Impellers with .2 < 900 are backward-curved orbackward-swept vanes， and for .2 > 900， the vanes are forward-curved or forward-swept. They have different characteristics of theoretical head-flowrelationship to eachother， as shown in Figure 6-9. Although in Figure 6-9the forward-curved head is the largest， in actual practice the head char-acteristics of all the impellers are similar to the backward-curved impeller. Table 6-1 shows the advantages and disadvantages of various impellers.
Table 6-1 The Advantages and Disadvantages of Various 1mpellers
T'pes of 1mpellers Advantages Disadvantages
Radial vanes
Backward-curved vanes
Forward-curved vanes
1. Reasonable compromise between low energy transfer and high absolute outlet velocity
2. No complex bending stress
3. Easy manufacturing
1. Low-outlet kinetic energy二low-diffuser inlet mach number
2. Surge margin is wide
1. High-energy transfer
1. Surge margin is relatively narrow
1. Low-energy transfer
2. Complex bending stress
3. Hard manufacturing
1. High-outlet kinetic energy二High-diffuser inlet mach number.
2. Surge margin is less than radial vanes
3. Complex bending stress
4. Hard manufacturing
The Euler equation， assuming simple one-dimensional flowtheory， is the theoretical amount of work imparted to each pound of fluid as it passesthrough the impeller， and it is given by
H二 1 [U1V81 U2V(6-1)
82
Ac
where:
H二work per lb of fluid
U2二impeller peripheral velocity
U1二inducer velocity at the mean radial station
V82二absolute tangential fluid velocity at impeller exit
V81二absolute tangential air velocity at inducer inlet
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