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Chapter 7
External Forced Convection
771
Air is cooled by an evaporating refrigerator. The refrigeration capacity and the pressure drop across
the tube bank are to be determined.
Assumptions
1
Steady operating conditions exist.
2
The surface temperature of the tubes is equal to the
temperature of refrigerant.
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 5
°
C (will be checked later) and 1 atm (Table A15):
k
= 0.02326 W/mK
ρ
= 1.316 kg/m
3
C
p
=1.006 kJ/kgK
Pr = 0.7375
μ
= 1.705
×
10
5
kg/ms
Pr
s
= Pr
@
Ts
= 0.7408
Also, the density of air at the inlet temperature of 0
°
C (for use in the mass flow rate calculation at the inlet)
is
ρ
i
= 1.292 kg/m
3
.
Analysis
It is given that
D
= 0.008 m,
S
L
=
S
T
= 0.015 m, and V
= 4 m/s.
Then the maximum velocity and the Reynolds number
based on the maximum velocity become
m/s
571
.
8
m/s)
4
(
008
.
0
015
.
0
015
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass

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