1.
Heat Exchanger Design
Anand 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]
2.
Heat Exchanger Classification
•Recuperative:
–Cold and hot fluid flow through the unit without mixing
with each other. The transfer of heat occurs through the
metal wall.
•Regenerative:
–Same heating surface is alternately exposed to hot and
cold fluid. Heat from hot fluid is stored by packings or
solids; 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 occur
simultaneously.
3.
Heat Exchanger Standards and
Codes
•British Standard BS-3274
•TEMA standards are universally used.
•TEMA standards cover following classes of
exchangers:
–Class R – designates severe requirements of petroleum
and other related processing applications
–Class C – moderate requirements of commercial and
general process applications
–Class B – specifies design and fabrication for chemical
process service.
4.
Shell and Tube Heat Exchanger
•Most commonly used type of heat transfer
equipment in the chemical and allied industries.
•Advantages:
–The configuration gives a large surface area in a small
volume.
–Good mechanical layout: a good shape for pressure
operation.
–Uses well-established fabrication techniques.
–Can be constructed from a wide range of materials.
–Easily cleaned.
–Well established design procedures.
5.
Types of Shell and Tube Heat
Exchangers
•Fixed tube design
–Simplest 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 upto
800
C.
•Floating head design
–More versatile than fixed head exchangers.
–Suitable for higher temperature differentials.
–Bundles can be removed and cleaned (fouling liquids)
6.
Design of Shell and Tube Heat
Exchangers
• 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 method
– Most widely used.
– Takes into account:
• Leakage through the gaps between tubes and baffles and the baffles and
shell.
• 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.
7.
Construction Details – Tube
Dimensions
•Tube diameters in the range 5/8 inch (16 mm) to 2
inch (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 tubes – BS 3606; Other tubes – BS 3274.
•Preferred tube lengths are 6 ft, 8 ft, 12 ft, 16 ft, 20
ft and 24 ft; optimum tube length to shell diameter
ratio ~ 5 – 10.
•¾ in (19 mm) is a good starting trial tube diameter.
8.
Construction Details – Tube
Arrangements
• Tubes usually arranged in equilateral triangular, square or rotated
square patterns.
• Tube pitch, Pt, is 1.25 times OD.
9.
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.
10.
Construction Details - Shell-
Bundle Clearance
11.
Construction Details – Tube
Count
• Bundle diameter depends not only on number of tubes but also number of
tube passes.
• Nt is the number of tubes
• Db is the bundle diameter (mm)
• D0 is tube outside diameter (mm)
• n1 and K1are constants
1/1
1
0
n
t
b
K
N
dD
=
12.
Construction Details - Baffles
• Baffles are 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%
13.
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
ΔTm is the mean temperature difference
•We are not doing a mechanical design, only a
thermal design.
mTUAQ ∆=
14.
Overall Heat Transfer Coefficient
• Overall coefficient given by:
h0(hi) is outside (inside) film coefficient
hod (hid) is outside (inside) dirt coefficient
kw is the tube wall conductivity
do (di) is outside (inside) tube diameters
iiidiw
i
od hd
d
hd
d
k
d
d
d
hhU
11
2
ln
111 00
0
0
00
++
++=
15.
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.
18.
Fouling Factors (Dirt Coeffs)
• Difficult to predict and usually based on past experience
19.
Mean Temperature Difference
(Temperature Driving Force)
• To determine A, ΔTm must be estimated
• True counter-current flow – “logarithmic temperature difference”
(LMTD)
mTUAQ ∆=
20.
LMTD
• LMTD is given by:
where T1 is the hot fluid temperature, inlet
T2 is the hot fluid temperature, outlet
t1 is the cold fluid temperature, inlet
t2 is the cold fluid temperature, outlet
−
−
−−−
=∆
12
21
1221
ln
)()(
tT
tT
tTtT
Tlm
23.
True Temperature Difference
• Obtained from LMTD using a correction factor:
ΔTm is the true temperature difference
Ft is the correction factor
• Ft is related to two dimensionless ratios:
lmtm TFT ∆=∆
)(
)(
12
21
tt
TT
R
−
−
=
)(
)(
11
12
tT
tt
S
−
−
=
24.
Temp Correction Factor Ft
Temperature correction factor, one shell pass, two or more even tube passes
26.
Shell and Tube Fluid Velocities
•High velocities give high heat-transfer coefficients
but 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 1.5 – 2.5 m/s
–Shell side: 0.3 – 1 m/s
27.
