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Chapter 7
External Forced Convection
767
Combustion air is heated by condensing steam in a tube bank. The rate of heat transfer to air, the
pressure drop of air, and the rate of condensation of steam are to be determined.
Assumptions
1
Steady operating conditions exist.
2
The surface temperature of the tubes is equal to the
temperature of steam.
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 35
°
C (will be checked later) and 1 atm (Table A15):
k
= 0.02625 W/mK
ρ
= 1.145 kg/m
3
C
p
=1.007 kJ/kgK
Pr = 0.7268
μ
= 1.895
×
10
5
kg/ms
Pr
s
= Pr
@
Ts
= 0.7111
Also, the density of air at the inlet temperature of 20
°
C (for use in the mass flow rate calculation at the
inlet) is
ρ
i
= 1.204 kg/m
3
. The enthalpy of vaporization of water at 100
°
C is h
fg
= 2257 kJ/kgK (Table A
9).
Analysis
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Heat

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