Heat Exchanger Part 1 PDF
Heat Exchanger Part 1 PDF
Heat Exchanger Part 1 PDF
Prabal Talukdar
Associate Professor
Department of Mechanical Engineering
IIT Delhi
E-mail: prabal@mech.iitd.ac.in
p
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Heat Exchangers
Heat exchangers
g
are devices that facilitate the exchange
g
of heat between two fluids that are at different
temperatures while keeping them from mixing with each
other.
other
Heat exchangers are
commonly used in practice in a
wide range of applications,
from heating
g and air
conditioning systems in a
household, to chemical
processing and power
production in large plants
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Double pipe
Heat exchanger
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Shell-and-Tube
Shell
and Tube Heat Exch
Exch.
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Multipass
flow arrangement
i shell-and-tube
in
h ll
d b
heat exchangers
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R wall =
ln(D o Di )
2kL
1
ln(Do D i )
1
+
+
hi A i
2kL
ho\ A o
T
= UA s T = U i A i T = U o A o T
R
1
1
1
1
1
=
=
= R =
+ R wall +
UA s
U oA o
U iA i
h iA i
hoA o
Q =
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1
1
1
+
U hi ho
U Ui U o
Important Remarks
1
1
1
+
U hi ho
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Fouling Factor
The performance of heat exchangers
usually deteriorates with time as a result
of accumulation of deposits on heat
transfer surfaces. The layer of deposits
represents additional resistance to heat
transfer and causes the rate of heat
transfer in a heat exchanger to
decrease.
The net effect of these accumulations
on heat transfer is represented by a
fouling factor Rf , which is a measure
th th
thermall resistance
i t
iintroduced
t d
db
by
off the
fouling.
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Fouling Resistances
(Fouling Factors)
Thedepositionofscaleonheattransfersurfacereducesthe
h tt
heattransferrateandincreasethepressuredropand
f
t
di
th
d
d
pumpingpower.
Theoverallheattransfercoefficientconsideringfouling
g
g
resistanceontheinsideandoutside
U0 =
1 A0
R
R
1
1
+ f ,i + Rw + f ,o +
hi Ai
Ai
Ao ho Ao
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Q = m c C pc (Tc,out Tc,in )
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Heat Capacity
Heat capacity rate
C h = m h C ph
Cc = m c C pc
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Condenser/Boiler
Q = m h fgg
Mean temperature
.
Q = UA s T
U average can be calculated
As can be calculated
calculated.
How to calculate mean temperature?
One way is to calculate the Log mean temperature difference
In order to develop a relation for the equivalent average
temperature difference between the two fluids, consider a
parallel-flow double-pipe heat exchanger
g ((next slide))
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LMTD parallel-flow
Q = m h C ph dTh
.
Q = m c C pcdTc
dTh =
dTc =
m c C pc
m h C ph
1
1
dTh dTc = d (Th Tc ) = Q
+
.
.
m
C
m
C
h
c
ph
pc
Q = U (Th Tc )dA s
d (Th Tc )
1
1
= UdA s
+
.
.
Th Tc
m
C
m
C
h
c
ph
pc
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d (Th Tc )
1
1
+
= UdA s
.
.
Th Tc
m
C
m
C
h
c
ph
pc
Integrating from the inlet of the heat exchanger to its outlet, we obtain
.
.
Th ,in Tc,in
m
C
m
C
h
c
ph
pc
Q = UA s T lm
Tlm =
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T1 T2
ln( T1 / T2 )
m h C ph
Th ,in Th ,out
=
.
.
m c C pc
Q
LMTD
.
Q = UA s T lm
Tlm =
T1 T2
ln( T1 / T2 )
Q = UA s T lm
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T1 T2
ln( T1 / T2 )
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