Production of n-propanol Chapter V
5-1
CHAPTER V
HEAT EXCHANGER DESIGN
5.0 INTRODUCTION
The process of heat exchanger between two fluids that are at different temperature
and separated by a solid wall occurs in many engineering applications. The device
used to implement this exchange is termed a heat exchanger, and a specific
applications may be found in space heating and air-conditioning, power production,
waste heat recovery and chemical processing.
A heat exchanger is a device used to passively transfer heat from one
material to another. These materials may be liquid or gaseous, depending on the
situation in which the heat exchanger is being used. There are many models and
types of heat exchangers, but they essentially work based on the laws of
thermodynamics. One of those laws states that when an object is heated, the heat
energy contained within that object will diffuse outward to the surrounding
environment, until the heat energy in the object and in the environment have reach
equilibrium.
(Source: Fundamentals of Heat and Mass Transfer , 6th Edition)
5.1 TYPE OF HEAT EXCHANGER
Heat exchangers may be classified according to the following main criteria:
1. Recuperators and regenerators
2. Transfer process: direct contact and indirect contact
3. Geometry of construction: tubes, plates and extended surface
4. Heat transfer mechanisms: single phase and two phase
5. Flow arrangements: parallel, counter and cross flow
Production of n-propanol Chapter V
5-2
The simplest heat exchanger is one for which the hot and cold fluids move in
the same or opposite directions in a concentric tube heat exchanger (or double pipe)
construction. In the parallel-flow arrangement, the hot and cold fluids enter at the
same end, flow in the same direction, and leave at the same end. In the counter flow
arrangement, the fluids enter at opposite ends, flow in opposite directions and leave
at opposite ends. Alternatively, the fluids may move in cross flow (perpendicular
each other), by the tubular heat exchanger. The principal types of heat exchanger
used in the chemical process and allied industries are as below:
1. Double-pipe exchanger
2. Shell and tube exchangers
3. Plate and frame exchangers
4. Double-pipe exchanger
5. Shell and tube exchangers
6. Plate and frame exchangers
7. Plate-fin exchangers
8. Spiral Heat exchangers
9. Air cooled: cooler and condensers
10. Direct contact: cooling and quenching
The common configuration use is the shell-and-tube heat exchanger.
Specific forms differ according to the number of shell-and-tube passes, and the
simplest form. Baffles are usually installed to increase the convection coefficient of
the shell-side fluid by inducing turbulence and cross-flow velocity component. The
shell and tube exchanger is by far the most commonly used type of heat-transfer
equipment used in the chemical and allied industries.
(Source: Chemical Engineering Design 5thEdition)
5.2 TYPE OF SHELL AND TUBE EXCHANGER
The principal types of shell and tube exchanger are:
1. Fixed tube plate
2. U-Tube
3. Internal floating head without clamp ring (pull through design)
4. Internal floating head with clamp ring (split flange design)
5. External floating head, packed gland
6. Kettle reboiler with U-tube bundle
Production of n-propanol Chapter V
5-3
The characteristics of shell and tube exchanger types are listed in Table 5.1.
For this design, the shell and tube type of internal floating head (split-ring floating
head) has been chosen according to the advantages compared to the others.
Table 5.1: Comparison of types of shell and tube exchanger
Advantages Disadvantages
Fixed Tube plate
Simplest
Cheapest
Tube bundle cannot be
removed for cleaning.
No provision for differential
expansion of shell and tubes.
Limited to temperature
differences up to 80°C.
Limited to low shell pressure
up to 8 bar.
U-Tube (U-Bundle)
Requires only one tube
Limited in use to relatively
clean fluids as the tubes and
bundle are difficult to clean.
Too difficult to replace a tube.
Internal Floating Head
(Split-ring floating head)
Suitable for high
temperature differentials
The tubes can be rodded
end to end and the bundle
easily to remove and
repairs.
Easier to clean and can be
used for fouling liquids.
Separate the shell and
tube side fluid at the
floating head end.
Increase efficiency.
Internal Floating Head
(pull through design)
Same as Internal Floating Head (split-ring floating head)
Clearance between the
outermost tubes in the bundle
and the shell must be made
greater than the fixed and U-
Production of n-propanol Chapter V
5-4
tube designs.
External Floating Head
Floating head joint is located
outside the shell, and the
shell sealed with a sliding
gland joint employing stuffing
box and makes a danger of
leaks through the gland.
Limited to about 20 bar.
The shell side is not suitable
for flammable or toxic
materials.
Kettle Reboiler
Same as U-tube Same as U-tube
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
5.3 CHEMICAL ENGINEERING DESIGN
Figure 5.1: Heat exchanger E101
Step to Design Heat Exchanger
1. Define the duty: heat-transfer rate, fluid flow-rates, temperatures.
2. Collect together the fluid physical properties required: density, viscosity,
thermal conductivity.
3. Decide on the type of exchanger to be used.
4. Select a trial value for the overall coefficient, U.
5. Calculate the mean temperature difference.
6. Calculate the area required from equation.
7. Decide the exchanger layout.
8. Calculate the individual coefficients.
Stream 15 Stream 17
Heat Exchanger (E101)
Production of n-propanol Chapter V
5-5
E-1
9. Calculate the overall coefficient and compare with the trial value. If the
calculated value differs significantly from the estimated value, substitute the
calculated for the estimated value and return to step 6.
10. Calculate the exchanger pressure drop; if unsatisfactory return to steps 7 or
4 or 3, in that order of preference.
Heat Load Of Heat Exchanger
Figure 5.2: Heat Exchanger ( E-101 )
Duty for this heat exchanger (E-101) is obtained from the equation 1.1
)( 21 TTmCQ p (5.1)
)70150(246.33600
/20890
s
hkg
KW87.1506
The duty of tube-side is equal to the duty of shell-side. From this value of duty
calculated, the flow rate of the cooling water can be determined. This is done by
using equation 5.2.
