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IZMIR INSTITUTE OF TECHNOLOGY
Department of Mechanical
Engineering
THERMODYNAMICS I
ME 206
Experiment 2
Constant Speed Centrifugal
Compressor Energy Balance
Lab Handout
April, 2014
Energy Lab.
Room Z35
Res. Ast. C.TURHAN
2014
1. Introduction
Compressors are devices used to increase the pressure and the kinetic energy of a fluid
for utilizing it in desired purposes like in heating-ventilating-air conditioning applications.
Work is supplied to these devices from an external source through a rotating shaft to
compress the fluid in it to very high pressures.
The objective of the experiment is to utilize the conservation of mass and energy laws for
steady state conditions on a centrifugal air compressor and to find its first law efficiency.
2. Theory and Principles
In the experiment, a centrifugal compressor is utilized to compress air to a higher
pressure. The general relationship between the various forms of energy, based on the 1 st
Law of Thermodynamics applied to a unit mass of fluid flowing through a control volume,
which is the compressor itself in this experiment, is expressed as
 Ws  ke  pe  P  F
1
where  Ws is the mechanical shaft work performed on the fluid, ke is the change of
kinetic energy of the fluid, pe is the change in potential energy of the fluid, p is the
change in pressure energy, and F is the frictional energy loss as heat to the surroundings
or in raising the temperature of the fluid itself as it travels from inlet to outlet of the
compressor. When the equation 1 is written explicitly, it becomes
v 2 v 2 

1   g ( z  z )  p2  p1  F
 Ws   2

2 1
2




2
where v , g , z , p ,and  represent the velocity of the fluid in m/s, gravitational acceleration in
m/s2, elevation in m, pressure in N/m2, and the density of the fluid in kg/m 3, respectively,
subscripts 1 and 2 refer to the compressor inlet and outlet.
The first three terms on the right hand side of the equation 2 represent useful work Wa .
v 2 v 2 

1   g ( z  z )  p2  p1
Wa   2

2 1
2




3
The term Wa represents the actual work performed in changing the energy stages of a unit
mass of the fluid. This may alternatively be represented as the compressor total pressure
ptc , by converting the units from work per unit mass to pressure by multiplication two
sides of equation 3 with  and it yields
  v 2  v 2   
 2
1  
ptc   
  p2  p1
2






4
The change in potential head of the fluid is ignored as the change in atmospheric pressure
between the inlet and outlet is negligible. Friction losses have also been ignored, and for
the purposes of the following experiments it is assumed that the fluid is incompressible ( 
is constant).
The mechanical power input, Pm , to the compressor can also be expressed as
Pm  2 .n.t / 60
5
where n is the revolution of the shaft per minute, t is the motor torque in Nm. The
compressor power output, Pu , based on Pm and volume flow rate may be expressed as
Pu  Q v  ptc
6
where Q v is the volume flow rate of the air in m3/s and it is expressed as follows
C d 2  2 p0
Q v  d
 ptc
4
7
where C d , d ,  , p0 indicate the coefficient of discharge for the orifice, the orifice diameter
in m, the density of the air in kg/m 3, and the differential pressure across the orifice in N/m,
respectively.
According to equation 5 and equation 6, the efficiency of the compressor is

Pu
 100
Pm
8
3. Experimental Set-up
The experiment is conducted with the experimental setup of “Armfield FM42 Centrifugal
Compressor Demonstration Unit” which consists of “Armfield IFD7 Interface Device” and a
compatible PC running “Armfield FM42-304 software”.
The experimental setup includes one temperature sensor and two pressure transducers at
the inlet and one pressure transducer at the outlet of the centrifugal compressor. At the
outlet of the compressor, there is a rotary outlet aperture to adjust the air outlet resistance.
The compressor speed can be increased or reduced by the software to obtain different
pressurization levels of the air and compare different operation conditions.
In Figure 1, the experimental setup and its instrumentation are shown.
Air Inlet
Air Outlet
Figure 1. Experimental setup
4. Procedure
 Close the rotary outlet aperture to give significant system resistance, for instance
2/3 closed.
 Set the compressor speed to maximum 100%.
 Click the “Go” button on the software to take a sample.
 Using the discharge as a guide, select incremental values for discharge that will
give 10-15 individual steps between minimum and maximum velocity.
 Reduce the compressor setting gradually to reduce the discharge by approximately
the increment chosen. Allow the flow to stabilize then click “Go”.
 Open the aperture to increase the flow rate to the first increment. Click “Go” to take
a sample. Repeat for each increment until the aperture is fully open.
 If time permits, repeat the above procedure for compressor speeds of 80%, 60%,
40% and 20%.
 Note measurements for each trial.
Measured Parameters
Inlet Temperature
Orifice Differential Pressure p0
1
2
3
Compressor Differential Pressure p2  p1
Compressor Speed n
Motor Torque t
5. Results
 Calculate the following parameters.
Air Density 
Volume Flow Rate Q v
Compressor Total Pressure ptc
Mechanical Power (Input) Pm
Compressor Power (Output) Pu
Efficiency 
 Plot graph of compressor total pressure against volume flow rate.
 Plot graph of compressor efficiency against volume flow rate.
 Plot graph of mechanical power against volume flow rate.
 Plot graph of compressor power output against volume flow rate.
 Describe the compressor performance characteristics of the compressor.
 Make your comment on the results.
6. References
[1]
[2]
Cengel Y., Boles M. A., Thermodynamics: An Engineering Approach, McGraw-Hill,
Fifth Edition
Instruction Manual Armfield FM42