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