Chemical Engineering April 2019 - 43

is referred to as standard, or normal,
flow, and calculates the volume flow
for the given mass flow at a " standard "
condition. For example, this
could be a temperature of 60°F and
pressure equal to 14.7 psia. For either
volume flow term, the mass flow
would be identical.
Inducer
Diffuser
Inlet
Shaft
Impeller
Measurements and calculations
The data gathered to determine
performance include temperatures,
pressures, flow and gas composition.
Temperatures and pressures are relatively
straightforward to collect using
a variety of available sensors. However,
flow can be a bit more complex
to determine. Such measurements
often rely on additional measurements,
but there are still a number of
sensor options available. Gas composition
is probably the least accessible
data to obtain, either requiring
an online gas chromatograph (rarely
available in most installations), or
sampling of the fluid for analysis by a
laboratory after the testing.
In addition to these base measurements,
some common performance
parameters that will be generated include
the work, head and efficiency.
Work and head, discussed earlier,
lead to the definition of efficiency,
which can be defined in many different
ways.
First, there is the thermodynamic
efficiency that only involves the actual
Diffuser vanes
Collector
Outlet
Diffuser vanes
FIGURE 2. Centrifugal compressors have a rotating impeller that imparts head to a fluid
compression process, and relates
the actual head to an ideal head.
The ideal head can also be defined
in multiple ways; the two most common
are isentropic and polytropic.
It is critical to understand which of
these two definitions is being used
in a given case. Isentropic efficiency
calculations assume that the thermodynamic
process is adiabatic, and
compares the actual head to the ideal
head, with entropy at the suction and
discharge assumed to be the same.
Polytropic efficiency compares the
actual head to the head conceptually
calculated by breaking the entire
compression step into a number of
smaller steps, with some heat added
at each step. The two processes converge
at low pressure ratios, yielding
identical results. One benefit of using
polytropic efficiency is that for multistage
machines, if all stages have the
same polytropic efficiency, then the
entire machine polytropic efficiency
is equal to the stage polytropic efficiency.
Overall machine isentropic efficiency
in this case will be lower than
the stage isentropic efficiency.
Another way that efficiency can be
defined is the mechanical efficiency
of the compressor. This usually involves
additional measurements to
evaluate the mechanical work going
into the compressor, usually through
the use of a torque-meter. This allows
calculation of the compressor
efficiency as the actual head compared
to the mechanical work going
into the compressor.
Finally, a total system efficiency can
be generated if measurements are
taken of the total energy being put
into the system. For electric-motordriven
compressors, the total energy
can be determined based on electrical
measurements and knowledge of
the motor. For engine- and gas-turRadial
bladed impeller
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APRIL 2019
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Chemical Engineering April 2019

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