By: Tanuj Gupta (Summer Intern)

Heat Exchanger Design

October 18, 2014JULY 27, 2015

Agenda

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Introduction to heat exchangers

Types of heat exchangers

Construction of heat exchanger

Key variable for designing a heat exchanger

Other important design parameters

Roadmap to reduce pressure drop and increase heat transfer

Steps involved in designing a heat exchanger

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2

3

4

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Introduction to heat exchangers

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Heat Transfer

Heat exchanger is a device in which two fluid streams, are brought into thermal Contact in order to effect the transfer of heat from hot fluid to cold fluid.

Hot Cold

Thermal contact Direct Indirect

Most heat exchangers are of ‘indirect contact’ type in which hot and cold fluids are separated by a barrier such as tube wall, plate, etc.

&

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Types Of Heat Exchangers

Heat Exchanger

Indirect Contact

Direct Contact

Shell and Tube

Double Pipe

Extended Surface

Single Pass

Multi-Pass

Non-Compact

Compact

Indirect Contact

Indirect Contact

Indirect Contact

Tubular

Plate

Shell & Tube type has been detailed in this

document

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It consists of a shell that encloses shell side fluid.

Shell and tube heat exchangers consist of series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes .

 A set of tubes is called the tube bundle and can be made up of several types of tubes.

Baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length.

Construction of Shell and Tube Heat Exchanger (1/2) 1 2 3 4 5 6 7

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Construction of Shell and Tube Heat Exchanger (2/2)

Auxiliary Components

Relief Valve

Vent Valve

Drain Valve

Isolation Valve

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1

Key decision variables for designing a heat exchanger

Which fluid on which side

Tube arrangement

Shell arrangement

Channel/head types

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3

4

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Which Fluid on which side?

• Dirty fluid, high fouling• High pressure• Expensive materials• Relative temperatures (lowest)

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Tube & Shell arrangement (1/4)Tube arrangement

Fixed Tube sheet U-Tube

– 1st choice, most common

– True counter-current flow possible

– Tubes are replaceable and cleanable

– Any number of tube passes possible

– Expansion Joint is needed in case of large

differential Temperature

– Shell side cleaning not possible

– Favored by big temperature difference

between tube and shell side, allows thermal

expansion

– Bundle is removable

– Only one header, one tube sheet

– True counter-current flow not possible

– Normally only clean tube side fluids

– Tube sheet temperature difference shall be

less than 120°CFloating Head

• Allows thermal expansion

• Facilitates cleaning on both sides

• T per tube pass < 30°C

• Very Costly

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E- Type Shell

F-Type Shell

G-Type

Temperature-cross problem occurs with multi-pass tubes

Pressure drop criteria sometimes is not met.

Relatively high shell side pressure drop.

Leakages across the longitudinal baffle.

Longitudinal baffles are difficult to seal with the shell especially when reinserting the shell after maintenance.

DISADVANTAGES Most commonly used shell type Least expensive and simple geometry True counter-current flow possible with

one tube pass No temperature-cross problem with one

tube pass

Gives pure counter-current flow with two tube passes.

Required less number of shells compared to E-type.

Better thermal efficiency Closer to counter-current flow with even

tube passes Required less number of shells compared to

E-type and longitudinal plate offers better flow distribution

ADVANTAGES + -

Tube & Shell arrangement (2/4)Shell arrangement

Leakage problem should be considered.

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H- Type Shell

J-Type Shell

X-Type Shell

Complicated geometry due to more nozzles and piping

More expensive

Most expensive.

Suitable for high shell side volume flows Low shell side pressure drop Better thermal efficiency Closer to counter-current flow with even

tube passes Required less number of shells

compared to E-type

Low shell side pressure drop Suitable for high shell side volumes Simpler and cheaper than H-type Normally single segmental baffles only

Applicable to very high shell side volume flows

Very low shell side pressure drop

Tube & Shell arrangement (3/4)Shell arrangement

DISADVANTAGESADVANTAGES + -

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Thermal Efficiency is low.

Start with

E

Problem with Pressure drop

Use Double Segmental Baffles

H

G

X

Problem with fouling factor

J

Tube & Shell arrangement (4/4)Shell selection thumb rules

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Solution 1

Solution 2Solution 1

Solution 2

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Channel/head types

Front End

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– Select B type as default

– Select A type if

• tube side is low pressure and

• tube side fluid is dirty (e.g. CW) and

• diameter is > 0.4 m

– Select N type over A type if

• Diameter is large enough for entry/maintenance

C and D Type are used for very high design pressures.

Rear End

Three general types:

L, M, N - Fixed tube sheet

U - U-tube

S, T, P, W - Floating head

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Some Important Design Parameters (1/3)

1. Baffles

2. Tube Layout

3. Tube Pitch

4. No. of Tubes

5. Baffle Spacing

6. Diameter of Shell and Tubes

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Some Important Design Parameters (2/3)

Baffling

BafflesSupporting tubesMaintain VelocityVibration Prevention

Types: Single segmental, Double segmental, Triple segmental.

Baffle spacing

Centre to centre distance b/w bafflesMaximum can be equal to the inner diameter of the shell

Baffle cut

Permit shell side fluid to flow across baffleCan vary between 15% to 45%.Optimum range is 20% to 35%

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Some Important Design Parameters (3/3)

Tube Layout and Tube Pitch

Four tube layout patterns: • Triangular(30), • Rotated Triangular(60), • Square (90) • Rotated Square (45).

Tube pitch: It is the shortest distance between two adjacent tubes.

Triangular Pattern

• More Tubes• High Turbulence• High Heat Transfer Coefficient• Mechanical cleaning of Shell side not possible

Square Pattern

• Less No. of tubes• Less turbulence• Less Heat transfer coefficient.

• Can be used for dirty shell side fluid

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Comparison of Triangular and Square pattern

Tube side– Decrease number of tube passes– Increase tube diameter– Decrease tube length and increase shell diameter and

number of tubes Shell side

– Increase the baffle cut– Increase the baffle spacing– Increase tube pitch– Use double or triple segmental baffles

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Roadmap to reduce pressure drop

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Delta P is proportional to Square of velocity* length of tube* number of tube passes

Increase heat transfer coefficient

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Roadmap to increase heat transfer

Tube Side

Shell Side

Surface Area

• Increase number of tubes• Decrease tube outside diameter

• Decrease the baffle spacing• Decrease baffle cut

• Increase tube length• Increase shell diameter à increased number of tubes• Employ multiple shells in series or parallel

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Heat exchanger design

First decide the TEMA Shell Type

Allocate The fluid on shell side and tube side

First guess and then estimate Tube Count.

Decide the no. of baffles and the baffle spacing. Hence we get tube length.

Keep adjusting the baffle spacing according to the shell dia. Remember CBS<=RBS or FBS

If pressure drop criteria is not met try changing the baffle type .

Remember that increasing tube pitch is the least recommended option.

If pressure drop criteria is still not met then change the shell type and accordingly change everything

Heat Exchanger Designed!!!!

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References

Binay K. Dutta (Heat Transfer)

Haldor Topsøe IDM No. 4-1385/E WBS 53 Rev. 8 Calculation of shell and tube heat exchangers

‘Effectively design shell and tube heat exchangers’ paper Mr. Rajiv Mukherjee (February 1998)

Thank You