Pressure Drop
• As the process fluids move through the heat exchanger there is
associated pressure drop.
• For liquids: viscosity < 1mNs/m2
35kN/m2
Viscosity 1 – 10 mNs/m2
50-70kN/m2
28.
Tube-side Heat Transfer
Coefficient• For turbulent flow inside conduits of uniform cross-section, Sieder-Tate
equation is applicable:
C=0.021 for gases
=0.023 for low viscosity liquids
=0.027 for viscous liquids
μ= fluid viscosity at bulk fluid temperature
μw=fluid viscosity at the wall
14.0
33.08.0
PrRe
=
w
CNu
µ
µ
f
ei
k
dh
Nu = µ
ρ et du
=Re f
p
k
C µ
=Pr
29.
Tube-side Heat Transfer
Coefficient
• Butterworth equation:
• For laminar flow (Re<2000):
• If Nu given by above equation is less than 3.5, it should be taken as 3.5
505.0205.0
PrRe −−
= ESt
pt
i
Cu
hNuSt
ρ==
PrRe
( )2
Pr)(ln0225.0exp0225.0 −=E
14.033.0
33.0
Pr)(Re86.1
=
w
e
L
d
Nu
µ
µ
30.
Heat Transfer Factor, jh
•“j” factor similar to friction factor used for pressure
drop:
•This equation is valid for both laminar and turbulent
flows.
14.0
33.0
PrRe
=
w
h
f
ii
j
k
dh
µ
µ
32.
Heat Transfer Coefficients for
Water
•Many equations for hi have developed specifically
for water. One such equation is:
where hi is the inside coefficient (W/m20
C)
t is the water temperature (0
C)
ut is water velocity (m/s)
dt is tube inside diameter (mm)
2.0
8.0
)02.035.1(4200
i
t
i
d
ut
h
+
=
33.
Tube-side Pressure Drop
where ΔP is tube-side pressure drop (N/m2
)
Np is number of tube-side passes
ut is tube-side velocity (m/s)
L is the length of one tube
m is 0.25 for laminar and 0.14 for turbulent
jf is dimensionless friction factor for heat
exchanger tubes
2
5.28
2
t
m
wi
fpt
u
d
L
jNP
ρ
µ
µ
+
=∆
−
35.
Shell-side Heat Transfer and
Pressure Drop
•Kern’s method
•Bell’s method
36.
Procedure for Kern’s Method
• Calculate area for cross-flow As for the hypothetical row of
tubes in the shell equator.
pt is the tube pitch
d0 is the tube outside diameter
Ds is the shell inside diameter
lB is the baffle spacing, m.
• Calculate shell-side mass velocity Gs and linear velocity, us.
where Ws is the fluid mass flow rate in the shell in kg/s
t
bst
s
p
Ddp
A
)( 0−
=
s
s
s
A
W
G =
ρ
s
s
G
u =
37.
Procedure for Kern’s Method
• Calculate the shell side equivalent diameter (hydraulic diameter).
– For a square pitch arrangement:
– For a triangular pitch arrangement
0
2
0
2
4
4
d
dp
d
t
e
π
π
−
=
2
42
1
87.0
2
4
0
2
0
d
d
p
p
d
t
t
e
π
π
−×
=
38.
Shell-side Reynolds Number
• The shell-side Reynolds number is given by:
• The coefficient hs is given by:
where jh is given by the following chart
µ
ρ
µ
eses dudG
==Re
14.0
3/1
PrRe
==
w
h
f
es
j
k
dh
Nu
µ
µ
40.
Shell-side Pressure Drop
• The shell-side pressure drop is given by:
where jf is the friction factor given by following chart.
14.02
2
8
−
=∆
w
s
Be
s
fs
uL
d
D
jP
µ
µρ
46.
t
bst
s
p
Ddp
A
)( 0−
=
2
42
1
87.0
2
4
0
2
0
d
d
p
p
d
t
t
e
π
π
−×
=
47.
iiidiw
i
od hd
d
hd
d
k
d
d
d
hhU
11
2
ln
111 00
0
0
00
++
++=
(Figure 10 in notes)
(Table 3 in notes)
48.
2
5.28
2
t
m
wi
fpt
u
d
L
jNP
ρ
µ
µ
+
=∆
−
14.0
2
2
8
−
=∆
w
s
Be
s
fs
uL
d
D
jP
µ
µρ
(Figure 12 in notes)
49.