Cooling water flow rate, ṁw ))(( 12 ttCp
Q
water (5.2)
))3060(2.4(
87.1506
KW
skg /0.12
The inlet and outlet temperature of the tube have been assumed. The inlet
temperature of water is 30oC and the outlet temperature of water is 60oC due to
commonly used in industry. Table 5.3 shows the physical properties of the
component in the tube and shell side. All the physical properties are taken based on
the mean temperature.
Stream 17
( 70oC)
Stream 15
( 150oC)
Production of n-propanol Chapter V
5-6
Table 5.3: Physical Properties in Shell and Tube Side
Physical Properties Shell Side Tube Side
Temperature, T (°C)
(a) Inlet 150 30
(b) Outlet 70 60
(c) Mean 110 45
Specific Heat, Cp ( kJ/kg°C) 3.246 4.200
Thermal Conductivity, k ( W/m°C) 0.1150 0.6376
Density, (kg/m3) 13.704 995.818
Viscosity, (mNs/m2) 0.0001621 0.5986
Duty, Q (kW) 1506.87 1506.87
Flow rate (kg/s) 5.80 12.0
(Source:Fundamentals of Heat And Mass Transfer Sixth Edition )
To design or to predict the performance of heat exchanger, it is essential to relate
the total heat transfer rate to quantities such as the inlet and outlet fluid temperature,
the overall heat transfer coefficient and the total surface area for heat transfer.
The logarithmic mean temperature different (LMTD) method has been
choose. It is because the fluid inlet temperature is known and the outlet temperature
is specified.
(source: Fundamentals of Heat and Mass Transfer , 6th Edition)
Counter-flow arrangement is selected as the temperature difference is
greater compared to cross flow. For the LMTD involved, the following assumptions
are made:
1. The overall coefficient of heat transfer is constant
2. The rate of flow of each fluid is constant
3. There is no phase change during cooling process
4. There is an equal amount of cooling surface in each pass
(source: Fundamentals of Heat and Mass Transfer , 6th Edition)
Production of n-propanol Chapter V
5-7
5.3.1 Determination of Heat Transfer Area
To find the heat transfer area of heat exchanger, the true temperature different mT
must known first. By relating the total heat transfer rate q to the temperature
different between the hot and cold fluid, expression 5.3 can be produce.
ch TTT (5.3)
Since T varies with the position in the heat exchanger, it is necessary to
work with equation 1.4, where mT is an true temperature difference. From
equation 5.4, heat transfer area of heat exchanger can be obtained.
mTUAq (5.4)
(source: Fundamentals of Heat and Mass Transfer , 6th Edition)
5.3.1.2 Determination of Log Mean Temperature Difference
The form of mT may be determined by applying an energy balance to the
differential elements of length dx and surface area dA in the hot and cold fluids. The
energy balances and the subsequent analysis are subject to the following
assumption:
1. The heat exchanger is insulated from its surrounding, in which case only
heat exchange between the hot and cold fluid.
2. Axial conduction along the tubes is negligible.
3. Potential and kinetic energy changes are negligible.
4. The fluid specific heat is constant.
5. The overall heat transfer coefficient is constant.
The specific heat may change as a result of temperature variations, and the
overall heat transfer coefficient may change because of variations in fluid properties
and flow conditions. However, in many applications such variations are not
significant, and it is reasonable to work with average ofcCp ,
hCp and U for heat
exchanger. Applying all this assumption, equation 5.5 can be obtained.
)(
)(ln
)()(
12
21
1221
tT
tT
tTtTTlm
(5.5)
Production of n-propanol Chapter V
5-8
where;
1T = inlet shell-side fluid temperature, °C
2T = outlet shell-side fluid temperature, °C
1t = inlet tube-side temperature, °C
2t = outlet tube-side temperature, °C
(source: Fundamentals of Heat and Mass Transfer , 6th Edition)
)3070(
)60150(ln
)3070()60150(
lmT
CT o
lm 66.61
5.3.1.3 True Temperature Difference of Heat Exchanger
The usual practice in design of heat exchanger is to estimate the true temperature
different from the log mean temperature difference by applying the correction factor,
tF to allow for the departure from true counter current flow. True temperature
difference is shows in equation 5.6. The correction factor is the function of the shell
and tube fluid temperature, and the number of tube and shell passes. It is normally
correlated as a function of two dimensionless temperature ratio which are equation
5.7 and equation 5.8.
lmtm TFT (5.6)
)(
)(
12
21
tt
TTR
(5.7)
)3060(
)70150(
R
67.2
)(
)(
11
12
tT
ttS
(5.8)
)60150(
)3060(
S
33.0
Production of n-propanol Chapter V
5-9
Based on the value of R and S calculated, the correction factor tF can be
found. In order to find the correction factor, two shell passes and four tube passes
on the heat exchanger have been choose. This is due to, the value of tF cannot be
obtained when one shell pass and two tube passes is used. In addition, an
economic exchanger design cannot normally be achieved if the correction factor tF
falls below about 0.75. in these circumstances, an alternative type of exchanger
should be considered that gives a closer approach to true counter current flow. The
use of two side shell pass and four tube passes will give a closer approach to true
counter current flow.
