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Heat Exchanger DesignAnand V P Gurumoorthy
Associate Professor
Chemical Engineering Division
School of Mechanical & Building Sciences
VIT University
[EDITED BY AFRAZ ]
ICET PU[UNIVERSITY OF THE PUNJAB LAHORE]
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Heat Exchanger Classification
Recuperative:Cold and hot fluid flow through the unit without mixing
with each other. The transfer of heat occurs through themetal wall.
Regenerative:Same heating surface is alternately exposed to hot and
cold fluid. Heat from hot fluid is stored by packings orsolids; this heat is passed over to the cold fluid.
Direct contact:Hot and cold fluids are in direct contact and mixing occurs
among them; mass transfer and heat transfer occursimultaneously.
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Heat Exchanger Standards andCodes
British Standard BS-3274
TEMA standards are universally used.
TEMA standards cover following classes ofexchangers:Class Rdesignates severe requirements of petroleum
and other related processing applications
Class Cmoderate requirements of commercial and
general process applicationsClass Bspecifies design and fabrication for chemical
process service.
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Shell and Tube Heat Exchanger
Most commonly used type of heat transferequipment in the chemical and allied industries.
Advantages:
The configuration gives a large surface area in a smallvolume.
Good mechanical layout: a good shape for pressureoperation.
Uses well-established fabrication techniques.
Can be constructed from a wide range of materials.
Easily cleaned.
Well established design procedures.
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Types of Shell and Tube HeatExchangers
Fixed tube designSimplest and cheapest type.
Tube bundle cannot be removed for cleaning.
No provision for differential expansion of shell and tubes.
Use of this type limited to temperature difference upto800C.
Floating head design
More versatile than fixed head exchangers.
Suitable for higher temperature differentials.
Bundles can be removed and cleaned (fouling liquids)
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Design of Shell and Tube HeatExchangers
Kern method:Does not take into account bypass and leakage streams.Simple to apply and accurate enough for preliminary design
calculations.Restricted to a fixed baffle cut (25%).
Bell-Delaware methodMost widely used.Takes into account:
Leakage through the gaps between tubes and baffles and the baffles andshell.
Bypassing of flow around the gap between tube bundle and shell.
Stream Analysis method (by Tinker)More rigorous and generic.Best suited for computer calculations; basis for most commercial
computer codes.
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Construction DetailsTubeDimensions
Tube diameters in the range 5/8 inch (16 mm) to 2inch (50 mm).
Smaller diameters (5/8 to 1 inch) preferred since
this gives compact and cheap heat exchangers. Larger tubes for heavily fouling fluids.
Steel tubesBS 3606; Other tubesBS 3274.
Preferred tube lengths are 6 ft, 8 ft, 12 ft, 16 ft, 20
ft and 24 ft; optimum tube length to shell diameterratio ~ 510.
in (19 mm) is a good starting trial tube diameter.
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Construction DetailsTubeArrangements
Tubes usually arranged in equilateral triangular, square or rotatedsquare patterns.
Tube pitch, Pt, is 1.25 times OD.
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Construction Details - Shells
Shell should be a close fit to the tube bundle to reduce bypassing.
Shell-bundle clearance will depend on type of heat exchanger.
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Construction Details - Shell-Bundle Clearance
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Construction DetailsTubeCount Bundle diameter depends not only on number of tubes but also number of
tube passes.
Ntis the number of tubes
Dbis the bundle diameter (mm)
D0is tube outside diameter (mm)
n1and K1 are constants
1/1
1
0
n
t
b
K
NdD
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Construction Details - Baffles
Bafflesare used:To direct the fluid stream across the tubes
To increase the fluid velocity
To improve the rate of transfer
Most commonly used baffle is the single segmental baffle.
Optimal baffle cut ~ 20-25%
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Basic Design Procedure
General equation for heat transfer is:
where Q is the rate of heat transfer (duty),
U is the overall heat transfer coefficient,
A is the area for heat transfer
Tmis the mean temperature difference
We are not doing a mechanical design, only athermal design.
mTUAQ
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Overall Heat Transfer Coefficient
Overall coefficient given by:
h0 (hi) is outside (inside) film coefficient
hod(hid) is outside (inside) dirt coefficient
kwis the tube wall conductivitydo(di) is outside (inside) tube diameters
iiidiw
i
od hd
d
hd
d
k
d
dd
hhU
11
2
ln111 00
00
00
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Individual Film Coefficients
Magnitude of individual coefficients will depend on:Nature of transfer processes (conduction, convection,
radiation, etc.)
Physical properties of fluids
Fluid flow rates
Physical layout of heat transfer surface
Physical layout cannot be determined until area is
known; hence design is a trial-and-error procedure.
