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DESIGN OF HEAT EXCHANGER
4.1 CHEMICAL DESIGN
According to Ramesh K. Shah et al, 2003, In chemical plant industry, heat exchanger is
one of the most and main important equipment. Heat exchanger is a device that transfers thermal
energy (enthalpy) between two or more fluids for solid and fluid or between particulates and fluid
at different range of temperature with thermal contact.
Most of the usage and application of the heat exchanger involves cooling, heating,
condensing, evaporating, concentrating, crystallizing, sterilizing, distilling and etc. In heat
exchanger, there are two ways of heat can be transferred which is indirect and direct transfer. The
indirect transfer, heat exchange is done using energy storage and rejection passing through the
exchanger surface.
On the other hand, for the direct transfer of heat exchanger, the fluid inside does not mix
since the fluid is separated by the walls designed in the heat exchanger. Some notes need to be
taken into consideration in designing heat exchanger. Most important and major factor in
designing heat exchanger is to know the details of fluid, phase and choosing the type of the heat
exchanger. More explanation to design the heat exchanger will be discussed further.
4.1.1 Design Specification
The liquid mixture from distallation column is low temperature. The main objective of
the heat exchanger is to heat up the temperature until 75 C from 43 C. At this stage, the first
heat exchanger heat up the stream of 2601.11 kg/hr of liquid mixture before entering the
polymerization reactor. Hence, the design of the heat exchanger is based on this flow rate rather
than of the fresh feed.
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4.1.2 Selection of Equipment
More than one type of heat exchanger is used in chemical plant and there is various types
of them. Every heat exchanger used has its own supremacy and weakness. The heat exchanger
type and specification will be described shortly in table 4.1 below.
Heat exchanger
type
Specification
Shell and tube
heat exchanger
The most common heat exchanger use in the world.
Can be easily clean.
The configuration can give a large surface area in a small
volume.
Not compact compare to other type of heat exchanger.
Can be constructed from a wide range of materials.
Well established design procedures.
Spiral and tube
Heat exchanger
Extremely compact and can handle most type of the fluid
Can be easily clean and the turbulence in the channel is high
Mostly use in the dirty process fluid and slurries
Has a high heat transfer rate compare straight tube
But ,applicable only for a small capacity duty
Mechanical cleaning of the tube is very limited and sometimes is
impossible
Very limited pressure drop 20 bar and temperature of 400C
Plate fin heat
exchanger
Low temperature approach can be used as low as 1C,compared
to shell and tube heat exchanger,5C to 10C
Plate will prefer more when cost of material is high
Suitable for high viscous fluid
But, it has a very limit operation temperature,260C
Also provide a very high pressure drop compare to the shell and
tube and heat exchanger.
Table 4.1 Summary Specification of Heat Exchanger Types
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After considering all the descriptions and specifications, shell and tube type is chosen
since it is the most widely used and can be designed for virtually any application. Furthermore, it
is relatively cheap as compared to the air-cooled and plate fin heat exchanger and it is sufficient
for this application.
Essentially, a shell and tube heat exchanger is compilation of bundle of tubes which is
compact together in a cylindrical shell. The ends of the tubes are fitted into tube sheets, which
separate the shell side and tube side fluids. Baffles are introduced inside the shell to direct the
fluid flow and support the tubes. The assembly of baffles and tubes is hold on each other with the
support of rods and spacers which can be represent in Figure 4.2 (Coulson &Richardsons
Volume 6)
Figure 4.1 Baffle Spacers and Tie Rods
4.1.3 Selection of Shell-Tube Type of Heat Exchanger
Referring to the Tubular Exchanger Manufacture Association (TEMA) classification, the heat
exchangers are as follows:
Type Reason of selection
Front and stationary head
Types
Type A: Channel and
removal cover
Removal cover without
break the flanges
Shell types Type E: One pass shell The most commonly used
Rear ends, head types Type M: Floating head with Used extensively in
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backing devices
petroleum service
Table 4.2 Selection on the types of heat exchanger
Internal Floating Head Exchanger is selected due to the following advantages:
Permitting differential movement between shell and tube and complete tube bundle
withdrawal.
Separate the shell and tube side fluid at the floating head end.
