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Homework #7 Solutions:
1.
449C
It can be used for any kind of process of an ideal gas.
2.
454C
The energy required is
mc
p
D
T
, which will be the same in both cases.
This is
because the
c
p
of an ideal gas does not vary with pressure.
3.
461
Neon is compressed isothermally in a compressor. The specific volume and
enthalpy changes are to be determined.
Assumptions
At specified conditions, neon behaves as an ideal gas.
Properties
The gas constant of neon is
R
= 0.4119 kJ/kg
⋅
K and the constantpressure
specific heat of neon is 1.0299 kJ/kg
⋅
K (Table A2
a
).
Analysis
At the compressor inlet, the specific volume is
/kg
m
207
.
1
kPa
100
K)
273
K)(20
/kg
m
kPa
(0.4119
3
3
1
1
=
+
⋅
⋅
=
=
P
RT
v
Similarly, at the compressor exit,
/kg
m
2414
.
0
kPa
500
K)
273
K)(20
/kg
m
kPa
(0.4119
3
3
2
2
=
+
⋅
⋅
=
=
P
RT
The change in the specific volume caused by the compressor is
/kg
m
0.966
3

=

=
=
D
207
.
1
2414
.
0
1
2

Since the process is isothermal,
kJ/kg
0
=
D
=
D
T
c
h
p
4.
466
A resistance heater is to raise the air temperature in the room from 7 to 23°C
within 15 min. The required power rating of the resistance heater is to be determined.
Assumptions
1
Air is an ideal gas since it is at a high temperature and low pressure
relative to its critical point values of 141
°
C and 3.77 MPa.
2
The kinetic and potential
energy changes are negligible,
D
D
ke
pe
@
@
0
.
3
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 Summer '08
 thermo

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