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PROBLEM 11.10
KNOWN:
Heat exchanger with two shell passes and eight tube passes having an area 925m
2
; 45,500
kg/h water is heated from 80
°
C to 150
°
C; hot exhaust gases enter at 350
°
C and exit at 175
°
C.
FIND:
Overall heat transfer coefficient.
SCHEMATIC:
ASSUMPTIONS:
(1) Negligible losses to surroundings, (2) Negligible kinetic and potential energy
changes, (3) Constant properties, (4) Exhaust gas properties are approximated as those of atmospheric
air.
PROPERTIES:
Table A6
, Water
( 29
( 29
c
T
8
0
15
0
C/
2
388K:
=+°=
c = c
p,f
= 4236 J/kg
⋅
K.
ANALYSIS:
The overall heat transfer coefficient follows from Eqs. 11.9 and 11.18 written in the
form
m,CF
U
q/AFT
=∆
l
where F is the correction factor for the HXer configuration, Fig. 11.11, and
m,CF
T
∆
l
is the log mean
temperature difference (CF), Eqs. 11.15 and 11.16.
From Fig. 11.11, find
( 29
( 29
( 29
( 29
h,
i
o
c,
o
c,i
c,
o
c,
i
i
c,i
T
T
TT
35
0
17
5
C
15
0
8
0C
R
2.
5
P
0.26
T
T
15
0
8
0
C
T
T
35
0
8


°
°
=
=
=
=
==


°

find F
≈
0.97.
The logmean temperature difference, Eqs. 11.15 and 11.17, is
( 29
( 29 ( 29
( 29 ( 29
12
35
0
15
17
5
8
T
141.1 C.
n
T
/T
n
35
0
150/ 17
5
80

°
 °
∆
∆
∆
=
=
=°
∆∆
l
l
l
From an overall energy balance on the cold fluid (water), the heat rate is
( 29
c
c
c,
o
c,i
q m
c
=
&
( 29
6
q
45,500kg/
h 1h/3600
s
4236J/kgK 15
0
8
3.74
81
0
W.
=
××
⋅

°
=×
Substituting values with A = 925 m
2
, find
6
22
U
3.74
8 1
0 W/925
m
0.9
7 141.1
K
29.6W/
m
K.
=
×
×
×
=⋅
<
COMMENTS:
Compare the above result with representative values for airwater exchangers, as
given in Table 11.2.
Note that in this exchanger, two shells with eight tube passes, the correction
factor effect is very small, since F = 0.97.
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View Full Document PROBLEM 11.56
KNOWN:
Inlet temperature and flow rates for a concentric tube heat exchanger.
Hot fluid outlet
temperature.
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This note was uploaded on 08/08/2008 for the course ME 364 taught by Professor Rothamer during the Spring '08 term at Wisconsin.
 Spring '08
 Rothamer
 Heat Transfer

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