Hence, based on Figure E.1(Appendix E), with the value of R is 2.67 and
the value of S is 0.33, the correction factor is 0.88. By using equation 5.6, true
temperature difference was calculated.
lmtm TFT
66.6188.0
Co26.54
5.3.1.4 Overall Coefficient
The most essential part of any heat exchanger analysis is determination of the
overall heat transfer coefficient,U . This overall heat transfer coefficient is defined in
terms of the total thermal resistance to heat transfer between two fluids. For this
case, gases is taking as a hot fluid and water as a cold fluid. Based on table E2
(Appendix E),the overall coefficient for this heat exchanger is 160 W/m2 oC
By inserting the value of heat load q , true temperature difference mT , and
overall heat transfer coefficient, U into equation 5.4, the heat transfer area can be
calculated.
mTUAq
mTU
qA
)26.54()/160(
1506870
2 CCmW
W
oo
257.173 m
Production of n-propanol Chapter V
5-10
5.3.2 Tube Exchanger
The most basic and the most common type of heat exchanger construction is the
tube and shell. This type of heat exchanger consists of a set of tubes in a container
called a shell. The fluid flowing inside the tubes is called tube side fluid and the fluid
flowing on the outside of the tubes is the shell side fluid. There are several factor
that must be take into account before allocate the suitable fluid in the shell and tube
side. In this case there is no phase change in both fluid. All the factor is shown in
table 5.4.
(source:www.engineersedge.com/heat_exchanger/tube_shell)
Table 5.4 : General consideration of fluid allocation in shell and tube
Factor Description
Corrosion The more corrosive fluid should be allocated to the tube
side. This will reduce the cost of expensive alloy or clad
components. It is advantageous in cooling to connect the
stream to the tubes of the cooler rather than the shell. In
this way, since the stream may be corrosive, the action can
be confined to the tube side alone, whereas if the steam is
introduced into the shell, both may be damaged.
Fouling The fluid that has the greatest tendency to foul the heat-
transfer surface should be placed in the tubes. This will
give better control over the design fluid velocity, and the
higher allowable velocity in the tubes will reduce fouling.
Also the tubes will be easier to clean.
Fluid temperature If the temperatures are high enough to require the use of
special alloys placing the higher temperature fluid in the
tubes will reduce the overall cost. At moderate
temperatures, placing the hotter fluid in the tubes will
reduce the shell surface temperatures, and hence the need
for lagging to reduce heat loss, or for safety reasons.
Operating
pressures
The higher pressure stream should be allocated to the
tube-side. High-pressure tubes will be cheaper than a high-
pressure shell.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Production of n-propanol Chapter V
5-11
Factor Description
Pressure drop For the same pressure drop, higher heat-transfer
coefficients will be obtained on the tube-side than the shell-
side, and fluid with the lowest allowable pressure drop
should be allocated to the tube-side.
Viscosity The higher heat-transfer coefficient will be obtained by
allocating the more viscous material to the shell-side,
providing the flow is turbulent.
Stream flow rate Allocating the fluids with the lowest flow-rate to the shell-
side will normally give the most economical design.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Based on the factors in Table 5.4, it can be conclude, the fluid that want to
be cool down was allocate in shell side which is gasses, and the cooling fluid which
is water will be allocate in the tube side.
5.3.2.1 Number Of Tube
Before the number of tube in the heat exchanger is determine, the arrangement of
tube inside the heat exchanger must be known first. The tubes in an exchanger are
usually arranged in an triangular pattern as shown in Figure 5.3 . The triangular
pattern gives higher heat transfer rates. This type of tube arrangement was
commonly used in industry.
The recommended tube pitch (distance between tube centres) is 1.25 times
the tubes outside diameter, and this will normally be used unless process
requirements dictate otherwise.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Figure 5.3.: Triangular tubes pattern
Flow
Pt
Production of n-propanol Chapter V
5-12
The preferred lengths of tubes for heat exchanger that commonly used in
industries are 16ft (4.88mm). For diameter size of tube, 16-25 mm is preferred for
most duties, as they will give more compact and therefore cheaper exchanger.
Hence, take the value of outside and inside diameter in this range. 20 mm for
outside diameter and 16 mm for inside diameter have been choose for this purpose.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Area of one tube ( neglecting thickness of tube sheet ) :
Lda o (5.9)
88.402.0 a
2307.0 m
Hence, the total number of tubes are :
a
AN t (5.10)
2
2
307.0
57.173
m
m
565
Taking the number of passes of tube is 4 as mention earlier, therefore the number of
tubes per pass, pN are :
4
tp
NN (5.11)
4
565
141
5.3.2.2 Tube Side Velocity
Tube cross sectional area, 4
2
ics
dA
(5.12)
4
)016.0( 2
410011.2
Production of n-propanol Chapter V
5-13
Area per pass, pcsp NAA (5.13)
14110011.2 4
2028.0 m
Water mass velocity, p
tt
A
mV (5.14)
20284.0
/12
m
skg
2/57.428 smkg
Tube side velocity,
tt
Vu (5.15)
3
2
/818.995
/57.428
mkg
smkg
sm /430.0
5.3.2.3 Tube Side Heat Transfer Coefficient
Reynolds number, t
ittt
du
Re (5.16)
where ;
tRe = Reynold number of fluid in tube side
t = fluid (water) density of tube side, 995.818 kg/m3
tu = fluid velocity of tube side, m/s
t = fluid dynamic viscosity of tube side, Ns/m2
id = inside diameter of tube side, m
Therefore;
23
3
/105986.0
016.0/430.0/818.995Re
mNs
msmmkgt
11445
Prandtl number, t
tp
k
C Pr (5.17)
Production of n-propanol Chapter V
5-14
where;
Pr = Prandtl number
pC = fluid heat capacity, J/kgoC
tk = fluid thermal conductivity of tube side, W/moC
therefore;
CmW
mNsCkgJo
o
/6376.0
/105986.0/102.4Pr
233
94.3
And ratio of, 305016.0
88.4
m
m
d
L
i
(5.18)
Then, find the heat transfer factor,hj . It is often convenient to correlate heat transfer
data in terms of a heat transfer. The hj value can be obtained from Figure E.3
(Appendix E) based on the Reynolds number and the ratio ofidL / . Hence, the heat
transfer factor is 0.0039.
