This preview shows pages 1–2. Sign up to view the full content.
HW 9 Solution
1.
724
Air is compressed steadily by a compressor. The air temperature is maintained constant by heat rejection to the
surroundings. The rate of entropy change of air is to be determined.
Assumptions
1
This is a steadyflow process since there is no change with time.
2
Kinetic and potential energy changes are
negligible.
3
Air is an ideal gas.
4
The process involves no internal irreversibilities such as friction, and thus it is an
isothermal, internally reversible process.
Properties
Noting that
h =
h
(
T
)
for ideal gases, we have
h
1
=
h
2
since
T
1
=
T
2
= 25
°
C.
Analysis
We take the compressor as the system. Noting that the enthalpy of air remains constant, the energy balance for
this steadyflow system can be expressed in the rate form as
out
in
out
in
energies
etc.
potential,
kinetic,
internal,
in
change
of
Rate
(steady)
0
system
mass
and
work,
heat,
by
nsfer
energy tra
net
of
Rate
out
in
0
Q
W
E
E
E
E
E
±
±
±
±
²
²³
²
²´
µ
±
µ
±
±
=
=
=
Δ
=
−
Ê
Therefore,
kW
30
in
out
=
=
W
Q
±
±
Noting that the process is assumed to be an isothermal and internally
reversible process, the rate of entropy change of air is determined to be
kW/K
0.0101
−
=
−
=
−
=
Δ
K
298
kW
30
sys
air
out,
air
T
Q
S
±
±
AIR
T
= const.
P
2
P
1
Q
·
30 kW
2.
729
R134a enters an evaporator as a saturated liquidvapor at a specified pressure. Heat is transferred to the refrigerant
from the cooled space, and the liquid is vaporized. The entropy change of the refrigerant, the entropy change of the cooled
space, and the total entropy change for this process are to be determined.
Assumptions
1
Both the refrigerant and the cooled space involve no internal irreversibilities such as friction.
2
Any
temperature change occurs within the wall of the tube, and thus both the refrigerant and the cooled space remain isothermal
during this process. Thus it is an isothermal, internally reversible process.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview. Sign up
to
access the rest of the document.
 Fall '07
 RAMUSSEN

Click to edit the document details