6.2.7
The polytropic exponent is not a constant during the compression cycle.
However, for most practical applications, it can be assumed to be a constant.
Therefore, along a polytropic path, the ratio of the reversible work input to
the enthalpy rise is constant.

PIP REEC001
COMPLETE REVISION
Compressor Selection Guidelines
April 2013
Process Industry Practices
Page 31 of 44
6.2.8
The exponent n is related to the ratio k of specific heats Cp/Cv and the
polytropic efficiency
p approximately as follows:
n -1
n
k -1
k
1
p
6.2.9
The work input per unit mass is typically referred to as polytropic head and
can be evaluated as follows:
Polytropic Head, H
=
Z
R T
MW(n -1) / n
P
P
- 1
p
avg
s
d
s
n 1
n
Where:
Z
avg
= Average Compressibility Factor of the gas
R
= Universal Gas Constant(8314 J/kg-°K)
= (1545 ft-lbf/lbm-°R)
MW = Gas Molecular Weight
T = Absolute Temperature of the Gas
P = Absolute Gas Pressure
s, d = Subscripts that refer to suction/inlet and discharge conditions,
respectively
6.2.10
The gas power can be calculated using the following relationship:
Gas Power = C
wH
p
p
Where:
C = a constant depending on units used = 1/3600 for SI units
(1/33,000 for U.S. Customary Units)
w = Mass Flow Rate
6.2.11
The shaft power is the sum of the gas power and frictional losses in bearings,
seals and gearing (if present).
6.2.12
Typically, the adiabatic process should be suitable to define the compression
of gases such as air that exhibit ideal gas behavior. In all other cases, the
polytropic process should be used. The following should be noted:
a.
Adiabatic Head is less than Polytropic Head
b.
Adiabatic Efficiency is less than Polytropic Efficiency

PIP REEC001
COMPLETE REVISION
Compressor Selection Guidelines
April 2013
Process Industry Practices
Page 32 of 44
6.2.13
The performance of centrifugal compressors is typically represented by head-
flow curves and efficiency-flow curves. A typical example of these curves is
shown in Figure 11.
6.2.13.1 Some manufacturers may also provide curves of pressure ratio and
shaft power versus volumetric flow rate.
6.2.13.2 The end points of the curves represent two important limits of
centrifugal compressors: surge and stonewall or choke point,
defined as follows:
a.
Surge is characterized by cyclic variation (and even reversal)
of gas flow and discharge pressure, and it occurs if the flow
is reduced below the surge point. It is typically accompanied
by abnormal noise and vibration and it can lead to significant
damage if the compressor’s operating condition is not
changed quickly to increase the flow.
b.
A stonewall or choke point condition is encountered if the
gas flow reaches sonic conditions somewhere in the
compressor passages. In this condition, flow through the
compressor cannot be increased further.

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- Summer '09
- ramana rao
- Thermodynamics, Gas compressor, compressor inlet