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Typical Overall Coefficients
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Typical Overall Coefficients
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Fouling Factors (Dirt Coeffs)
Difficult to predict and usually based on past experience
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Mean Temperature Difference(Temperature Driving Force)
To determine A, Tmmust be estimated
True counter-current flowlogarithmic temperature difference(LMTD)
mTUAQ
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LMTD
LMTD is given by:
where T1is the hot fluid temperature, inlet
T2is the hot fluid temperature, outlet
t1is the cold fluid temperature, inlett2is the cold fluid temperature, outlet
12
21
1221
ln
)()(
tTtT
tTtTTlm
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Counter-current FlowTemperature Proflies
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1:2 Heat ExchangerTemperature Profiles
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True Temperature Difference
Obtained from LMTD using a correction factor:
Tmis the true temperature difference
Ftis the correction factor
Ftis related to two dimensionless ratios:
lmtm
TFT
)(
)(
12
21
tt
TTR
)(
)(
11
12
tT
ttS
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Temp Correction Factor Ft
Temperature correction factor, one shell pass, two or more even tube passes
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Fluid Allocation: Shell or Tubes?
Corrosion
Fouling
Fluid temperatures
Operating pressures Pressure drop
Viscosity
Stream flow rates
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Shell and Tube Fluid Velocities
High velocities give high heat-transfer coefficientsbut also high pressure drop.
Velocity must be high enough to prevent settling of
solids, but not so high as to cause erosion. High velocities will reduce fouling
For liquids, the velocities should be as follows:Tube side: Process liquid 1-2m/s
Maximum 4m/s if required to reduce fouling
Water 1.52.5 m/s
Shell side: 0.31 m/s
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Pressure Drop
As the process fluids move through the heat exchanger there isassociated pressure drop.
For liquids: viscosity < 1mNs/m235kN/m2
Viscosity 110 mNs/m250-70kN/m2
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Tube-side Heat TransferCoefficient For turbulent flow inside conduits of uniform cross-section, Sieder-Tateequation is applicable:
C=0.021 for gases
=0.023 for low viscosity liquids
=0.027 for viscous liquids
= fluid viscosity at bulk fluid temperature
w=fluid viscosity at the wall
14.0
33.08.0PrRe
w
CNu
f
ei
kdh
Nu
etduRef
p
k
C Pr
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Tube-side Heat TransferCoefficient
Butterworth equation:
For laminar flow (Re
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Heat Transfer Factor, jh
j factor similar to friction factor used for pressuredrop:
This equation is valid for both laminar and turbulentflows.
14.0
33.0PrRe
w
h
f
ii jk
dh
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Tube Side Heat Transfer Factor
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Heat Transfer Coefficients forWater
Many equations for hihave developed specificallyfor water. One such equation is:
where hiis the inside coefficient (W/m2 0C)
t is the water temperature (0C)utis water velocity (m/s)
dt is tube inside diameter (mm)
2.0
8.0
)02.035.1(4200
i
ti
duth
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Tube-side Pressure Drop
where P is tube-side pressure drop (N/m2)
Npis number of tube-side passes
utis tube-side velocity (m/s)
Lis the length of one tube
mis 0.25 for laminar and 0.14 for turbulentjfis dimensionless friction factor for heat
exchanger tubes
25.28
2
t
m
wi
fpt
u
d
LjNP
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Tube Side Friction Factor
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Shell-side Heat Transfer andPressure Drop
Kerns method
Bells method
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Procedure for Kerns Method
Calculate area for cross-flow Asfor the hypothetical row oftubes in the shell equator.
ptis the tube pitchd0is the tube outside diameter
Dsis the shell inside diameter
lBis the baffle spacing, m.
Calculate shell-side mass velocity Gsand linear velocity, u
s.
where Wsis the fluid mass flow rate in the shell in kg/s
t
bsts
p
DdpA
)(0
s
ss
A
WG
ss
Gu
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Procedure for Kerns Method
Calculate the shell side equivalent diameter (hydraulic diameter). For a square pitch arrangement:
For a triangular pitch arrangement
0
2
0
2
44
d
dp
d
t
e
2
42
187.0
24
0
2
0
d
dp
p
d
tt
e
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Shell-side Reynolds Number
The shell-side Reynolds number is given by:
The coefficient hsis given by:
wherejhis given by the following chart
eses dudG Re
14.0
3/1PrRe
w
h
f
es jk
dhNu
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Shell Side Heat Transfer Factor
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Shell-side Pressure Drop
The shell-side pressure drop is given by:
where jfis the friction factor given by following chart.