Nusselt number, 33.0PrRe tht jNu (5.19)
33.0)94.3(114450039.0
18.70
Tube side heat transfer coefficient:
i
tti
d
kNuh
(5.20)
m
CmW o
016.0
/6376.018.70
CmWo2/67.2796
5.3.3 Bundle and Shell Diameter
The bundle diameter depends not only the number of tubes but also on the number
of tube passes, as space must be left in the pattern of tubes on the tube sheet to
accommodate the pass partition plates.
Production of n-propanol Chapter V
5-15
An estimation of the bundle diameter bD can be obtained from equation 5.21
which is an empirical equation based on standard tube layouts. The constant for use
in the equation, for triangular and square pattern are given in table E4 (Appendix E).
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
By choosing the triangular pattern of tube and 4 tube passes, the value of 1K is
0.175 and the value of 1n is 2.285. Hence ,
1
1
1
nt
obK
NdD
(5.21)
285.2
1
175.0
565020.0
m
m687.0
The clearance required between the outermost tubes in the bundle and the shell
inside diameter will depend on the type of exchanger and the manufacturing
tolerance. Split ring floating head exchangers have been choose for efficiency and
ease of cleaning. Based on figure E5 (Appendix E) the shell clearance is 64mm.
Hence, shell inside diameter,
bs DD shell clearance (5.22)
064.0687.0
751.0
5.3.3.1 Shell-side Heat Transfer Coefficient
The complex flow pattern on the shell side, and the greater number of variables
involved, make it difficult to predict the shell-side heat transfer corfficient and
pressure drop with complete assurance.
Kern’s method was choose to determine these heat transfer in shell side.
This method is base on experimental work on commercial exchanger with standard
tolerances and will give a reasonably satisfactory prediction of the heat transfer
coefficient for standard designs.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Production of n-propanol Chapter V
5-16
In order to calculate the heat transfer on the shell side, the number of baffle
spacing must be estimate first. Baffle spacing are used in the shell to direct the fluid
stream across the tubes, to increase the fluid velocity and so to improve the rate of
transfer. The most commonly used type of baffle is the single segmental baffle
spacing. Take the baffle spacing equal to 5 because this spacing should give good
heat transfer.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Therefore, take the baffle spacing equal to 5, hence :
5
sb
DI (5.23)
5
751.0 m
150.0
For triangular pitch, tube pitch ot dp 25.1 (5.24)
020.025.1
025.0
Hence, cross flow area ;
t
bsots
p
IDdpA
)( (5.25)
025.0
150.0751.0)020.0025.0( sA
202253.0 m
The shell side equivalent diameter ( hydraulic diameter ).
)917.0(10.1 22
ot
o
e dpd
d (5.26)
))020.0(917.0025.0(020.0
10.1 22
20142.0 m
Production of n-propanol Chapter V
5-17
Volumetric flow rate on shell side ;
m
s (5.27)
3/704.13
1
3600
120890
mkgs
h
h
kg
sm /423.0 3
Therefore, shell side velocity ;
s
ss
Au
(5.28)
2
3
02253.0
/423.0
m
sm
sm /77.18
Reynolds number, s
esss
du
Re (5.29)
23
3
/101621.0
0142.0/77.18/704.13
mNs
msmmkg
22532
Prandtl number, s
ss
k
Cp Pr (5.30)
CmW
mNsCkgJo
o
/1150.0
)/101621.0)(/10246.3(Pr
233
6.4Pr
In order to find the heat transfer coefficient of shell, the baffle cut must select first.
Baffle cut is used to specify the dimension of a segmental baffle, expressed as a
percentage of the baffle disc diameter. Baffle cut from 15% to 45% are used.
Generally , a baffle cut of 20% to 25% will be the optimum, giving good heat transfer
rates, without excessive pressure drop. From this selection of baffle cut, the heat
transfer factor,nj can be determine.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Based on Figure E.6 (Appendix E), at baffle cut percent equal to 25% and Reynold
number equal to 22532, heat transfer factornj equal to 0.004.
Production of n-propanol Chapter V
5-18
Hence, shell-side heat transfer coefficient,
3/1
Re rsn
f
es Pjk
dh (5.31)
where;
sh = heat transfer coefficient of shell-side, W/m2°C
ed = inner diameter of tube-side, m
fk = thermal conductivity of shell tube, W/m°C
nj = heat transfer factor of shell-side
sRe = Reynolds number of shell side
Pr = Prandtl number of shell-side
Therefore,
e
renf
sd
PRjkh
33.0
0142.0
)6.4(22532004.01150.0 33.0
CmWo2/76.1207
5.3.4 Overall Heat Transfer
Taking material of construction is carbon steel, wk = 55 W/m°C
Overall heat transfer coefficient,
0003.01
2
ln
0002.011
sw
i
o
o
i
o
to hk
d
dd
d
d
hU (5.32)
0003.076.1207
1
)55(2
016.0
020.0ln020.0
016.0
020.00002.0
67.2796
1
WCmo
/1077612.1 23
CmWUo
o
2/03.563
This is above the initial estimation of 160 W/m2 0C. The number of tube could
possibly be reduced, but first check the pressure drop.
Production of n-propanol Chapter V
5-19
5.3.5 Pressure Drop
In many applications, the pressure drop available to drive the fluids through the
exchanger will be set by the process conditions. When the designer is free to select
the pressure drop, an economic analysis can be made to determine the exchanger
design that gives the lowest operating costs. The value that suggested in designing
of this heat exchanger are shown in table 5.5.
Table: 5.5 : Allowable pressure drop
Phase Allowable Pressure Drop
Liquid 35kN/m2
Gas 0.4-0.8kN/m2
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
5.3.5.1 Tube Side Pressure Drop
Based on Figure E.7 (Appendix E) at Reynolds number , 11445 the tube friction
factor fj equal to 0.0048.
