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Chapter 7
External Forced Convection
768
Water is preheated by exhaust gases in a tube bank. The rate of heat transfer, the pressure drop of
exhaust gases, and the temperature rise of water are to be determined.
Assumptions
1
Steady operating conditions exist.
2
The surface temperature of the tubes is equal to the
temperature of steam.
3
For exhaust gases, air properties are used.
Properties
The exit temperature of air, and thus the mean temperature, is not known. We evaluate the air
properties at the assumed mean temperature of 250
°
C (will be checked later) and 1 atm (Table A15):
k
= 0.04104 W/mK
ρ
= 0.6746 kg/m
3
C
p
=1.033 kJ/kgK
Pr = 0.6946
μ
= 2.76
×
10
5
kg/ms
Pr
s
= Pr
@
Ts
= 0.7154
Also, the density of air at the inlet temperature of 300
°
C (for use in the mass flow rate calculation at the
inlet) is
ρ
i
= 0.6158 kg/m
3
.
Analysis
(
a
) It is given that
D
= 0.021 m,
S
L
=
S
T
= 0.08 m, and V
= 4.5 m/s.
Then the maximum velocity and the Reynolds number
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Heat

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