Access to the tubes and at the stationary end is obtained by removing the stationary head
cover or complete head.
The inside of the tubes may be cleaned in situ and complete bundle may remove for
cleaning the outside of the tubes or repairs.
The split-backing ring floating head type accommodates a smaller number of tubes than
fixed tube sheet and U tube types having the same shell diameter.
4.1.4 Selection of Shell or Tube Side for the Fluids
Fluid in shell side: Steam water
Fluid in tube side: Organic mixture
It is advisable that the Organic mixture flows in the pipe and the water flows in the opposite
direction in the annular space between the two pipes. With this arrangement, the outer surface of
the equipment will be at the lowest possible temperature and thus, heat loss to surroundings will
be minimal. The organic mixture should be place in the tube side rather than shell side as this
confines the organic mixture to the tube side and shell side will not affected.
4.1.5 Fouling
Thermal resistance of the fouling layers on the inside and outside heat transfer surfaces should be
taken into consideration in the calculation of the overall heat coefficient. The fouling layers
increase in thickness with time during operation and have lower thermal conductivity than the
fluids or the tube material, thereby increasing the overall thermal resistance.
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4.1.6 Process Background of Heat Exchanger Heater, H-102
These sections discuss the design of suitable heat exchanger after the distillation process stage.
The heat exchanger to be design is first heat exchanger (H-102). The purposed of heat exchanger
is to increase high temperature from distillation column (T-102) at 43 C until 75 C to the
polymerization reactor (R-103). A comprehensive design study is to determine the heat exchanger
mechanical details and physical characteristics and anticipated performance. This section contains
the operating criteria, the equipment selection, and the result of the thermal design and
mechanical design of heat exchanger.
Figure 4.2 Process conditions for heat exchanger
Name of Equipment : Heater H-102
Equipment Purpose : To heat up the process in the shell to achieve the desire temperature by
using steam water in the tube.
Type : Shell and Tube, Internal Floating Head
4.1.6.1 Specification and Chemical Properties
The process liquid will be in the tube side which will be heat up by the steam water from outside
that will be flowed through the shell side of the heat exchanger. The value of chemical properties
for both stream are as shown below.
Component Organic mixture (Tube) Water steam (Shell)
Type Cold Hot
Temperature inlet, (C) t1 = 43 T1 = 90
Temperature outlet, (C) t2 = 75 T2 = 54
Specific heat, Cp (kJ/kg.C) 17.82 4.200
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Thermal conductivity (W/m.C) 0.6376 0.1150
Density (kg/m3) 19.2667 8.9194
Viscosity (Ns/m2) 3.36 10-5 1.3 10-3
Table 4.3 Data information of process condition
4.1.6.2 Mean Temperature Difference (LMTD)
Counter flow arrangement is selected, as the temperature difference is greater as compared to
cross flow. For the LMTD involved, the following assumptions are made:
The overall coefficient of heat transfer is constant
The rate of flow of each fluid is constant
There is an equal amount of cooling surface in each pass
Figure 4.3 Temperature profiles for counter current flow
Organic mixture Water
In Out In Out
t1 (C) t2 (C) T1 (C) T2 (C)
43 75 90 64
T1
T2
t2
t1
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Where,
T1 = Inlet shell side fluid temperature
T2 = Outlet shell side fluid temperature
t1 = Inlet tube side fluid temperature
t2 = Outlet tube side fluid temperature
True Temperature Difference,
Temperature correlation factor, Ft is determined in Figure 12.19 Ray Sinnote & Gavin Towler
Chemical Engineering Design, 5th Edition, 2009 (Figure D.1 Appendix D). At R = 0.81 and S =
0.68, Ft = 0.8. Therefore, it allowed to be install in 1 shell pass since the correction factor is high
enough and satisfy (Towler and Sinnot, 2009)
Therefore,
Where,
Tm = True temperature difference
Ft = Temperature correction factor
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4.1.6.3 Heat Transfer Area
From the shell side chemical properties, the value of power (Q) could be calculated
After Q value is obtained, the flow rate of steam water used can be calculated by using the same
equation.