Therefore, the pressure drop on tube side ;
25.28
2
t
m
wi
fpt
u
d
LjNP
(5.33)
Neglect the viscosity correction term,
m
w
equation 5.33 becomes;
25.28
2
t
i
fpt
u
d
LjNP
(5.34)
where;
pN = Number of tube passes, 4
fj = friction factor
L = tube length, 4.88m
id = inside diameter of tube, 0.016m
tu = fluid velocity in tube-side, 0.430m/s
= fluid density in tube-side, 995.818kg/m3
Production of n-propanol Chapter V
5-20
Therefore,
∆Pt =
2
/43.0/818.9955.2
016.0
88.40048.084
23 smmkg
m
mPt
= 5233.62 N/m2
The pressure drop is in range of specification.
5.3.5.2 Shell-side Pressure Drop
Based on figure E8 (Appendix E) at Reynolds number 22532 , the tube friction
factor fj equal to 0.044.
Therefore, shell-side pressure drop;
1402
28
.
w
s
Be
sfs
u
l
L
d
DjP
(5.35)
Neglect the viscosity correction term,
m
w
equation 5.35 becomes;
28
2
s
Be
sfs
u
l
L
d
DjP
(5.36)
where;
L = tube length, 4.88m
bI = baffle spacing, 0.150m
ed = equivalent diameter,0.0142 m
su = fluid velocity in shell-side, 18.77m/s
Therefore,
2
/77.18/7035.13
150.0
88.4
0142.0
751.0044.08
23 smmkg
m
m
m
mPs
= 1462.02 kN/m2
This value of pressure drop is exceeding specification and need to be modified. In
order to doing this, the shell side velocity must be reduced by increasing the baffle
spacing.
Production of n-propanol Chapter V
5-21
Hence, the new baffle spacing is ;
bI = old baffle spacing/0.3 (5.37)
3.0
150mm
mm500
By using the new value of baffle spacing, the new properties in shell side are;
20751.0 mAs
smus /63.5
6758Re s , 007.0nj , 053.0fj
CmWho
s
2/92.633
barP 47.0
This pressure drop is in a range of specification.
New overall heat transfer coefficient,
0003.01
2
ln
0002.011
sw
i
o
o
i
o
to hk
d
dd
d
d
hU
0003.092.633
1
)55(2
016.0
020.0ln020.0
016.0
020.00002.0
67.2796
1
WCmo
/105256.2 23
CmWUo
o
2/95.395
Production of n-propanol Chapter V
5-22
Table 5.6: Summary of Chemical Design of Heat Exchanger
Parameters SI unit
Process condition:
Heat load, Q
Heat transfer coefficient assume, Uass
Heat transfer coefficient calculate, Ucalc
Heat transfer area
∆Tlm
∆Tm
1.506 x 102 kW
160 W/m2.°C
395.95 W/m².°C
173.57 m2
61.66°C
54.26°C
Shell side
Inlet temperature, T1
Outlet temperature, T2
Flow rate,
m s
Shell side velocity, us
Diameter of shell, Ds
Bundle diameter, Db
Equivalent diameter, de
Shell passes
Heat Transfer Coefficient, hs
Pressure drop, ∆Ps
150°C
70°C
20890 kg/h
5.63 m/s
0.751 m
0.687 m
0.0142 m
2
633.92 W/m2.°C
47000 N/m2
Tube side: Water
Inlet temperature, t1
Outlet temperature, t2
Flow rate,
m t
Tube velocity, ut
Tube length
Outer diameter, do
Inner diameter, di
Birmingham Wire Gage (BWG)
Tube pitch, pt
Number of tube, Nt
Tube per pass
Tube passes, Np
Heat transfer coefficient, hi
Pressure drop, ∆Pt
30°C
60°C
12.0 kg/s
0.430 m/s
4.88 m
0.020 m
0.016 m
16
0.0238 m
565
150
4
2796.67 W/m2.°C
5233.62 N/m2
Production of n-propanol Chapter V
5-23
5.4 MECHANICAL DESIGN OF HEAT EXCHANGER
The mechanical design is a function of the equipment, the operating pressure and
temperature the equipment dimension, the opening and connection and the material
of construction. The main details of mechanical design include the followings:
Operating and design temperature and pressure
Material of construction
Corrosion allowance
Shell side
1. Shell thickness
2. Head and closures
3. Nozzles
4. Flanges
Tube side
1. Tube thickness
2. Nozzles
3. Flanges
Insulator thickness
Weight load heat exchanger
1. Vessel weight
2. Tubes weight
3. Weight of mixture to fill the shell vessel
4. Weight of water to fill the tubes
5. Weight of insulator
Support design
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Production of n-propanol Chapter V
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Previous calculations from chemical engineering design will be used in the
calculation of mechanical design.
