Handbook of Turbomachinery Second Edition Revised - Ventech!

components of gas velocity upstream and downstream of a blade row are
discussed.
The principles involved in the selection of tentative shapes for blades
can now be considered; axial and centrifugal compressors are often treated
separately in this discussion. Factors governing the compromises necessary
for practical multistage units are noted.
Estimating the behavior of a compressor at various inlet flow
conditions and rotating speeds is the next subject to be discussed. Here
again, some separate procedures are given for axial and centrifugal
compressors.
The subjects of unsteady flows and excessive losses in useful energy are
reviewed and methods of coping with these problems are suggested.
Mechanical stresses and vibrations that may be troublesome are identified
and relevant references are cited. The subject of compressor noise is briefly
noted.
Bear in mind that compressor design is not an exact science.
Experimental development is nearly always required to make the best use
of a particular configuration when the desired pressure ratio is about 3 or
higher. Development testing is therefore the final subject included in this
chapter. Emphasis is placed on the objectives of the tests and the
precautions that should be taken. Details about instrumentation and other
testing techniques are left to publications dealing with these particulars.
COMPRESSOR MAPS
Compressors can and do operate over a range of speeds and gas flows. This
ability is shown by the curves of Fig. 5, which presents atypical compressor
map. Each of the coordinates and parameters of this figure can be shown to
be equal to a dimensionless variable multiplied by some constant.
Corrected Weight Flow and Pressure Ratio
Corrected weight flow, W ffiffiffiffi p
y1=d1,
is the abscissa of Fig. 5. This variable is
almost a standard in the United States and is proportional to the rate of
mass flow of the gas entering the compressor (see Symbols). Note that this
term is also proportional to W ffiffiffiffiffiffi p
TT=P1,
which is frequently used in other
countries. Both of these terms are functions of the Mach number of the flow
at the compressor inlet. The ordinate is the ratio of the total pressure at the
compressor outlet to that at the inlet. One of the parameters is the corrected
speed, N= ffiffiffiffi p p ffiffiffiffiffi
y1;
it too is often replaced by another term, N= T1,
which is
proportional to it.
Copyright © 2003 Marcel Dekker, Inc.