3.3 Design of Heat Exchanger (HE-102)
3.3.1 Introduction
This section is discussed to select and design a suitable heat exchanger before the
reactor (R-101). The purposed of heat exchanger is to increase low temperature from
premix vessel at 32ºC until 200ºC to the reactor. A comprehensive design study is to
determine the heat exchanger chemical and mechanical details and also physical
characteristics that contribute to the performance of heat exchanger. This section
contains the operating criteria, the equipment selection, the thermal design , chemical
design and mechanical design of heat exchanger.
Heat exchangers are equipment primarily for transferring heat between hot and
cold streams. Heat exchanger use hot fluid in order to heat a cooler fluid and vice
versa. Hot fluid and cold fluid used are depends on either the exchanger is cooler,
heater, condenser or vaporizer. The most common hot fluid and cold fluid used are
steam and cooling water respectively. The two fluids are not mix together. Barrier like
tube wall or metal wall will separated both fluids (Lugwig, 2001).
Heat exchangers are used in a wide variety of applications including power
plants, nuclear reactors, refrigeration and air conditioning systems, automotive
industries, heat recovery systems, chemical processing, and food industries
(Salimpour, 2009). Common examples of heat exchangers are shell and tube
exchangers, automobile radiators, condensers, evaporators, air preheaters, and cooling
towers. If no phase change occurs in any of the fluid in the exchanger, it is sometimes
referred to as a sensible heat exchanger (Ravagnani, 2005).
Figure1: Design procedure for shell and tube heat exchanger
3.3.2 Selection of Equipment
Heat exchangers come in a wide variety of types and sizes. Here are the most
common used in industry:
Shell and tube heat exchanger
Plate and Frame heat exchanger
Double pipe heat exchanger
a) Shell and tube heat exchanger
A shell and tube heat exchanger is a class of heat exchanger that is commonly found
in oil refineries and other larger chemical processing plant. The shell and tube heat
exchanger is designed to allow two fluids of different starting temperatures to flow
through it. A first fluid flows through the tubes (the tube side), while the second fluid
flows in the shell (the shell side) but outside the tubes. Heat is transferred between the
two fluids through the tube walls, either from tube side to shell side or vice versa.
The fluids may be either liquids or gases on either the shell or the tube side. In
order to transfer heat efficiently, a large heat transfer area is generally used, requiring
many tubes, which are usually disposed horizontally inside the tank-like shell
structure (Al-Hadrami, 2009). The major components of this exchanger are tubes (or
tube bundle), shell, stationary or front end head, rear end head, baffles, and tube
sheets (R.K.Shah et.al, 2004).
b) Plate heat exchanger
Plate heat exchangers consist of a stack of parallel thin plates that lie between heavy
end plates. Each fluid stream passes alternately between adjoining plates in the stack,
exchanging heat through the plates. The plates are corrugated for strength and to
enhance heat transfer by directing the flow and increasing turbulence. These
exchangers have high heat-transfer coefficients and area, and they often provide very
high effectiveness. However, they have relatively low pressure capability (Dean A.
Bartlett).
c) Double pipe heat exchanger
Double pipe exchangers are generally. The heat exchangers usually consist of
concentric pipes. One fluid flows in the inner pipe and the other flows counter
currently in the annulus between the pipes. This is perhaps the simplest heat
exchanger. Flow distribution is better and cleaning process is done easily by
disassembly. Stack of double-pipe or multitube-type heat exchangers are also used in
some process applications with radial or longitudinal fins (R.K.Shah et.al, 2004).
Table 1 below shows the comparisons between three types of heat exchanger.
Table 1: Comparisons between the heat exchanger types
Heat Exchanger
Type
Advantages Limitation
Shell And Tube
Heat Exchanger
Single phases, condensation or
boiling can be accommodated in
either the tubes or the shell, in
vertical or horizontal positions.
A great variety of materials of
construction can be used and may
be different for the shell and tubes.
heavy fouling, corrosive, or viscous
fluids can be accommodated
The equipment is readily
dismantled for cleaning or repair.