Figure 5.4: Part of heat exchanger
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Table 5.7: Part of heat exchanger
Number Description
1 Shell
2 Channel head
3 Channel cover
4 Nozzle
5 Tube
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
5.4.1 Design Pressure
The heat exchanger must be design to withstand the maximum pressure to which it
is likely to be subjected in operation. The design pressure is normally taken at 10%
above normal working operation. The purpose is to avoid spurious operation during
minor process upsets.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Production of n-propanol Chapter V
5-25
By taking a safety factor of 10%;
For shell-side; Pi = Po x 1.1
= 20 bar
= 2 N/mm2 x 1.1
= 2.2 N/mm2
For tube-side; Pi = Po x 1.1
= 20 bar
= 2 N/mm2 x 1.1
= 2.2 N/mm2
Table 5.8: Design pressure of shell and tube
Parameter Shell side Tube side
Operating pressure, bar 20 20
Design pressure, N/mm2 2.2 2.2
5.4.2 Design Temperature
The strength of metals decreases with increasing temperature so the maximum
allowable design stress will depend on the material temperature. The design
temperature at which the design stress is evaluated should be taken as the
maximum working temperature of material, with due allowance for any uncertainty
involved in predicting vessel wall temperature, therefore taking a safety factor of
10% to cover uncertainties in temperature prediction.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
By taking a safety factor of 10%;
For shell-side; Ti = To x 1.1
= 150°C x 1.1
= 165°C
For tube-side; Ti = To x 1.1
= 60°C x 1.1
= 66°C
Production of n-propanol Chapter V
5-26
Table 5.8: Design temperature of shell and tube
Parameter Shell side Tube side
Operating temperature, °C 150 60
Design temperature, °C 165 66
5.4.3 Material of Construction
Selection of a suitable material must take into account the suitability of the material
for fabrication as well as the compatibility of the material with the process
environment. The material which is fit to the chemical and mechanical requirements
and the same time the most economical should be selected. Carbon steel has been
choosing as a material of construction due to more cheaply than stainless steel and
high corrosion resistance. A few factors that should be considered while choosing
the material of construction are:
1. Corrosion resistance
2. Operating conditions
3. Economic feasibility
4. Suitability for fabrication
5. Process safety
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
5.4.4 Welded Joint Efficiency and Corrosion Allowance
The strength of welded joint will depend on type of joint and the quality of the
welding. The soundness of weld is then checked by visual inspection and by non-
destructive testing called radiography. The welded joint factor, J is taken as 1.0.
The corrosion allowance is additional thickness of metal added to allow for
material lost by corrosion and erosion or scaling. The allowance is based on
experience with the material of construction under similar service condition to those
for the purposed design. A minimum corrosion allowance used is 2 mm for carbon
steel material of construction.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Production of n-propanol Chapter V
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5.4.5 Design Stress (Nominal Design Strength)
For the design purpose, it is necessary to decide a value for the maximum allowable
stress (nominal design stress) that can be accepted in the material of construction.
The allowable stress for the selected material of construction at the design
temperature shows in Table 5.10
Table 5.10: Design stress for material construction
Material used Design stress ,f (N/mm2)
Shell: Carbon steel 105 @ 200°C
Tube: Carbon steel 125 @ 100°C
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
5.4.6 Minimum Practical Wall Thickness
This is required to ensure that any vessel is sufficiently rigid to withstand its own
weight, and any incidental loads. From previous calculation in chemical engineering
design, the internal diameter of shell, Ds = 0.751 m. For a cylindrical shell, the
minimum thickness required to resist internal pressure can be determined as
follows:
Minimum wall thickness, i
ii
PJf
DPe
2 (5.38)
where;
iP = internal design pressure of shell, N/mm2
iD = Shell diameter, mm
J = Joint factor (J=1)
f = Design stress of shell, N/mm2
Therefore,
22
2
/2.2/10512
751/2.2
mmNmmN
mmmmNe
mm95.7
Adding corrosion allowance of 2mm;
mmmme 295.7
mm95.9
Production of n-propanol Chapter V
5-28
5.4.7 Minimum Thickness of Tube Wall
The minimum thickness required for the tube: i
ii
PJf
DPe
2 (5.39)
where;
iP = internal design pressure of tube, N/mm2
iD = internal tube diameter, mm
J = Joint factor (J=1)
f = Design stress of tube side, N/mm2
Therefore;
22
2
/2.2/10512
16/2.2
mmNmmN
mmmmNe
mm17.0
Adding corrosion allowance of 2mm;
mmmme 217.0
mm17.2
5.4.8 Head and Closure
There are several types of head and closure as describe in Table 5.11. For this
design, ellipsoidal heads was chosen since the operation pressure is less than 15
bar and this types of heads most economical. The standard ellipsoidal heads are
manufactured with a major and minor axis ratio of 2:1.
Table 5.11: Selection of head and closure
Types of head Advantages
Hemispherical heads Suitable for high pressure
Higher cost
The strongest shape
Ellipsoidal heads Most economical for operation above
15 bar
Torispherical Suitable for operation above 15 bar
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Production of n-propanol Chapter V
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5.4.8.1 Ellipsoidal Head
Minimum thickness required, i x P.-s x J x f
iDiPe=
202 (5.40)
where;
iP = internal design pressure of shell, N/mm2
iD = Shell diameter, mm
J = Joint factor (J=1)
f = Design stress of shell, N/mm2
Therefore;
22
2
/2.22.0/10512
751/2.2
mmNmmN
mmmmNe
mm88.7
Adding corrosion allowance of 2mm;
mmmme 288.7
mm88.9
5.4.8.2 Channel covers (Closures)
Minimum thickness required, 2
1
e
i
epD
PDCe (5.41)
where;
pC = design constant that depend on the edge constraint
eD = nominal plate diameter, mm
