Handbook of Turbomachinery Second Edition Revised - Ventech!

Observe that each of the curves for constant N= ffiffiffiffi p
y1 is terminated on
the left by another curve known as the surge line. Attempts to operate to the
left of this line produce effects ranging from pulsating flow to a complete
deterioration of pressure ratio and efficiency. Prolonged operation here can
be disastrous. At lower corrected speeds, an unsteady flow known as
rotating stall is encountered in the region indicated by the dashed lines.
Operating here has caused blades and vanes to vibrate and fail.
Note that the corrected weight flow generally increases with corrected
speed, but the rate of change diminishes at the higher speeds. This could
invite trouble if proper attention is not paid to this trend at the beginning of
the design.
Compressor Adiabatic Efficiency, Z ad
This is another parameter of Fig. 5, and it is defined in the section on
symbols. Note that this quantity is equal to the ratio of the ideal energy (that
of an isentropic process) required to produce the given pressure ratio to the
energy actually needed. Other definitions of efficiency are sometimes used.
Two of these are
1. Polytropic efficiency, Z p, which is the average efficiency that each
small increment of pressure rise must experience in order to achieve the
observed overall adiabatic efficiency. It is often used in Great Britain.
2. Compression efficiency, Z*. Reference to Symbols shows that the
denominator for this efficiency is the same as that for Z ad, but the ideal
energy is that which could be recovered if the gas at the compressor outlet
were expanded isentropically to the total pressure at the compressor inlet.
This definition has some theoretical and some empirical merits that are
noted later.
For general information, an idea of the relative values of these efficiencies is
provided by Table 1, using the ideal thermodynamic properties of air. It is
clear that any cited efficiency should be defined. One should be mindful that
Reynolds number, based on some characteristic dimension, is also an
important operating variable—the useful operating range, pressure ratio,
and efficiency deteriorate when Reynolds number becomes too small.
Wassel [4] and Bullock [5] discuss this subject, which is important when
compressors are small or when they operate with low-density gases such as
air at high altitudes.
It is also important to realize that an effort was made to supply
uniform flow to the compressor while the data for the map were being
generated. The useful range of mass flow rate, the pressure ratio, and the
efficiency deteriorate when the entering flow is nonuniform or distorted.
Copyright © 2003 Marcel Dekker, Inc.