fluid_mechanics

686 Chapter 12 ■ Turbomachines
Compressors are pumps that add energy to the fluid, causing a significant pressure rise and
a corresponding significant increase in density. Compressible flow turbines, on the other hand, remove
energy from the fluid, causing a lower pressure and a smaller density at the outlet than at
the inlet. The information provided earlier about basic energy considerations 1Section 12.22 and
basic angular momentum considerations 1Section 12.32 is directly applicable to these turbomachines
in the ways demonstrated earlier.
As discussed in Chapter 11, compressible flow study requires an understanding of the principles
of thermodynamics. Similarly, an in-depth analysis of compressible flow turbo-machines
requires use of various thermodynamic concepts. In this section we provide only a brief discussion
of some of the general properties of compressors and compressible flow turbines. The interested
reader is encouraged to read some of the excellent references available for further information
1e.g., Refs. 1–3, 18–202.
P 0 out, psia
Multistaging is
common in highpressure
ratio
compressors.
500
400
300
200
100
0
0
PR = 2
P 0 in = 14.7 psia
1 2 3 4
Stage, n
5
12.9.1 Compressors
Turbocompressors operate with the continuous compression of gas flowing through the device. Since
there is a significant pressure and density increase, there is also a considerable temperature increase.
Radial-flow 1or centrifugal2 compressors are essentially centrifugal pumps 1see Section 12.42
that use a gas 1rather than a liquid2 as the working fluid. They are typically high pressure rise, low
flowrate, and axially compact turbomachines. A photograph of the rotor of a centrifugal compressor
rotor is shown in Fig. 12.33.
The amount of compression is typically given in terms of the total pressure ratio, PR p 02p 01 ,
where the pressures are absolute. Thus, a radial flow compressor with PR 3.0 can compress standard
atmospheric air from 14.7 psia to 3.0 14.7 44.1 psia.
Higher pressure ratios can be obtained by using multiple stage devices in which flow from the
outlet of the preceding stage proceeds to the inlet of the following stage. If each stage has the same
pressure ratio, PR, the overall pressure ratio after n stages is PR n . Thus, as shown by the figure in
the margin, a four-stage compressor with individual stage PR 2.0 can compress standard air from
p 0 in 14.7 psia to p 0 out 2 4 14.7 235 psia. Adiabatic 1i.e., no heat transfer2 compression of a
gas causes an increase in temperature and requires more work than isothermal 1constant temperature2
compression of a gas. An interstage cooler 1i.e., an intercooler heat exchanger2 as shown in Fig. 12.34
can be used to reduce the compressed gas temperature and thus the work required.
Relative to centrifugal water pumps, radial compressors of comparable size rotate at much
higher speeds. It is not uncommon for the rotor blade exit speed and the speed of the absolute flow
leaving the impeller to be greater than the speed of sound. That such large speeds are necessary
for compressors can be seen by noting that the large pressure rise designed for is related to the differences
of several squared speeds 1see Eq. 12.142.
The axial-flow compressor is the other widely used configuration. This type of turbomachine
has a lower pressure rise per stage, a higher flowrate, and is more radially compact than a
centrifugal compressor. As shown in Fig. 12.35, axial-flow compressors usually consist of several
stages, with each stage containing a rotor/stator row pair. For an 11-stage compressor, a compression
ratio of PR 1.2 per stage gives an overall pressure ratio of 1.2 11 7.4. As the gas
p 02p 01
F I G U R E 12.33 Centrifugal
compressor rotor. (Photograph courtesy of
concepts NREC.)