Maximum pressure: 500 psig
(35 bar) maximum operating
temperature: 260°C (500°F)
Require large plot (footprint)
area to remove the bundle
The cost of the heat transfer
surface is relatively high
Plate Heat
Exchanger
it can be fully disassembled for
cleaning
high overall heat transfer coefficient
low heat transfer surface cost in
$/ft2
Maximum pressure: 360psig
(2.5MPa)
Maximum operating
temperature : 260°C (500°F)
Pressure drop is very high
compared to shell and tube
exchanger
Not suitable for erosive
duties.
Double Pipe
Heat Exchanger
Easy to obtain counter-current flow
Can handle high pressure
Easy to maintain
The straight length is limited
to a maximum of about 20 ft
used for the small capacity
application where the total
heat transfer surface area is
50m2 (500ft2) or less
By considering all the specifications of each type of the heat exchanger, shell and tube
heat exchanger have the high specification in terms of high temperature and pressure
also its material which easily to maintain compared to the other type of heat
exchanger.
3.3.2.1 Types of Shell and Tube Heat Exchanger
There are various types of shell and tube heat exchanger used in industries, each of it
can give its own advantages and disadvantages. In shell and tube heat exchanger,
basically it has three common types which are U-tube, fixed plates and floating head.
Table 2 below shows the different specification of each type from shell and tube
exchanger.
Table 2: Types of shell and tube heat exchanger (Coulson & Richardson, volume 6,
1999)
Types Advantages Limitation
Fixed tube
sheet design
The simplest and cheapest type of
shell and tube exchanger
The tube bundle cannot be
removed for cleaning
There is no provision for
differential expansion of the shell
and tubes.
This type is limited to temperature
differences up to about 80°
U-Tube Cheaper than the floating-head
types
The tubes and bundle are difficult
to clean
It is also more difficult to replace a
tube in this type
Floating
head
More versatile than fixed head
and U-tube exchangers
They are suitable for high-
temperature differentials
The tubes can be rodded from end
to end and the bundle removed
Easier to clean and can be used
for fouling liquids
The clearance between the
outermost tubes in the bundle and
the shell roust be made greater
than in the fixed and U-tube
designs
From the Table 2, it can be conclude that, the best type of shell and tube exchanger is
floating head type heat exchanger. Floating type has the highest specification among
the other types. This type of heat exchanger can be apply in the chemical and
hydrocarbon condenser plant, thus the maintenance for this heat exchanger are cheap
and easy due to its compatibility compartment. When dealing with cleaning, the tube
and shell in floating head can be done using the chemical and mechanical cleaning
method thus increased its efficiency as one of the heat exchanger medium. An STHE
is divided into three parts, the front head, the shell, and the rear head. Figure 2
illustrates the TEMA nomenclature for the various construction possibilities
Figure 2: TEMA designations for shell-and-tube heat exchangers (Rajiv Mukherjee,1998)
Table 3: The summarize of selection on type of shell and tube heat exchanger
Type Reason
Front end stationary
head types
Type A: channel and
removable cover
Removable cover without
breaking the flanges
Shell types Type E : one pass shell The most commonly used
Excellent for
application
Type S : floating head with
backing devices
Figure 3: Internal floating head with clamp ring (based on figures from BS 3274: 1960)
Tube side selection
a) Dimension
Steel tubes for heat exchangers are covered by BS 3606. Table 4 show the selection on
tube side characteristic.
Table 4: Selection On Tube Characteristic (Sinnott And Towler,5th Edition)
Tube Characteristic Reason
Material of construction Stainless steel The composition of the fluid is
corrosive
Tube length selected 5.00 meter It provides an adequate heat
transfer surface area and pressure
drop is below the allowable
pressure drop
The outer diameter selected , D
The inner diameter selected , Di
19 mm
14.83 mm
Its common tube used
Allocation stream Stream 8
b) Tube arrangements
The tubes in heat exchanger are usually arranged in an equilateral triangular, square, or
rotated square pattern, see figure 4. For the design of this shell and tube exchanger, the
square pitch pattern was use due to the heavily fouling fluids in the shell side, where it is
necessary to mechanically clean the outside of the tubes
Figure 4: Tube pattern
c) Tube-side passes
The fluid in the tube is usually directed to flow back and forth in a number of "passes"
through groups of tubes arranged in parallel, to increase the length of the flow path. The
number of passes is selected to give the required tube-side design velocity. The 2 side
passes was selected in this design.