Bell’s Method
• In Bell’s method, the heat transfer coefficient and pressure drop are
estimated from correlations for flow over ideal tube banks.
• The effects of leakage, by-passing, and flow in the window zone are
allowed for by applying correction factors.
50.
Bell’s Method – Shell-side Heat
Transfer Coefficient
where hoc is heat transfer coeff for cross flow
over ideal tube banks
Fn is correction factor to allow for no.
of vertical tube rows
Fw is window effect correction factor
Fb is bypass stream correction factor
FL is leakage correction factor
Lbwnocs FFFFhh =
51.
Bell’s Method – Ideal Cross Flow
Coefficient
• The Re for cross-flow through the tube bank is given by:
Gs is the mass flow rate per unit area
d0 is tube OD
• Heat transfer coefficient is given by:
µ
ρ
µ
00
Re
dudG ss
==
14.0
3/10
PrRe
=
w
h
f
oc
j
k
dh
µ
µ
52.
Bell’s Method – Tube Row
Correction Factor
• For Re>2100, Fn is obtained as a function of Ncv (no. of tubes between
baffle tips) from the chart below:
• For Re 100<Re<2100, Fn=1.0
• For Re<100,
18.0'
)( −
∝ cn NF
53.
Bell’s Method – Window
Correction Factor
• Fw, the window correction factor is obtained from the
following chart:
where Rw is the ratio of bundle cross-sectional area in the
window zone to the tube bundle cross-sectional area
(obtained from simple formulae).
54.
Bell’s Method – Bypass
Correction Factor
• Clearance area[Ab] between the bundle and the shell
• For the case of no sealing strips, Fb as a function of Ab/As can be
obtained from the following chart
)( bsBb DDA −=
55.
Bell’s Method – Bypass
Correction Factor
•For sealing strips, for Ns<Ncv/2 (Ns is the number of
baffle strips)
where α=1.5 for Re<100 and α=1.35 for Re>100.
−−=
3/1
2
1exp
cv
s
s
b
b
N
N
A
A
F α
56.
Bell’s Method – Leakage
Correction Factor
• Tube-baffle clearance area Atb is given by:
• Shell-baffle clearance area Asb is given by:
where Cs is baffle to shell clearance and θb is the angle subtended by baffle chord
• AL=Atb+Asb
where βL is a factor obtained from following chart
)(
2
8.0 0
wttb NN
d
A −=
π
)2(
2
b
ss
sb
DC
A θπ −=
+
−=
L
sbtb
LL
A
AA
F
)2(
1 β
58.
Shell-side Pressure Drop
• Involves three components:
– Pressure drop in cross-flow zone
– Pressure drop in window zone
– Pressure drop in end zone
59.
Pressure Drop in Cross Flow
Zone
where ΔPi pressure drop calculated for an equivalent ideal tube
bank
Fb’ is bypass correction factor
FL’ is leakage correction factor
where jf is given by the following chart
Ncv is number of tube rows crossed
us is shell-side velocity
''
Lbic FFPP ∆=∆
14.02
2
8
−
=∆
w
s
cvfi
u
NjP
µ
µρ
61.
Bell’s Method – Bypass
Correction Factor for Pressure
Drop
α is 5.0 for laminar flow, Re<100
4.0 for transitional and turbulent flow, Re>100
Ab is the clearance area between the bundle and shell
Ns is the number of sealing strips encountered by bypass
stream
Ncv is the number of tube rows encountered in the cross-
flow section
−−=
3/1
' 2
1exp
cv
s
s
b
b
N
N
A
A
F α
62.
Bell’s Method – Leakage Factor
for Pressure Drop
where Atb is the tube to baffle clearance area
Asb is the shell to baffle clearance area
AL is total leakage area = Atb+Asb
βL’ is factor obtained from following chart
+
−=
L
sbtb
LL
A
AA
F
)2(
1 ''
β
64.
Pressure Drop in Window Zones
where us is the geometric mean velocity
uw is the velocity in the window zone
Ws is the shell-side fluid mass flow
Nwv is number of restrictions for cross-flow in window
zone, approximately equal to the number of tube rows.
2
)6.00.2(
2
' z
wvLw
u
NFP
ρ
+=∆
swz uuu =
ρw
s
w
A
W
u =
65.
