Design and Construction

BY- Prerona Das 10010744

Pritom Sharma 10010745

Rijumoni Boro 10010746

Rohitash Meena 10010747

Introduction A heat exchanger is a device in which two fluid streams, one

hot and another cold, are brought into ‘thermal contact’ in order to effect transfer of heat from the hot fluid stream to the cold.

It provides relatively large area of heat transfer for a given volume of the equipment.

They are in frequent use in the chemical process industries as well as in the refrigeration,cryogenic,waste-heat recovery, metallurgical and manufacturing applications.

The driving force for the operation of a heat exchanger is the temperature difference between the fluids.

The Indian code for heat exchanger design is IS 4503 and the British code is BS 3274.

The heat exchanger ‘design code’ for mechanical design calculations is TEMA (US code).

Basis of classification

Contactingtechnique

Construction

Surface compactness

Flow

arrangement

Classification of heat exchangers

H

Indirect contact

Heat

exchangers

Direct contact

Tubular [double-pipe, shell and tube, spiral tube]

Plate [plate and frame (gasketed and welded plate),

spiral plate]

Extended surface [plate-fin, tube-fin]

Regenerative

Single pass [parallel flow, counter-flow, cross –flow]

Multi-pass [parallel flow, counter-flow, split-flow,

divided flow]

Non-compact [surface area density < 700 m2/m3]

Compact [surface area density > 700 m2/m3]

Contacting technique

Construction

Flow arrangement

Surface compactness

Fig: Classification of heat exchangers

TEMAHeat

ExchangerTubular Exchanger Manufacture’s

Association(TEMA) is the most

widely used ‘standard’ or

‘stipulated’ heat exchanger ‘design

code’.This is a US code and is

used together with ASME Section

VIII(for the design of unfired

pressure vessels).The TEMA code

specifies the mechanical design

procedure, tolerances allowed and

the dimensions of the various parts

of an exchanger.

Front Head Type

A-Type B-Type C-Type

Shell type

E-Type F-Type

J-Type K-Type

Rear End Head Types

M-Type

Fixed Tubesheet

S-Type

Floating Head

T-Type

Pull-Through

Floating Head

Double pipe heat exchanger

A typical double pipe heat

exchanger basically consists of a

tube or pipe fixed concentrically

inside a larger pipe or tube.

They are used when the flow rates

of the fluids and the heat duty are

small (less than 500 kW).

These are simple to construct, but

may require a lot of physical space

to achieve the desired heat transfer

area.

Construction of double pipe

Straight construction

It has single sections of inner and outer pipes.

It requires more space.

Hairpin construction

It has two sections each of the inner and outer pipes.

It is more convenient because it requires less space.

Several hairpins may be connected in series to obtain large

heat transfer area.

All the return bends of the inner pipe are kept outside the

jacket and do not contribute to the heat transfer area.

Hairpin heat exchanger

ComponentsPacking & gland

The packing and gland provides sealing to the annulus and support the inner pipe.

Return bendThe opposite ends are joined by a U-bend through welded joints.

Support lugsSupport lugs may be fitted at these ends to hold the inner pipe

position.

FlangeThe outer pipes are joined by flanges at the return ends in order that the assembly may be opened or dismantled for cleaning and maintenance.

Union jointFor joining the inner tube with U-bend.

Flow arrangements

Co-current flow

(Fluids flow in same direction)

Counter-current flow

(Fluids flow in opposite direction)

The Dirt factor or Fouling factor

Deposition of any undesired material on heat transfer surfaces

is called fouling, and the heat transfer resistance offered by the

deposit is called the fouling factor or dirt factor, commonly

denoted by Rd.

Fouling increases the overall thermal resistance and lowers the

overall heat transfer coefficient of heat exchangers.

The fouling factor is zero for a new heat exchanger.

It can be only be determined from experimental data on heat

transfer coefficient of a fouled exchanger and a clean

exchanger of similar design operated at identical conditions.

Types of fouling

Chemical fouling Corrosion fouling

Crystallization fouling Biological fouling

Log MeanTemperature

evaluation

1

2

12

lnT

T

TTTLn

∆ T1

∆ T2

∆ A

A

1 2T 1 T 2

T 4 T 5

T 6T 3

T 7

T 8 T 9

T 10

P ara ll e l Fl ow

731 TTTTT in

c

in

h

1062 TTTTT out

c

out

h

Co-current flow

T1

A

1 2

T2

T3

T6

T4 T6

T7T8

T9

T10

Wall

T 1T 2

T 4 T 5

T 3

T 7 T 8 T 9

T 10

T 6

Co un t e r - C u r re n t F l ow

1062 TTTTT in

c

out

h

731 TTTTT out

c

in

h

Counter-current flow

Log Mean Temperature

Difference Correction Factor

The Logarithmic Mean Temperature Difference(LMTD) is

valid only for heat exchanger with one shell pass and one

tube pass. For multiple number of shell and tube passes

the flow pattern in a heat exchanger is neither purely co-

current nor purely counter-current. Hence to account for

geometric irregularity, Logarithmic Mean Temperature

Difference (LMTD) has to be multiplied by a Mean

Temperature Difference (MTD) correction factor(F) to

obtain the Corrected Mean Temperature Difference

(Corrected MTD) or the effective driving force.