d) Shells sides selection
The shell diameter must be selected to give as close a fit to the tube bundle as is practical
to reduce bypassing round the outside of the bundle. 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 tolerances, typical values. Table 5 shows the selection
on shell characteristic. The British standard BS 3274 covers exchangers from 6 in. (150
mm) to 42 in (1067 mm) diameter.
Table 5: Selection on shell characteristic (Sinnott And Towler,5th Edition)
Selected Shell Characteristic
Material of construction Stainless steel The composition of the
fluid is high corrosive
Shell pass 1
Allocation stream Stream 8
3.3.3 Chemical Design of Heat Exchanger
Figure 5: Model input and output of heat exchanger, E-102
Table 6: Properties of inlet and outlet process stream of E-102
Parameter Cold (tube side) Hot (shell side)
Temperature in (°C) 32 255
Temperature out (°C) 200 216
Mass flow rate (kg/h) 2775 5990
Cp (kJ/kg°C) 1.878 90.89
Phase liquid liquid
**(IMPORTANT NOTE: The properties of each stream are abstracted from ASPEN
simulation due to the unavailability of chemical data and properties for certain
component for the streams at various temperature and pressure.)
Assumptions:
1. Heat exchanger is insulated from its surrounding, in which case only heat
exchange is between the hot fluid and the cold fluid.
2. Axial conduction along the tubes is negligible.
3. Potential energy and kinetic energy changes are negligible.
4. Fluid specific heat is constant.
5. Overall heat transfer coefficient is constant.
3.3.3.1 Determination of Physical Properties
Table 7: Physical properties of the shell side fluid (Steam)
Properties Inlet Outlet Mean
Temperature (°C) 110.0 92.0 101.0
Specific Heat
(kJ/kg°C) 4.380 4.320 4.350
Thermal Conductivity
(w/m°C) 0.043 0.027 0.035
Density (kg/m3) 875.660 452.080 663.870
viscosity (Ns/m²) 0.0001 0.0001 0.0001
Table 8: Physical properties of tube side fluid (mixture of methyl palmitate and butane)
Properties Inlet Outlet Mean
Temperature (°C) 24.000 100.000 62.000
Specific Heat
(kJ/kg°C) 1.878 2.487 2.183
Thermal Conductivity
(w/m C) 0.104 0.072 0.088
Density (kg/m3) 865.800 733.000 799.400
viscosity (Ns/m²) 0.817 0.398 0.607
**Notes: all of the value was get from the ASPEN simulation
3.3.3.2 Heat Load in Shell and Tube Side
Calculation Value Unit Heat load,
Q=m×Cp s×(t 2−t 1)
Where,Q=27753600
× 2.183 ×(100−24)
m = mass flow rate (kg/h)
Cp = specific heat (kJ/kg.°C )
127.89 kW
3.3.3.3 Overall coefficient
For E-102, the overall coefficient is in range of 350 - 950 W/m2°C. Refer figure 12.1.
Thus, take the first U, assume as 350 W/m2°C.