iP = internal pressure of shell, N/mm2
sf = design stress of shell side, N/mm2
Values for the design constant, Cp and the nominal plate diameter, De are given in
the design codes and standards for various arrangements of flat end closures (BS
5500, clause 3.5.5). From Coulson & Richardson’s Chemical Engineering Volume 6,
plates welded to the end of the shell with a fillet weld, angle of fillet 45oC and depth
equal to the plate thickness, take Cp as 0.4 and De=Di;
Production of n-propanol Chapter V
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Therefore,
2
1
105
2.27514.0
e
Adding corrosion allowance of 2mm;
mmmme 248.43
mm48.45
5.4.9 Weight Loads
5.4.9.1 The Shell Weight
For preliminary calculation, the approximate weight of a cylindrical vessel with dome
ends, and uniform wall thickness, can be estimated using the equation below:
Vessel weight, 310)8.0( tDLgDCW mmmvv (5.41)
where;
Wv = Total weight of the shell, N
Cv = Factor for the weight of nozzles for vessels with only a few internal
fittings (1.08)
L = Length of tube, m
g = gravitational acceleration, 9.81 m/s2
t = wall thickness, mm
m = density of vessel material, 7854 kg/m3
Dm = mean diameter of vessel, m
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
)10( 3 tDD im (5.42)
)1095.9751.0( 3
mm76.0
Therefore;
)00995.0))(76.0(8.088.4)(/81.9)(76.0)(/7854()08.1( 23 msmmmkgWv
N12730
Production of n-propanol Chapter V
5-31
5.4.9.2 Weight of Tubes
Weight of tubes, gddNW miott )(22
(5.43)
where;
Nt = number of tubes,
do = outside diameter of tube, m
di = inside diameter of tube, m
m = density of tube material, kg/m3
Therefore;
2322 /81.9/7854)))016.0()02.0((565 smmkgmmWt
N19693
5.4.9.3 Weight of fluid to fill the shell
Weight of fluid (gas), 4
2 x gs x L x ρsπ x D
Wg= (5.44)
where:
Ds = diameter of shell side, m
L = length, m
s = density of shell-side, kg/m3
g = gravitational acceleration, m/s2
Therefore;
4
/81.9/7035.1388.4751.0 232smmkgmm
Wg
N6.290
Production of n-propanol Chapter V
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5.4.9.4 Weight of water to fill the tube
Weight of coolant,
4
22gLρddxN
W tiot
water
(5.45)
where:
t = density of water in tube, kg/m3
Therefore;
4
)/81.9)(/818.995)(88.4()016.0()02.0(565 2322 smmkgmmmWwater
N3.3046
5.4.9.5 Weight of Insulator
Material used as insulator is mineral wool. From Coulson & Richardson’s Chemical
Engineering Volume 6, the density of mineral wool insulation is 130 kg/m3.
Approximate volume of insulation, iDLeV (5.46)
where:
V = Approximate volume of insulation, m3
L = Length of tube, m
ei = Thickness of insulator, m
Volume of insulation, )0187.0)(88.4)(751.0( mmV
3215.0 m
Weight of Insulator, gVW ii (5.47)
where:
Wi = weight of insulation, kgm/s2
i = insulation density, kg/m3
Therefore;
233 /81.9/130215.0 smmkgmWi
N2.274
Production of n-propanol Chapter V
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The total weight of heat exchanger;
iwatergtvT WWWWWW (5.48)
NNNNN 2.2743.30466.2901969312730
N1.36034
kN03.36
5.4.10 Baffles
Baffles are used in the shell to direct the fluid flow across tube and increase the fluid
velocity. When the fluid velocity increases, the rate of heat transfer is also improved.
The assembly of baffles and tubes inner diameter hold together by support rods and
spacers. The most commonly used type of baffle is the single-segmental baffle.
Baffle used to specify the dimensions of a segmental baffle. Generally, baffle cut of
20%-25% will be optimum. The value will give good heat transfer rate without
excessive drop. The function of baffles are to support the tubes for structural rigidity,
preventing tube vibration and sagging to divert the flow across the bundle to obtain a
higher heat transfer coefficient.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Baffle diameter = Ds – 3.2 mm (5.49)
= 751 mm – 3.2 mm
= 747.8 mm
Tolerance = Ds + 0.8 mm (5.50)
= 751 mm + 0.8 mm
= 751.8 mm
Baffle spacing, IB = Ds/5 (5.51)
= 751 mm /5
= 150 mm
Baffle modification; = 150mm/0.3
= 500mm
Production of n-propanol Chapter V
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Number of baffle, Nb = (L/IB) -1 (5.52)
= (4880 mm/500 mm) -1
= 8.76 ≈ 9 baffles
Baffle Thickness = 4.76 mm (Adapted from British Standard (DIN 38025)
5.4.11 Nozzle (Branches)
Nozzles are used for entering and leaving the inlet and outlet stream of heat
exchanger. The nozzles are for channel side and the shell side of heat exchanger.
Standard steel pipe will be used for the inlet and outlet nozzles are obtained from
Perry’s Handbook (Table 10-18). It is important to avoid flow restrictions at the inlet
and outlet nozzles. It is also to prevent excessive pressure drop flow induced
vibration of the tubes. Material of construction for nozzle will be the same as the
heat exchanger body.
(source:Perry's Chemical Engineering Handbook)
5.4.11.1 Shell-side Nozzles
Table5.12: Properties for the shell-side
Properties Inlet Outlet
Temperature, °C 150 70
Density, , kg/m3 13.70 13.70
Flow rate, m, kg/s 5.80 5.80
Fluid velocity, u, m/s 5.63 5.63
Flow area, A, m2 [A = m/ x u] 2.38 2.38
Inside diameter, mm 751 751
By referring to the standard properties of steel pipe from Table 10-18
(Perry’s Handbook), the standard nominal pipe size was taken as 30 in. The
properties at this nominal size are show below:
Production of n-propanol Chapter V
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Table 5.13: Properties of pipe of shell-side
Nominal
size, in
Outside diameter,
OD, in
Schedule
no.
Inside diameter,
ID, in
Flow area,
ft2
30 30 5S 29.5 4.746
5.4.11.2 Tube-side Nozzles
Table 5.14: Properties of tube-side
Properties Inlet Outlet
Temperature, °C 30 60
Density, , kg/m3 995.81 995.81
Flow rate, G, kg/s 12.00 12.00
Fluid velocity, u, m/s 0.43 0.43
Flow area, A, m2 [A = G/ x u] 0.0052 0.0052
Inside diameter, m [(4 x A/)1/2] 0.0813 0.0813
Inside diameter, mm 81.3 81.3
By referring to the standard properties of steel pipe from Table 10-18
(Perry’s Handbook), the standard nominal pipe size was taken as 4.0 in. The
properties at this nominal size are show below:
Table 5.15: Properties of pipe of tube-side
Nominal size,
in
Outside
diameter, OD, in
Schedule
no.