Assume that Qtube = Qshell,
Thus, 3.7727 kg/s of steam water are needed to heat up the cold stream of 43 C to 75 C.
The overall coefficient that suitable with this heat exchanger condition will be in range 500
1000 W/m2.C for organic solvent, determined from Table 19.1 Ray Sinnote & Gavin Towler
Chemical Engineering Design, 5th Edition, 2009 (Table D.2 Appendix D), the starting value was
selected to be 500 W/m2.oC.
Assume that U = 500 W/m2C
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Where,
q = Heat transferred per unit time, W
U = The overall heat transfer coefficient, W/m2.C
A = Heat transfer area, m2
Tm = the mean temperature difference, C
4.1.6.4 Number of Tube Calculation
TEMA design standard allows tube diameters in the range of 6.4 mm to 50 mm, but the common
used diameter in industry within 16 mm to 50 mm. While the favorable tubes length for heat
exchanger are 1.83 m, 2.44 m, 3.66 m, 4.88m, 6.10 m and 7.32 m (Towler and Sinnot, 2009). The
optimum tube length to shell diameter ratio usually fall within the range of 5 to 10. The following
tube dimension has been selected show in table below.
Selected tube characteristic Reasons
Tube length selected: 4.88 m It provides an adequate heat transfer surface
area and pressure drop is below the allowable
pressure drop.
The outer diameter selected, Do: 20 mm
The inner diameter selected, Di: 16 mm
It is a common tube used
Pitch selected; 23.81 mm (Triangular pitch) It cause lower pressure drop compare the
square pitch.
Birmingham wire gage (BWG): 16 It provides flow area and wall thickness to
withstand significant pressure drop.
Table 4.4 Selection of Tube
Area of tube,
Thus, the number of tube is
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The fluid in the tube usually directed to flow back and forth in a number of passes through
groups of tubes arranged in parallel in order to increase the length of the flow path. The number
of passes is selected to give the required tube side design velocity. In this design, single passes
are chosen to decrease cost of construction.
Tube per pass = 151 tubes
4.1.6.5 Area per Pass
4.1.6.6 Heat Transfer Factor, jh
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From Reynolds Number, it shown that the flow is turbulent.
So,
From figure 12.23 Ray Sinnote & Gavin Towler Chemical Engineering Design, 5 th Edition 2009
(Figure D.3 Appendix D), the value for heat transfer factor is jh = 3.9 10-3. Therefore,
For turbulent flow,
Where,
hi = inside coefficient, W/m2.C
de = equivalent diameter (or hydraulic mean), m
kf = fluid thermal conductivity, W/m.oC
= fluid viscosity at the bulk fluid temperature, Ns/m2
w = fluid viscosity at wall
Neglecting the,
the result becomes:
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4.1.7 Bundle and Shell Diameter Calculation
4.1.7.1 Tube Arrangements
Usually, tubes in heat exchanger are arranged I equilateral triangular or square or rotated square
pattern. A square or rotated square pattern arrangement is used for heavily fouling fluid, where it
is necessary to mechanically clean the outside of the tubes. The triangular pattern gives higher
heat transfer rates than square pattern. Therefore, this heat exchanger goes to triangular pattern.
Figure 4.4 Tube pattern
The recommended tube pitch (distance between tube centers) is 1.25 times the tube outside
diameter and this will normally be used unless process requirement dictate otherwise.
Thus, triangular pitch,
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From Ray Sinnote & Gavin Towler Chemical Engineering Design, 5 th Edition 2009, at Table 12.4
(Table D.4 Appendix D), the triangular pitch properties are listed below by referred to number of
tube passes.
The number of tube passes = 1
K1 = 0.319
n1 = 2.142
Thus, the bundle diameter is
4.1.7.2 Bundle Clearance and Shell Diameter
Type of heat exchanger that been chosen is split-ring floating head. The bundle clearance for this
type of exchanger determined by referring to Figure 12.12 in Ray Sinnote & Gavin Towler
Chemical Engineering Design, 5th Edition 2009 (Figure D.5 Appendix D). It is 64mm (0.064m)
Therefore, the shell diameter is
4.1.8 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.
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Kerns 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.
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.