3.3.3.4 Type and dimension
T1 – t2
T2 – t1
Log mean temperature Difference, LMTD
(T1−t2 )−(T2−t1 )
ln (T 1−t 2
T 2−t 1
)=
(110−100 )−( 91.94−24 )
ln( 110−10091.94−24 )
Where,
T1 : inlet shell-side fluid temperature,
30.24
°C
T2 : outlet shell-side fluid temperature ,
t1 : inlet tube-side temperature,
t2 : outlet tube-side temperature
R=T 1−T 2
t 2−t 1
=110−91.94100−24
S=t2−t1
T 1−t 1
=100−24110−24
*R : two dimensionless temperature ratios
*S : measure of the temperature efficiency of the
exchanger
0.238
0.884
Refer figure 12.19, the temperature correction factor, Ft
Thus mean temperature , Tm = Ft x LMTD
¿0.56 ×30.24
0.56
16.93 °C
The following assumptions are made in the derivation of the temperature correction
factor, Ft in addition to those made for the calculation of the log mean temperature
difference:
1. Equal heat transfer areas in each pass
2. A constant overall heat-transfer coefficient in each pass
3. The temperature of the shell-side fluid in any pass is constant across any
cross section
4. There is no leakage of fluid between shell passes
3.3.3.5 Heat Transfer Area
Heat transfer area
A0=Q
U 0× LMTD=
127.89 × ( 103 )350×16.93
21.58m2
3.3.3.6 Layout and Tube Size
Length, L 5.0 m
Outer diameter, D0 0.01905 m
Inner diameter, Di 0.01483 m
Tube pattern Square
Tube Pitch, pt 1.25 ×0.01905
=0.024
m
3.3.3.7 Number of Tubes
Area of one tube:
Assumption : neglect the thickness of tubeA=π × D0× L
¿ π× 0.01905 ×5
0.299 m2
Number of tubes,
N t=A0
A=21.58
0.299Number of tube per pass,
N p=N t
2=72
2From Table 12.4,
Number of passes,K1
n1
72
36
2
0.249
2.207
Tube cross sectional area,
At=π4
× Di2=
π4
× 0.014832
Area per pass,
0.00017
0.00623
m2
m2
Ap=N p× A t=36 ×0.000173.3.3.8 Bundle and Shell Diameter
From Table 12.4, for 2 tube passes: K1= 0.249 n1= 2.207Bundle diameter,
Db=d0×[ N t
k1 ]1
n1=0.01905 ×[ 72
0.249 ]1
2.207
For a Pull-through floating head exchanger the typical shell clearance from Figure 12.12 is 88 mm, so the shell inside diameter
Ds = 0.248 + 0.088 = 0.243 m
So, the shell inside diameter was following the British standard BS 3274, because it’s still in range of the BS.
0.248
0.337
m
m
3.3.3.9 Tube-Side Heat Transfer Coefficient
Volumetric flow rate at tube side,
V t=mt
ρ= 2775
799.4 × 3600Where,mt = mass flow rate in the tube side (kg/s)ρ = density in the tube side (kg/m3)
0.001 m3/s
Tube side velocity,
U t=V t
A p
= 0.00 10.00623 1.0 m/s
Reynolds number, ℜ=ρ× U t × d0
μ
¿ 799.4 ×1 ×0.014830.000608
19516.18
Prandtl number,
Pr=Cp × μ
k f
=(2.183 ×103 )× 0.000608
0.0881
15.05
Where, kf = Thermal Conductivity (w/m °C)
LDi
= 50.01483 337.15
Refer to Figure 12.23, tube side heat transfer factor, jh = 0.004
Heat transfer coefficient for tube side,
hi=jh×ℜ× Pr0.33× k f
d i
¿ 0.004 ×19 516.18 ×15.0 50.33 ×0.0880.01483
1191.52W/m2.°C
3.3.3.10 Shell-Side Heat Transfer Coefficient
Baffle spacing,
Bs=D s
5=0 .337
5 0.067 m
For square tube pitch,pt=1.25 × D0
¿1.25 ×0.019050.024 m
Shell cross flow area,
A s=p t−D0
pt
× D s× B s
¿0.024−0.01905
0.024× 0 .337×0.0 67
Where pt = tube pitch, D0= tube outside diameter,Ds = shell inside diameter, m,Bs = baffle spacing, m.