Inside diameter,
ID, in
Flow area,
ft2
3.33 4.0 5S 3.834 0.08017
5.4.11.3 Flanged for Nozzle
Flanged joints are used for connecting pipes and instruments to vessel, for
manholes cover and for removal vessel head when ease of access is required.
Flanged may also be used on the vessel body, when it is necessary to divide the
vessel into sections for transport maintenance. Flanged joints are also used to
connect pipe to requirements such as pumps and valves. Flanges range size from a
few millimeters diameter for small pipes to several meters diameter for those used
as body or head flanges on vessels. Flanges dimension must be able to withstand
Production of n-propanol Chapter V
5-36
the hydrostatic ends loads and the bolt loads necessary to ensure tight joint in
service.
For the design of this heat exchanger, welding-neck flange are used. It is
because welding-neck flanges have a long trapped hub between the flange ring and
the welded joint. This gradual transition of the section reduces the discontinuity
stresses between the flange and branch. It is also can increase the strength of the
flange assembly.
Welding-neck flanges are suitable for extreme service conditions, where
flange are likely to be subjected to temperature, shear and vibration loads. They will
normally be specified for the connections and nozzles on process vessels and
process equipment. The dimensions of flanged for nozzle for nominal size 80 and
300 mm are show in Table 5.16.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Table 5.16: Dimensions of flanged for nozzle
Type
Nom. size
Pipe, o.d. d1
Flange Raised
face Bolting Drilling Neck
D b h1 d4 f No d2 k d3 Shell-side
300 323.9 440 22 44 365 4 M20 12 22 395 355
Tube-side
80 88.9 190 16 34 128 3 M16 4 18 150 110
(All units in mm)
d4
k
D
de
d3
d1
Production of n-propanol Chapter V
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Figure 5.5: Typical standard flange design
5.4.12 Design of support saddles
The saddles must be designed to withstand the load imposed by the weight of the
vessel and contents. They are constructed of bricks or concrete, or are fabricated
from steel plate. The contact angel should not less than 120oC, and will not normally
be greater than 150oC. Wear plate often welded to the shell wall to reinforce the wall
over the area contact with the saddles.
(Source: Coulson & Richardson’s Chemical Engineering Volume 6, 1999)
Table 5.17: Dimensions for saddle support
Vessel
diameter,
(m)
Maximum
weight,
(kN)
Dimensions, (m) mm
V Y C E J G t1 t2 Bolt
dia.
Bolt
holes
0.8 50 0.58 0.15 0.70 0.29 0.225 0.095 8 5 20 25
Figure 5.6: Standard steel saddles
Production of n-propanol Chapter V
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Table 5.18: Summary of mechanical design of shell and tube heat exchanger
PARAMETER SPECIFICATION
Design Pressure
+10% above normal working of operations.
Shell : 2.2 N/mm2
Tube : 2.2 N/mm2
Design Temperature
+ 10% to cover uncertainties in prediction.
Shell : 165°C
Tube : 66°C
Material of construction Shell : Carbon steel
Tube : Carbon steel
Design Stress
Take above or nearest the design temperature
Shell : 105 N/mm2@200°C
Tube : 125 N/mm2@100°C
Minimum Thickness Shell : 9.95 mm
Tube : 2.17 mm
Head and closure
Head: Ellipsoidal head type
Closure: Channels cover type
Thickness : 9.88 mm
Thickness : 44.48 mm
Weight Load
Weight of shell
Weight of tubes
Weight of gas to be filled the vessel
Weight of water to be filled in the tube
Weight of insulator
Total weight
12.73 kN
16.69 kN
0.29 kN
3.05 kN
0.27 kN
36.03 kN
Production of n-propanol Chapter V
5-39
REFERENCES
R K Sinnott. Third Edition, 1999. Coulson & Richardson’s Chemical Engineering
Volume 6. Elsevier Butterworth Heinemann. 634-869.
Frank P. Incropera & David P. Dewitt, Fifth Edition, 2002. Fundamentals of Heat and
Mass Transfer. John Wiley & Sons, U.K. 924
Robert H. Perry & Don W. Green. 1997. Perry’s Chemical Engineers Handbook.
Seventh Edition. Mc Graw Hill.
Carl L. Yaws. Chemical Properties Handbook (Physical Thermo, Environment,
Transport, Safety and Health for Organic and Inorganic Chemicals). Mc
Graw Hill.
Yunus A. Cengel & Michael A. Boles. Third Edition, 1998. Thermodynamics An
Engineering Approach. McGraw-Hill.
Dr. Brian Spulding & J.Tab Orela. 1990. Heat exchanger Theory and Design
Handbook. McGraw-Hill.
Production of n-propanol Chapter V
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APPENDIX E
Production of n-propanol Chapter V
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Figure E.1: Temperature correction factor : two shell passes and four tube
passes
Production of n-propanol Chapter V
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Table E.2: Typical Overall Coefficient
Production of n-propanol Chapter V
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Figure E.3: Tube Side Heat Transfer Factor
Production of n-propanol Chapter V
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Table E.4: Constant for Tube Arrangement
Figure E.5: Shell-bundle Clearance
Production of n-propanol Chapter V
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Figure E.6: Shell Side Heat Transfer Factor
Production of n-propanol Chapter V
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Figure E.7: Tube Side Friction Factor
Production of n-propanol Chapter V
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Figure E.8: Shell Side Friction Factor
Production of n-propanol Chapter V
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