Solutions of homework assignment 16
1.
(a)
The ideal process for all three devices is the reversible adiabatic (i.e., isentropic) process. The adiabatic
efficiencies of these devices are defined as
energy
kinetic
exit
c
insentropi
energy
kinetic
exit
actual
and
,
input
work
actual
input
work
c
insentropi
,
output
work
c
insentropi
output
work
actual
N
C
T
(b)
Yes. Because the entropy of the fluid must increase during an actual adiabatic process as a result of
irreversibilities. Therefore, the actual exit state has to be on the right
‐
hand side of the isentropic exit
state
2.
A steam power plant operates between the pressure limits of 5 MPa and 20 kPa. The ratio of the
turbine work to the pump work is to be determined.
Assumptions
1
Liquid water is an incompressible substance.
2
Kinetic and potential energy changes are
negligible.
3
The process is reversible.
4
The pump and the turbine are adiabatic.
Properties
The specific volume of saturated liquid water at 20 kPa is
v
1
=
f @
20 kPa
= 0.001017 m
3
/kg
(Table A
‐
5).
Analysis
Both the compression and expansion processes are reversible and adiabatic, and thus
isentropic,
s
1
=
s
2
and
s
3
=
s
4
. Then the properties of the steam are
4
@ 20 kPa
4
4
@ 20 kPa
3
3
34
2608.9 kJ/kg
20 kPa
7.9073 kJ/kg K
.
5 MPa
4318.9 kJ/kg
g
g
hh
P
ss
sat vapor
P
h
Also,
1
=
f @
20 kPa
= 0.001017 m
3
/kg.
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 Summer '07
 RAMUSSEN
 Energy, Entropy, kPa, kJ/kg, potential energy changes, mass flow rate

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