0.005 m2
Equivalent diameter,
de=1.10D0
×( pt 2−0.917 do
2)
¿ 1.100.01905
×(0.0242−(0.917 ×0.019052))0.014 m
Volumetric flow rate on shell side,
V s=ms
ρ= 590 0
663.87 × 36000.002 m3/s
Shell side velocity,
U s=V s
A s
= 0.0020.00 5
0.545 m/s
Reynolds number,
ℜ=ρ× U s ×de
μ
¿ 663.87 ×0.545 ×0.0140.0001 48941.43
Prandtl number,
Pr=Cp × μ
k f
=( 4.35 × 103 )× 0.0001
0.03 5
12.47
Baffle cut chosen , 25%Refer to Figure 12.29, shell side heat transfer factor, jh= 0.003Heat transfer coefficient for cold stream,
hs=jh ×ℜ× Pr0.33 ×k f
de
¿ 0.00 3× 48941.43 ×12.470.33× 0.0350.01 4
870.56 W/m2.°C
3.3.3.11 Overall Coefficient
Outside fluid film coefficient, h0
Inside fluid film coefficient, hi
Outside fouling factor coefficent, hod
Inside fouling factor coefficent, hid
Thermal conductivity of the tube wall material, kw
Tube inside diameter,d0
Tube outside diameter, di
870.561191.52
3000500058
0.014830.01905
W/m2 ºCW/m2 ºCW/m2 ºCW/m2 ºCW/m ºC
mm
Calculated overall heat transfer coefficient,
1U 0 ,calc
= 1h0
+ 1hod
+d0 ln
d0
di
2 k w
+[ d0
d i
×( 1h i
+ 1hid
)]Thus ,
U0=1
1870.56
+1
3 000+
0.01483 ln0.014830.01905
(2×58 )+[ 0.01483
0.01905×( 1
1191.52+
15000 )]
350 W/m2 ºC
The U0,calc value is same the initial estimate of 350 W/m2.°C, thus the design has adequate area for the duty.
3.3.3.12 Tube Side Pressure Drop
Reynolds number,
ℜ=ρ× U t × d0
μ
¿ 799.4 ×1 ×0.014830.00060 7
19516.18
From Figure 12.24,tube side friction factor, jf =0.004
Tube side pressure drop,Assumption: neglect viscosity correction
∆ pt=N p [8 jf [ LDi ] [ μ
μW ]−0.14
+2.5] ρ U t2
2
∆ pt=N p [8 jf [ LDi ]+2.5] ρU t
2
2
¿2 [ (8 ×0.004 × 337.15 )+2.5 ] [ 799.4 ×12 ]
*Within the specification for tube side fluid pressure drop
11054.410.1111.05
N/m2bar kPa
3.3.3.13 Shell Side Pressure Drop
Reynolds number,
ℜ=ρ× U s ×d0
μ
¿ 663.87 ×0. 545 ×0.0140.0001
48941.43
Baffle cut chosen, 25%
From Figure 12.30, shell side friction factor, jf = 0.035
Shell side pressure drop,
Assumption : neglect viscosity correction
∆ P=8 jf (D s
de)( L
I b)( ρ U s
2
2 )( μμw
)−0.14
∆ P=(8×0.035 )( 0.3370.014 )( 5
0.067 )( 663.87 × (0.5452 )2 )
155204.48
1.6
155.20
N/m2
bar
kPa
3.3.3.14 Conclusion
The proposed design is satisfied. With the pressure drop on shell and tube side is both
below the allowed pressure drop. The heat exchanger use are shell and tube exchanger
(pull through, floating head, one shell pass, two tube passes).Therefore there is some
scope for improving the design.
Table 6: Summarize of Chemical Design for Heat Exchanger
Parameter Value Units
Process condition,
Heat load, Q
Heat transfer coefficient,
U0,calc
127.89
350
kW
W/m2.°C
Tube side (hot stream)
Inlet temperature, T1
Outlet temperature, T2
Flow rate
Outside diameter, D0
Inside diameter, Di
Pitch, pt
Number of tube
Pressure drop, Δpt
24
100
2775
0.01905
0.01483
0.024
72
11.05
°C
°C
Kg/hr
m
m
m
kPa
Shell side (cold stream)
Inlet temperature, t1
Outlet temperature, t2
Flow rate
Shell inside diameter, Ds
Passes, Ns
Pressure drop, ΔPs
110
92.0
5900
0.337
2
51.03
°C
°C
Kg/hr
m
kPa