Where,

LMTD = Log mean temperature differenceCLMTD = Corrected Log mean

temperature differenceF = Correction factorTh1 = hot fluid inlet temperatureTh2 = hot fluid outlet temperatureTc1 = cold fluid inlet temperatureTc2 = cold fluid outlet

temperatureN = number of shell passes = shell

passes per shell x number of shell units in seriesP = temperature efficiencyR=capacity ratio

X=temperature ratio

Overall Heat Transfer

coefficients Calculate convective heat transfer coefficient for tube side (hi).

Calculate convective heat transfer coefficient for shell side (ho).

Outside surface area of tube (Ao)

Inside surface area of tube (Ai )

Mean surface area (Am)

Based on the outside tube area, clean overall heat transfer coefficient becomes

1/Uo = 1/ho + (Ao/Am) x (ro - ri / kw) + Ao/Ai(1/hi)

Based on the outside tube area,the relation for the overall heat transfer coefficient becomes

1/Ud = 1/ho +Rdo + (Ao/Am) x (ro - ri / kw) + (Ao /Ai) x Rdi +Ao /Ai(1/hi)

Energy Balance and Heat dutyThe Heat transfer rate taking into account the fouling or the dirt factor and LMTD correction factor is as follows:

Q = UdAFT∆Tm

Where,

Ud = the overall heat transfer coefficient that takes into

account the fouling or the dirt factor Rd.

FT ∆Tm = the true temperature difference.

If U is the clean overall coefficient, then by addition of heat resistances, we have

1/Ud = (1/U) + Rd

Overall resistance of the fouled exchanger = overall resistance of the clean exchanger + heat transfer resistance due to dirt or scaling on both sides of the tube.

An overall heat balance for the counter current double-pipe exchanger

may be written as follows:

Q=WcCpc(Tc1-Tc2) = Wh Cph(Th1-Th2)

Where, c=cold fluid T=Temperature

h=hot fluid Q=Heat duty or load duty of exchanger

Cp=Specific heat W=Flow rate of a stream

In this calculation, the heat exchange (gain or loss) with the ambient

medium, if any, is neglected.

Pressure drop calculations

where,f = friction factor

Gt = mass velocity of the fluid

L = length of the tube, m

g =9.8m/s2

pt = density of tube fluid

di= inside diameter of tuben =the number of tube passes

Φt = dimensionless viscosity ratio∆Pt =pressure drop

Φt=(viscosity at bulk temperature/viscosity at wall temperature)^mwhere m=0.14 for Re > 2100 and m= 0.25 for Re < 2100

Tube-side pressure drop

In a multi-pass exchanger, in addition to frictional loss the head

loss known as return loss has to be taken into account.

The pressure drop owing to the return loss is given by-

Where,

n=the number of tube passes

V=linear velocity of the tube fluid

The total tube-side pressure drop is

∆PT = ∆Pt + ∆Pr

Shell-side pressure drop

For an unbaffled shell the following equation may be used

Where,

L=shell length, m

N=number of the shell passes

ps=shell fluid velocity, m/s

Gs=shell-side mass velocity, kg/m2 s

DH=hydraulic diameter of the shell, m

Φs=viscosity correction factor for the shell-side fluid

Where,

do=the outer diameter of the tube, m

Ds=the inside diameter of the shell, m

Nt=the number of tubes in the shell

and

For a shell with segmented baffles,

Where,

Nb=the number of baffles

DH=the hydraulic diameter of the shell, m

The Reynolds number of the shell-side flow is given by

The Design Procedure Calculate the log mean driving force, LMTD.

Select the diameters of the inner and outer pipes.If the

allowable pressure drops for the individual streams are

given,they may provide a basis for selection of the pipe

diameters.

Calculate the inner fluid Reynolds number; estimate the

heat transfer coefficient hi from the Dittus-Boelter

equation or from jH factor chart.

Nu = hidi/k = 0.023(Re)0.8(Pr)0.3

Calculate the Reynolds number of the outer fluid flowing through the annulus.Use the equivalent diameter of the annulus.Estimate the outside heat transfer coefficient ho

using the equation or the chart mentioned above.

Calculate the clean overall heat transfer coefficient; calculate the design overall coefficient Ud using a suitable value of the dirt factor.

Calculate the heat transfer area A(for a counter flow double-pipe exchanger LMTD correction factor, F=1 Determine the length of the pipe that will provide the required heat transfer area.If the length is large use a number of hairpins in series.

Calculate the pressure drop of the fluids.Use the Reynolds number calculated above to determine the friction factor.

Shell and tube heat exchanger

Shell and tube heat

exhangers are one of the

most common heat exchange

equipment found in all plants.

They are the most versatile

type of heat exchangers.

