Heat Exchangers
What are heat exchangers for?
- Heat exchangers are practical devices used to transfer
energy from one fluid to another
- To get fluid streams to the right temperature for the next
process
* reactions often require feeds at high temp.
- To condense vapours
- To evaporate liquids
- To recover heat to use elsewhere
- To reject low-grade heat
- To drive a power cycle
Application: Power cycle
Steam Turbine
Boiler CondenserFeed water
Heater
Main Categories Of Exchanger
Wall separating streams Direct contact
- Most heat exchangers have two streams, hot and cold, but some have more than two
Heat exchangers
Recuperators Regenerators
Wall separating streamsWall separating streams Direct contacts
Recuperators/Regenerators
- Recuperative:
Has separate flow paths for each fluid
which flow simultaneously through the
exchanger transferring
the streams
- Regenerative
heat between
Has a single flow path which the hot
and cold fluids alternately pass
through.
Double Pipe
- Simplest type has one tube inside another - inner tube may have longitudinal fins on the outside
- However, most have a number of tubesin the outer tube - can have very manytubes thus becoming a shell-and-tube
Shell and Tube
- Typical shell and tube exchanger as used in the process industry
Shell-Side Flow
Plate-Fin Exchanger
- Made up of flat plates (parting sheets) and corrugated sheets which form fins
- Brazed by heating in vacuum furnace
Configurations
T2
t1
T1
t2
ParallelFlowT1
T2
t1 t2
Position
Tem
pera
ture
T2
t2
T1
t1
CounterFlowT1
T2
t2 t1Tem
pera
ture
Position
Basic flow arrangement in tube in tube flow
Heat Exchanger Analysis
Log mean temperature difference (LMTD) method
.Want a relation Q=UAΔTm
Where ΔTmis some mean ΔT between hot and cold fluid
Counterflow
Note Th,out can be <Tc,out
Parallel flow
T ' s ' cannot cross
Energy balance (counterflow) on element shown
hhcc
ch
cc
c
hh
h
ch
ccchhh
cmcmQdTTd
cm
QddT
cm
QddT
TTUdAQd
c
m
dTcmdTcmQd
11
(1) fromNow
)2(
EquationRate
heat specific
fluid of rateflow mass
)1(
2121
11
22
and
ratefer heat trans Total
11ln
21 Integrate
11
(2), from Subtract
cccchhhh
hhccch
ch
hhccch
ch
TTcmQTTcmQ
cmcmUA
TT
TT
dAcmcm
UTT
TTd
Qd
• Remember – 1 and 2 are ends, not fluids
• Same formula for parallel flow (but ΔT’s are different)
•Counterflow has highest LMTD, for given T’s therefore smallest area for Q.
Difference eTemperaturMean Log is LMTD
LMTD
/ln
2
1
put and mfor Substitute
12
12
222
111
c
UAQ
TT
TTUAQ
ENDTTT
ENDTTT
ch
ch
Condenser Evaporator
Multipass HX Flow Arrangements
- In order to increase the surface area for convectionrelative to the fluid volume, it is common to design formultiple tubes within a single heat exchanger.
- With multiple tubes it is possible to arrange to flow so thatone region will be in parallel and another portion in counterflow.
1-2 pass heat exchanger,indicating that the shell sidefluid passes through the unitonce, the tube side twice. Byconvention the number of shellside passes is always listedfirst.
- The LMTD formulas developed earlier are no longer adequate formultipass heat exchangers. Normal practice is to calculate the LMTD forcounter flow, LMTDcf, and to apply a correction factor, FT, such that
=FT ⋅LMTDCFΔθeff
- The correction factors, FT, can be found theoretically and presentedin analytical form. The equation given below has been shown to beaccurate for any arrangement having 2, 4, 6, .....,2n tube passes pershell pass to within 2%.
1Rfor ,1
:essEffectiven/1
/1
shell
shell
N
N
XR
XP
1R1RP2
1R1RP2ln1R
PR1
P1ln1R
F
2
2
2
T
1Rfor ,
1NPN
PP
shelloshell
o
11
12o
tT
ttP
1
1
o
o
P
RPX
12
21 ratioCapacity tt
TTR
T,t = Shell / tube side; 1, 2 = inlet / outlet
R=0.1
1.0
0.50.0 1.0P
R=10.0
FT
Effectiveness-NTU MethodNumber of Transfer Units (NTU) Method
havecan ratecapacity heat
C ,C oflesser with fluid only thethen
since and
fluid One
H.Ex. long infinitelyan for is where
:esseffectiven Define
? conditionsinlet
given for perform Ex. H. existing willHow
max
BA
A
,,max
max
max
T
TCTC
TcmTcmQ
TTTT
Q
Q
Q
BBA
BBAA
incinh
actual
max
min
minmax
min
max
min
min
in.cin.hmin
in.cin.hmin
maxminmax
C
C1
C
UA-exp
C
C1
C
C1
C
UA-exp-1
......... )LMTD(UAQ intoback Substitute
sT'outlet contain not does which for expressionWant
TTCQ or,
TTC
Q and TCQ i.e.
min
max
min
HEx.) of (size units transfer of No. and
,
C
UANTU
C
CNTU
Charts for each Configuration
incinh TTCQ ..min
Procedure:
Determine Cmax, Cmin/Cmax
Get UA/Cmin, from chart
• NTUmax can be obtained from figures in
textbooks/handbooks First, however, we must determine
which fluid has Cmin
• For the type of HEX used in this problem
Examination of the last equation, subject to values given, indicated that gas will have Cmin.
U
CNTUA
C
UANTU minmax
min
max
Effectiveness can be calculated using
°C⎪=38.0m2
m2°C180 W
1.4⎪4,882W⎪
Umin=
NTUmaxCA=
→ NTUmax=1.4ε=0.649
min=0.467⎪Cmax
C
⎪→
⎠
⎛ ⎞
⎪⎝
⎬⎪⎪⎪⎪⎪⎭
⎪⎪
⎪⎫
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