University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Expansion processes
Turbine or expanders, the expansion of a gas in a nozzle to produce a high-velocity stream is a
process that converts internal energy into kinetic energy. This kinetic energy is in turn
converted into shaft work when the stream impinges on blades attached to a rotating shaft.
Thus a turbine (or expander) consists of alternate sets of nozzles and rotating blades through
which vapor or gas flows in a steady-state expansion process whose overall effect is the
efficient.
The fluid in the turbine undergoes an expansion process that is reversible as well as adiabatic,
then the process is isentropic, and S 2 = S1 .
 H  WS
WS  m  H
WS (isentropic )  (H ) S
The shaft work │WS(isentropic) │ is the maximum that can be obtained from an adiabatic
turbine with given inlet conditions and given discharge pressure. Actual turbines produce less
work, because the actual expansion process is irreversible. We therefore define a turbine
efficiency as:

WS ( Actual )
WS (isentropic )
 
H
(H ) S
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University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Adiabatic expansion process in a turbine or expander
Throttling processes
It is a steady flow process across a restriction with a resulting drop on pressure a typical
example is the flow through a partially open valve (orifice) .This process occurs so rapidly and
such a small space that there is no time and no large area foe such heat transfer, so it is
assumed to be adiabatic.
H  Q  W
H  0
Joul-Thomson coefficient  J
 T 

 P  H
J  
If  J positive, this means temperature dropping, and if  J negative, this means temperature
raises during throttling .process.
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University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Compression processes
-Compressors
The compression of gases may be accomplished in equipment with rotating blades (like a
turbine operating in reverse) or in cylinders with reciprocating pistons. Rotary equipment is
used for high-volume flow where the discharge pressure is not too high. For high pressures,
reciprocating compressors are required.

WS (isentropic )
WS ( Actual )
 
(H ) S
H
Adiabatic compression process
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University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
-Pumps
Liquids are usually moved by pumps, generally rotating equipment. The same equations apply
to adiabatic pumps as to adiabatic compressors. For the calculation of (WS = AH) requires
values of the enthalpy of compressed (sub cooled) liquids, and these are seldom available. The
fundamental property relation, below which provides an alternative. For an isentropic process,
dH  VdP
constant S
WS (isentropi c)  (H ) S
P2
WS   VdP
P1
WS (isentropic )  (H ) S  V ( P2  P1 )
also useful are the following equations
dH  C P  V (1   )dP
dS  C p
dT
 VdP
T
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