Pressure Drop in End Zones
• Ncv is the number of tube rows encountered in the cross-flow section
• Nwv is number of restrictions for cross-flow in window zone,
approximately equal to the number of tube rows.
')(
b
cv
cvwv
ie F
N
NN
PP
+
∆=∆
66.
Bell’s Method – Total Shell-side
Pressure Drop
zoneswindowN
zonescrossflowNzonesendP
b
bs
+
−+=∆ )1(2
wbcbes PNPNPP ∆+∆−+∆=∆ )1(2
67.
Effect of Fouling
• Above calculation assumes clean tubes
• Effect of fouling on pressure drop is given by table above
68.
Condensers
• Construction of a condenser is similar to other shell and tube
heat 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 most
commonly used types of condenser.
sB Dl =
69.
Heat Transfer Mechanisms
• Filmwise condensation
– Normal mechanism for heat transfer in commercial condensers
• Dropwise condensation
– Will give higher heat transfer coefficients but is unpredictable
– Not 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 take
place entirely by conduction through the film.
• Nusselt model strictly applied only at low liquid and vapor
rates when the film is undisturbed.
• At higher rates, turbulence is induced in the liquid film
increasing the rate of heat transfer over that predicted by
Nusselt model.
70.
Condensation Outside Horizontal
Tubes
where (hc)1 is the mean condensation film coefficient, for a single tube
kL is the condensate thermal conductivity
ρL is the condensate density
ρv is the vapour density
μL is the condensate viscosity
g is the gravitational acceleration
Γ is the tube loading, the condensate flow per unit length of tube.
• If there are Nr tubes in a vertical row and the condensate is assumed to flow
smoothly from row to row, and if the flow is laminar, the top tube film coefficient
is given by:
3/1
1
)(
95.0)(
Γ
−
=
L
vLL
Lc
g
kh
µ
ρρρ
4/1
1)()( −
= rcNc Nhh r
72.
Condensation Outside Horizontal
Tubes
• In practice, condensate will not flow smoothly from tube to
tube.
• Kern’s estimate of mean coefficient for a tube bundle is
given by:
L is the tube length
Wc is the total condensate flow
Nt is the total number of tubes in the bundle
Nr is 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)( −
Γ
−
= r
hL
vLL
Lbc N
g
kh
µ
ρρρ
t
c
h
LN
W
=Γ
73.
Condensation Inside and Outside
Vertical Tubes
• For condensation inside and outside vertical tubes the Nusselt
model gives:
where (hc)v is the mean condensation coefficient
Γv is the vertical tube loading, condensate per unit tube perimeter
• Above equation applicable for Re<30
• For higher Re the above equation gives a conservative (safe)
estimate.
• For Re>2000, turbulent flow; situation analyzed by Colburn and
results in following chart.
3/1
)(
926.0)(
Γ
−
=
vL
vLL
Lvc
g
kh
µ
ρρρ
75.
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
of vapour) 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 totally
condensed. For these conditions:
• For design of condensers with condensation inside the tubes and downward vapor flow, coefficient
should be evaluated using Colburn’s method and Boyko-Kruzhilin correlation and the higher value
selected.
xJwhere
JJ
hh
v
vL
iBKc
−
+=
+
′=
ρ
ρρ
1
2
)(
2/1
2
2/1
1
43.08.0
PrRe021.0
=′
i
L
i
d
k
h
+
′=
2
1
)( v
L
iBKc hh
ρ
ρ
76.
Flooding in Vertical Tubes
•When the vapor flows up the tube, tubes should not
flood.
•Flooding should not occur if the following condition is
satisfied:
where uv and uL are velocities of vapor and liquid and
di is in metres.
•The critical condition will occur at the bottom of the
tube, so vapor and liquid velocities should be
evaluated at this point.
[ ] [ ] 4/14/12/14/12/1
)(6.0 vLiLLvv gduu ρρρρ −<+
77.
Condensation Inside Horizontal
Tubes
• 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 a
wide flow range.
78.
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 and
the higher value of mean coefficient should be selected.
3/1
)(
76.0)(
Γ
−
=
hL
vLL
Lsc
g
kh
µ
ρρρ
79.
• Condensation of steam
– For air-free steam a coefficient of 8000 W/m2
-0
C should be used.
• Mean Temperature Difference
– A 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 Tsat is 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
A particular slide catching your eye?
Clipping is a handy way to collect important slides you want to go back to later.
Be the first to comment