Therefore, take the baffle spacing equal to 5, hence:
Shell side mass velocity, Gs
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Linear velocity of shell, Us
The baffle cut is the height of the segment removed to form the baffle, express as a percentage of
the baffle disc diameter. The term baffle cut is used to specify the dimension of a segmental
baffle. The optimum baffle cut is about 20% to 25%, which giving good heat transfer rates
without excessive pressure drop. In this case, 25% is taken as the optimum for the baffle cut.
Based on the Figure 12.29 from Ray Sinnote & Gavin Towler Chemical Engineering Design, 5 th
Edition 2009 (Figure D.6 Appendix D), the value of heat transfer factor, jn, can be obtained by
Reynolds number, Re.
Therefore,
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Jn = 4.0 10-3
Neglecting the viscosity correction,
The overall coefficient and the individual coefficients relationship is given by:
Where,
ho = outside fluid film coefficient, W/m2.C
hi = inside fluid film coefficient, W/m2.C
hod = outside dirt coefficient (fouling factor), W/m2.C
hid = inside dirt coefficient, W/m2.C
kw = thermal conductivity of the tube wall material, W/m2.C
di = tube inside diameter, m
do = tube outside diameter, m
Parameter value
outside fluid film coefficient, ho W/m2.C 987.22
inside fluid film coefficient, hi W/m2.C 1736.6
outside dirt coefficient (fouling factor), hod W/m2.C 4000
inside dirt coefficient, hid W/m2.C 5000
thermal conductivity of the tube wall material, kw W/m2.C 55
tube inside diameter, di m 0.016
tube outside diameter, do m 0.020
Table 4.5 Value for calculate overall coefficient
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From the overall coefficient been assume at earlier stage to calculated coefficient, the
relative error is about 8.86 % errors, therefore our design is determined to be feasible in range of
overall coefficient from 250 - 750 W/m2.C.
4.1.9 Pressure drop
In order to determine the pressure drop on tube and shell side, the viscosity correction is
neglect as in fluid mechanism; a boundary layer is the layer of fluid in immediate vicinity of
bounding surface where the viscosity is significant. Allowable pressure drop varies with total
system pressure and the phase fluid (Thakore & Bhatt, 2007).
Allowable pressure drop:-
Tube side = 0.01 bar
Shell side = 0.64 bar
4.1.9.1 Tube side pressure drop
Re = 1.14 104
From Figure 12.24 Ray Sinnote & Gavin Towler Chemical Engineering Design, 5th Edition 2009
(Figure D.7 Appendix D), tube side friction factor, jf = 1.02 10-3
By neglecting viscosity correction:
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4.1.9.2 Shell side pressure drop
Re = 1.83 104
From Figure 12.30 Ray Sinnote & Gavin Towler Chemical Engineering Design, 5 th Edition 2009
(Figure D.8 Appendix D), the 25% baffle cuts were taken, the friction factor, jf = 2.87 10-4
By neglecting viscosity correction:
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4.1.10 Summary 0f Chemical Design
Item: Heater
Item no.: H-102
Design by:
Mohd Zhariff Bin Mohd Kamar
Function: heating cold stream to achieved desired temperature in order to enter the
polymerization reactor, R-103
Operation: Continuous
Type: Shell and Tube; Internal Floating Head
Heat Duty, Q: 411.98 kW
Heat transfer area: 46.21 m2
Tube Side
Fluid handle: Organic mixture
Flowrate: 19.2667 kg/s
Pressure: 19.50 bar
Temperature: 43 75 C
Heat transfer coefficient: 1736.63 W/m2C
Pressure drop: 0.01 bar
Tube Details
Inside diameter, di: 20 mm
Outside diameter, do: 16 mm
Length, L: 4.88 m
Bundle diameter:0.687 m
No of tube, Nt: 151 tubes
Shell Side
Fluid handle: Steam water
Flowrate: 3.7729 kg/s
Pressure: 19.80 bar
Temperature: 90 - 64 C
Heat transfer coefficient: 987.22 W/m2C
Pressure drop: 0.62 bar
Shell Details
Diameter, Ds: 0.715 m
Overall Coefficient, Uo: 544.30 W/m2C
Table 4.6 Chemical design summarization