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Chapter 1
Basics of Heat Transfer
142E
Air gains heat as it flows through the duct of an airconditioning system.
The velocity of the air at
the duct inlet and the temperature of the air at the exit are 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 222
°
F and 548 psia.
2
The kinetic and potential energy changes are negligible,
.
3
Constant specific heats at room temperature can be used for air,
C
ΔΔ
ke
pe
≅≅
0
p
= 0.2404 and
C
v
=
0.1719 Btu/lbm·R. This assumption results in negligible error in heating and airconditioning applications.
Properties
The gas constant of air is
R
= 0.3704 psia.ft
3
/lbm.R (Table A1). Also,
C
p
= 0.2404 Btu/lbm·R
for air at room temperature (Table A15E).
Analysis
We take the airconditioning duct as the system. This is a
control volume
since mass crosses the
system boundary during the process. We
observe
that this is a steadyflow process since there is no change
with time at any point and thus
Δ
Δ
mE
CV
CV
and
=
=
00
&
, there is only one inlet and one exit and thus
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Mass, Heat

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