14.02
28
w
s
Be
s
fs
uL
d
DjP
Shell Side Friction Factor
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Shell Side Friction Factor
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)(
)(
12
21
tt
TTR
)(
)(
11
12
tT
ttS
(Figure 8 in notes)
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mTUAQ
1/1
1
0
n
tb
K
NdD
(Figure 4 in notes)
(Figure 2)
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2.0
8.0)02.035.1(4200
i
ti
d
uth
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(Figure 9 in notes)
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t
bst
s p
Ddp
A
)( 0
2
42
187.0
24
0
2
0
d
dp
p
d
tt
e
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iiidiw
i
od hd
d
hd
d
k
d
dd
hhU
11
2
ln111 00
0
0
00
(Figure 10 in notes)
(Table 3 in notes)
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25.28
2
t
m
wi
fpt
u
d
LjNP
14.02
28
w
s
Be
sfs
uL
d
DjP
(Figure 12 in notes)
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Bells Method
In Bells method, the heat transfer coefficient and pressure drop areestimated from correlations for flow over ideal tube banks.
The effects of leakage, by-passing, and flow in the window zone areallowed for by applying correction factors.
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Bells Method Shell-side HeatTransfer Coefficient
where hocis heat transfer coeff for cross flow
over ideal tube banks
Fnis correction factor to allow for no.of vertical tube rows
Fwis window effect correction factorFbis bypass stream correction factor
FLis leakage correction factor
Lbwnocs FFFFhh
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Bells Method Ideal Cross Flow
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Bell s Method Ideal Cross FlowCoefficient
The Re for cross-flow through the tube bank is given by:
Gsis the mass flow rate per unit area
d0is tube OD
Heat transfer coefficient is given by:
00Re dudG
ss
14.0
3/10 PrRe
w
h
f
oc jk
dh
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Bells Method Tube RowCorrection Factor
For Re>2100, Fnis obtained as a function of Ncv(no. of tubes betweenbaffle tips) from the chart below:
For Re 100
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Bells Method WindowCorrection Factor
Fw, the window correction factor is obtained from thefollowing chart:
whereRwis the ratio of bundle cross-sectional area in thewindow zone to the tube bundle cross-sectional area(obtained from simple formulae).
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Bells Method BypassCorrection Factor
Clearance area[Ab]between the bundle and the shell
For the case of no sealing strips, Fbas a function of A
b/A
scan be
obtained from the following chart
)( bsBb DDA
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Bells Method BypassCorrection Factor
For sealing strips, for Ns
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Bells Method LeakageCorrection Factor Tube-baffle clearance area Atbis given by:
Shell-baffle clearance area Asb
is given by:
where Csis baffle to shell clearance and bis the angle subtended by baffle chord
AL=Atb+Asb
where Lis a factor obtained from following chart
)(2
8.0 0wttb NN
dA
)2(2
bss
sb
DCA
L
sbtbLL
A
AAF )2(1
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Coefficient for FL, Heat Transfer
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Shell-side Pressure Drop
Involves three components:
Pressure drop in cross-flow zone
Pressure drop in window zone
Pressure drop in end zone
C
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Pressure Drop in Cross FlowZone
where Pipressure drop calculated for an equivalent ideal tubebank
Fb is bypass correction factor
FL is leakage correction factor
wherejfis given by the following chart
Ncvis number of tube rows crossed
usis shell-side velocity
''
Lbic FFPP
14.02
28
w
scvfi
uNjP
F i ti F t f C Fl
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Friction Factor for Cross FlowBanks
Bells Method Bypass
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Bell s Method BypassCorrection Factor for Pressure
Drop
is 5.0 for laminar flow, Re100
Abis the clearance area between the bundle and shell
Nsis the number of sealing strips encountered by bypass
streamNcvis the number of tube rows encountered in the cross- flow section
3/1
' 21exp
cv
s
s
bb
N
N
A
AF
B ll M th d L k F t
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Bells Method Leakage Factorfor Pressure Drop
where Atbis the tube to baffle clearance area
Asbis the shell to baffle clearance area
ALis total leakage area = Atb+Asb
L is factor obtained from following chart
L
sbtbLL
A
AAF
)2(1
''
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Coefficient for FL
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Pressure Drop in Window Zones
whereusis the geometric mean velocity
uwis the velocity in the window zone
Wsis the shell-side fluid mass flow
Nwvis number of restrictions for cross-flow in windowzone, approximately equal to the number of tube rows.
2)6.00.2(
2
' z
wvLw
u
NFP
swz
uuu
w
swA
Wu
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Pressure Drop in End Zones
Ncv
is the number of tube rows encountered in the cross-flow section
Nwvis number of restrictions for cross-flow in window zone,approximately equal to the number of tube rows.
