Lecture 14 - Heat Exchangers1
Lecture 14
HEAT
EXCHANGERS
Lecture I
Lecture 14 - Heat Exchangers2
Heat Exchangers INTRODUCTION
Definition: A heat exchanger is a device
that facilitates transfer of heat from one
fluid stream to another.
Heat Exchangers are used in:
Power generation, refrigeration, heating, air-
conditioning, food processing, chemical
processing, oil refining and automobiles.
Heat Exchangers are classified into two
types:
1. Single-Stream Exchangers
The temperature of only one stream changes
in the exchanger (evaporators, condensers,..)
2. Two-Stream Exchangers
The temperature of both streams change in the
exchanger (radiators, oil coolers, ..)
Lecture 14 - Heat Exchangers3
Type of Exchangers
Single-Stream Two-Stream
Lecture 14 - Heat Exchangers4
Type of Exchangers, continued
Geometric Flow Configurations:
• Single-Stream: The temperature of only one
stream changes in the exchanger
• Parallel-Flow Two-Stream: Two fluid flows
parallel to each other in the same direction. It is
often constructed as a shell-and-tube exchanger
(Cocurrent)
• Counterflow Two-Stream: Two fluid flows
parallel to each other in opposite direction. It is
often constructed as a shell-and-tube exchanger.
(Counter-current)
Lecture 14 - Heat Exchangers5
Type of Exchangers, continued
Geometric Flow Configurations:
• Cross-flow Two-Stream: The two streams flow at
right angles to each other. Intermediate
Effectiveness. Example: Automobile Radiators.
• Cross-counterflow Two-Stream: The tubes can
pass twice or four times the shell. The more it
crosses the more effective it is.
•Multipass Two-Stream:When the tubes of a
shell-and-tube exchanger double back one or more
times inside the shell.
• Regenerators: The above configurations involve
steady flows and temperatures usually called
Recuperators. For Regenerators the two streams
flow alternately through a stored matrix of
substantial heat storage capacity. Regenerators can
have parallel, counter and cross flow configurations.
Lecture 14 - Heat Exchangers6
Lecture 14 - Heat Exchangers7
Fluid Temperature Behavior
Notations:
H denotes hot stream
C denotes cold stream,
TH and TC change depending on the configuration.
In parallel flow two stream (TH -TC) decreases
along the exchanger in the flow direction and
TC,out< TH,out.
In the Counterflow two-stream exchanger (TH -TC)
can increase, decrease or remain constant.
Lecture 14 - Heat Exchangers8
Lecture 14 - Heat Exchangers9
Energy Balance
The energy balance of a simple coaxial-tube parallel-
flow heat exchanger is:
)()(
: vapoursaturated withcondenser simplea For
direction of veirrespecti positive as ratesflow mass
thetakingexchanger couterflowfor applies also This
)()()()(
)()()()(
hen,constant t isheat specific theAssuming
stream. cold theand
streamhot thebetween ferefheat trans theis where
)()(
,,
,,,,
0,,,0,
0,,,0,
inCoutCCPfgHH
inCoutCCPoutHinHHP
CLCCPLHHHP
CLCCLHHH
TTcmhm
QTTcmTTcm
or
QTTcmTTcm
Q
Qhhmhhm
−=
=−=−
=−=−
=−=−
&&
&&&
&&&
&
&&&
Lecture 14 - Heat Exchangers10
Overall Heat Transfer Coefficient
Most heat exchangers involve tubes which
have an overall heat transfer coefficient, U.
perimeter tube theis where
21
2
)/ln(
211
,,
℘
πππ℘ ++=ooc
io
iic rhk
rr
rhU
A well known problem of heat exchangers
are fouling. Deposits of calcium or
magnesium on the surface alters the
conductivity of the surface and can also alter
the conductivity coefficient.
.resistance foulinghot and cold theare and where
11
fHR
fCR
RR
UU C
fC
H
fH
f ℑℑ℘℘ ++=
Lecture 14 - Heat Exchangers11
Fouling in a Finned Heat
Exchanger
For finned clean tube in a heat exchanger:
LAhLAhk
rr
rhU pocffoc
io
iic /)/(1
2
)/ln(
211
,,, +ηππ℘ ++=
Lecture 14 - Heat Exchangers12
Fouling Resistance for Heat
Exchangers
Lecture 14 - Heat Exchangers13
Approximate Overall H-T
Coefficient
Lecture 14 - Heat Exchangers14
fgCCoutHinHHp hmTTcm( && =− )()
:anceEnergy BalExchanger
,,
Single-Stream Steady-Flow ExchangerAnalysis of an Evaporator
Lecture 14 - Heat Exchangers15
To obtain gas temperature variation along the
exchanger we now consider a differential element
∆x.
Energy balance in this element shows that the heat
transfer across the tube wall must equal the gas flow
rate times its enthalpy decrease.
D
TTcmTTxUxxHxHpHHsatH
π=℘
−=−∆℘∆+
where
)||()( &
Dividing by ∆x , letting ∆x 0, and rearranging:
0)( =−+ ℘
satHcm
U
dx
dT TTpHH
H
&
Boundary conditions: x=0 : TH=TH,in
xcmU
satinHsatH
pHHeTTTT)/(
,)(
&℘−−=−
Single-Stream Steady-Flow ExchangerAnalysis of an Evaporator continued
Lecture 14 - Heat Exchangers16
Single-Stream Steady-Flow ExchangerAnalysis of an Evaporator continued
units.transfer
ofnumber theis and esseffectiven
exchanger theis where, 1
or
1,
:ratureexit tempe get the weLxFor
/
,
,
Ntu
e-ε
cme
TT
TT
-Ntu
LU
satinH
outHinH pHH
ε=
−=−
−
=
℘− &
The effectiveness is the actual heat transfer
rate divided by the maximum heat transfer
for an infinitely long exchanger. The larger
Ntu is the more effective the heat exchanger.
εεεε typically range from 0.6 to 0.9.
Lecture 14 - Heat Exchangers17
Example 8.4, page 780
In a pilot open-cycle ocean thermal
energy conservation plant, 1 kg/s of
warm sea water at 300 K enters an
evaporator maintained at 2619 Pa.
The water is injected through an array
of nozzles to give an estimated
transfer area and liquid-side heat
transfer coefficient of 0.80 m² and
17,000 W/m² K, respectively. At
what rate is vapor produced?
In Class Exercise
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