This type provides a large heat transfer surface in a small space.

They can operate at high pressures, are easy to clean and can be made

of a wide variety of materials.

Components

Shell

fluid inTube

fluid out 1516

1-Channel cover

2-Stationary head channel

3-Channel flange

4-Pass partition plate

5- Tube sheet

6-Shell flange

7-Tube

8-Shell

9-Baffles

10-Floating head backing device

11-Floating tube sheet

12-Floating head

13-Floating head flange

14-Stationary head bonnet

15-Heat exchanger support

16-Shell expansion joint

Shell-fluid

nozzle

Tube

fluid in

Shell fluid

out

The shell [item 8]The shell is the enclosure and passage of the shell-side fluid.

It has a circular cross-section.

The selection of the material depends upon the corrosiveness of

the fluid and the working temperature and pressure.

Carbon steel is a common material for the shell under moderate

working conditions.

The tubes [item 7]The tubes provide the heat transfer area in a shell and tube heat

exchanger.

Tubes of 19mm and 25mm diameter are more commonly used.

The tube wall thickness is designated in terms of BWG

(Birmingham wire gauge).

Tubes are generally arranged in a triangular or square pitch.

The tube sheets [item 5]The tube sheets are circular, thick metal plates which hold the

tubes at the ends.

The arrangement of tubes on a tube sheet in a suitable pitch is

called tube-sheet layout.

Two common techniques of fixing the ends of a tube to the tube

sheet are: (i)expanded joints and (ii) welded joints.

A few common joints between the tube and the tube sheet:

(a)Grooved joint (b)Plain joint (c)Belled or beaded joint (d)Welded

joint

The bonnet and the channel [item 14 and 2]The closure of heat exchanger is called bonnet or channel

depending upon its shape and construction.

A bonnet has an integral cover and a channel closure has a

removable cover.

The bonnet closure consists of a short cylindrical section with a

bonnet welded at one end and a flange welded at the other end.

The bonnet-type closure is replaced by a channel-type closure if a

nozzle is required to be fitted.

The pass partition plate [item 4]The channel is divided into compartments by a pass partition

plate.

The number of tube and shell-side passes can be increased by

using more pass partition plates for both the sides.

The number of passes in either the shell or the tube side indicates

the number of times the shell or the tube side fluid traverses the

length of the exchanger.

For a given number of tubes, the area available for flow of the

tube-side fluid is inversely proportional to the number of passes.

An even number of passes on any side is generally used (For

example,1-2,1-4,2-4,2-6 etc; 1-3,2-5 etc are not used).

2-4 pass heat exchanger

1-2 pass heat exchanger

NozzlesNozzles are small sections of pipes welded to the shell or the

channel which act as the inlet or outlet of the fluids.

The shell-side inlet nozzle is often provided with an

‘impingement plate’.

The impingement plate prevents impact of the high velocity inlet

fluid stream on the tube bundle.

Fig: Two types of impingement

plates.

A-The plates

B-Expanded nozzle

C-Nozzle flange

Baffles [item 9]A baffle is a metal plate usually in the form of the segment of a

circle having holes to accommodate tubes.

Segmental baffle is the most popular type of baffle.

Functions of shell-side baffles-(i)to cause changes in the flow

pattern of the shell fluid creating parallel or cross flow to the tube

bundle and (ii)to support the tubes.

A few types of baffles:

Disc and doughnut baffle

Rod baffle

Baffle cut

Baffle cut orientation

Segmented baffles

Tie rods and baffle spacersTie rods having threaded ends are used to hold the baffles in

position.

The baffle spacers maintain the distance or spacing between

successive baffles.

Flanges and gaskets [item 13]The flanges fixes the bonnet and the channel closures to the tube

sheets.

Gaskets are placed between two flanges to make the joint leak-

free.

Expansion joint [item 16]The expansion joint prevents the problem of thermal stress which

may occur when there is a substantial difference of expansion

between the shell and the tubes because of the temperature

difference between the two fluid streams.

Tube Layout

Design Procedure Perform the energy balance and calculate the exchanger

heat duty.

Obtain the necessary thermo physical property at mean

temperature (If the variation of viscosity is large then we

would do the same at the caloric temperature of hot and

cold fluid).

Select the tentative number of shell and tube passes;

calculate the LMTD and the correction factor FT.

Assume a reasonable value of Ud on outside tube area

basis. This is available in the literature.

Select tube diameter, its wall thickness(in terms of BWG or

SWG) and the tube length. Calculate the number of tubes

required to provide the area A calculated above.

Select the type, size, number and spacing of baffles.

Estimate the tube side and shell side heat transfer coefficient.

Calculate the clean overall coefficient U, select the dirt factor,

and then calculate Ud and the area on the basis of Ud.

Now compare the Ud and the area to that assumed earlier. If the

configuration gives 10% excess area than its fine. Otherwise

the configuration has to be changed.

Calculate the tube side and shell side pressure drop. If pressure

drop value is more than corresponding allowable value then

further adjustment in configuration will be necessary.