')(b
cv
cvwvie F
N
NNPP
B ll M th d T t l Sh ll id
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Bells Method Total Shell-sidePressure Drop
zoneswindowN
zonescrossflowNzonesendP
b
bs
)1(2
wbcbes PNPNPP )1(2
Eff t f F li
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Effect of Fouling
Above calculation assumes clean tubes
Effect of fouling on pressure drop is given by table above
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Condensers
Construction of a condenser is similar to other shell and tubeheat exchangers, but with a wider baffle spacing
Four condenser configurations:Horizontal, with condensation in the shell
Horizontal, with condensation in the tubes
Vertical, with condensation in the shell
Vertical, with condensation in the tubes
Horizontal shell-side and vertical tube-side are the mostcommonly used types of condenser.
sB Dl
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Heat Transfer Mechanisms
Filmwise condensationNormal mechanism for heat transfer in commercial condensers
Dropwise condensationWill give higher heat transfer coefficients but is unpredictableNot yet considered a practical proposition for the design of
condensers In the Nusselt model of condensation laminar flow is
assumed in the film, and heat transfer is assumed to takeplace entirely by conduction through the film.
Nusselt model strictly applied only at low liquid and vaporrates when the film is undisturbed.
At higher rates, turbulence is induced in the liquid filmincreasing the rate of heat transfer over that predicted byNusselt model.
Condensation Outside Horizontal
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Condensation Outside HorizontalTubes
where (hc)1is the mean condensation film coefficient, for a single tubekLis the condensate thermal conductivity
Lis the condensate density
vis the vapour density
Lis the condensate viscosity
g is the gravitational acceleration
is the tube loading, the condensate flow per unit length of tube.
If there are Nrtubes in a vertical row and the condensate is assumed to flowsmoothly from row to row, and if the flow is laminar, the top tube film coefficientis given by:
3/1
1)(95.0)(
L
vLLLc
gkh
4/1
1)()( rcNc Nhh r
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Condensation Outside Horizontal
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Condensation Outside HorizontalTubes
In practice, condensate will not flow smoothly from tube totube.
Kerns estimate of mean coefficient for a tube bundle isgiven by:
Lis the tube length
Wcis the total condensate flow
Ntis the total number of tubes in the bundle
Nris the average number of tubes in a vertical tube row For low-viscosity condensates the correction for the number
of tube rows is generally ignored.
6/1
3/1
)(95.0)(
rhL
vLLLbc Ng
kh
t
ch
LNW
Condensation Inside and Outside
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Condensation Inside and OutsideVertical Tubes
For condensation inside and outside vertical tubes the Nusseltmodelgives:
where (hc)vis the mean condensation coefficient
vis the vertical tube loading, condensate per unit tube perimeter
Above equation applicable for Re2000, turbulent flow; situation analyzed by Colburn andresults in following chart.
3/1
)(926.0)(
vL
vLLLvc
gkh
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Colburns Results
Boyko-Kruzhilin Correlation
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Boyko-Kruzhilin Correlation A correlation for shear-controlled condensation in tubes; simple to use.
The correlation gives mean coefficient between two points at which vapor quality, x, (mass fraction ofvapour) is known.
1,2 refer to inlet and outlet conditions respectively
In a condenser, the inlet stream will normally be saturated vapour and vapour will be totallycondensed. For these conditions:
For design of condensers with condensation inside the tubes and downward vapor flow, coefficientshould be evaluated using Colburns method and Boyko-Kruzhilin correlation and the highervalueselected.
xJwhereJJ
hhv
vLiBKc
1
2)(
2/12
2/11
43.08.0
PrRe021.0
i
L
id
k
h
2
1
)( v
L
iBKc hh
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Flooding in Vertical Tubes
When the vapor flows up the tube, tubes should notflood.
Flooding should not occur if the following condition is
satisfied:
where uvand uLare velocities of vapor and liquid anddiis in metres.
The critical condition will occur at the bottom of thetube, so vapor and liquid velocities should beevaluated at this point.
4/14/12/14/12/1 )(6.0 vLiLLvv gduu
Condensation Inside Horizontal
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Condensation Inside HorizontalTubes When condensation occurs, the heat transfer coefficient at any point along the
tube will depend on the flow pattern at that point.
No general satisfactory method exists that will give accurate predictions over awide flow range.
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Two Flow Models Two flow models:
Stratified flow
Limiting condition at low condensate and vapor rates
Annular flow
Limiting condition at high vapor and low condensate rates
For stratified flow, the condensate film coefficient can be estimated as:
For annular flow, the Boyko-Kruzhilin equation can be used
For condenser design, both annular and stratified flow should be considered andthe higher value of mean coefficient should be selected.
3/1
)(76.0)(
hL
vLLLsc
gkh
Condensation of steam
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For air-free steam a coefficient of 8000 W/m2-0C should be used.
Mean Temperature DifferenceA pure, saturated, vapor will condense at a constant temperature, at
constant pressure.
For an isothermal process such as this, the LMTD is given by:
where Tsatis saturation temperature of vapor
t1(t2) is the inlet (outlet) coolant temperature
No correction factor for multiple passes is needed.
2
1
12
ln
)(
tT
tT
tt
lm
sat
sat
T