7.23 A rigid tank contains an ideal gas that is being stirred by a paddle wheel. The temperature of the
gas remains constant as a result of heat transfer out. The entropy change of the gas is to be
determined.
Assumptions
The gas in the tank is given to be an ideal gas.
Analysis
The temperature and the specific volume
of the gas remain constant during this process.
Therefore, the initial and the final states of the gas
are the same. Then s
2
= s
1
since entropy is a
property.
Therefore,
0
sys
S
7.25 Heat is transferred directly from an energysource reservoir to an energysink. The entropy change
of the two reservoirs is to be calculated and it is to be determined if the increase of entropy principle is
satisfied.
Assumptions
The reservoirs operate steadily.
Analysis
The entropy change of the source and sink is given by
kJ/K
0.0833
K
600
kJ
100
K
1200
kJ
100
L
L
H
H
T
Q
T
Q
S
Since the entropy of everything involved in this process has increased, this transfer of heat is
possible
.
7.29 A reversible heat pump with specified reservoir temperatures is considered. The entropy change
of two reservoirs is to be calculated and it is to be determined if this heat pump satisfies the increase
in entropy principle.
Assumptions
The heat pump operates steadily.
Analysis
Since the heat pump is completely reversible, the
combination of the coefficient of performance expression, first Law,
and thermodynamic temperature scale gives
73
.
26
)
K
294
/(
)
K
283
(
1
1
/
1
1
COP
rev
HP,
H
L
T
T
The power required to drive this heat pump, according to the
coefficient of performance, is then
kW
741
.
3
26.73
kW
100
COP
rev
HP,
in
net,
H
Q
W
According to the first law, the rate at which heat is removed from the lowtemperature energy
reservoir is
kW
26
.
96
kW
741
.
3
kW
100
in
net,
W
Q
Q
H
L
The rate at which the entropy of the high temperature reservoir changes, according to the definition
of the entropy, is
kW/K
0.340
K
294
kW
100
H
H
H
T
Q
S
IDEAL GAS
40
C
Hea
t
20
H
100
L
Q
net
W
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View Full Documentand that of the lowtemperature reservoir is
kW/K
0.340
K
283
kW
26
.
96
L
L
L
T
Q
S
The net rate of entropy change of everything in this system is
kW/K
0
340
.
0
340
.
0
total
L
H
S
S
S
as it must be since the heat pump is completely reversible.
7.35E R134a is expanded in a turbine during which the entropy remains constant. The enthalpy
difference is to be determined.
Analysis
The initial state is superheated vapor and thus
EES)
(from
R
Btu/lbm
23281
.
0
Btu/lbm
95
.
129
F
175
psia
250
1
1
1
1
s
h
T
P
The entropy is constant during the process. The final state is also
superheated vapor and the enthalpy at this state is
EES)
(from
Btu/lbm
95
.
106
R
Btu/lbm
23281
.
0
F
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 Summer '07
 RAMUSSEN
 Thermodynamics, Entropy, Heat, Heat Transfer, entropy change

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