Kuppan T. Heat Exchanger Design Handbook

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I24

Chapter

.aa

y .

@.*a

0.Qe--

0.85

a.-

8.7s

0.70

0.65

--.--

0.1

0.2

0.3

0.4

8.5

0.6

0.7

0.8

8.9

Thermal effectiveness

Figure

Five

E,.z

shells in series; shell fluid mixed, tube fluid mixed between passes with stream symmetric.

a function of

for constant

(solid lines) and constant NTU (dashed lines).

1.0

125

Heat Exchanger T~e~ohydra~li~ F~nd~~e~tuls

8-3

8.4

8.5

.'I

8*8

Thermal effectiveness

Figure

Six

shells in

series;

shell

fluid

mixed, tube ~uid mixed between passes

with

stream

s~rnrne~~c.

function of

for

constant

(solid

lines)

and

constant

NTU

(dashed Iines).

I26

Chapter

OUTLET

INLET

NO2

ZLC

NO2

ZLE

RETURM

HEADER

PA;

ARRANGEMENT

0'5

0'4

0'1

P-

TEURRATURE

EFFICIENCY

P,,

Thermal effectiveness

0.1

0.4

4.0

u"./w,q

NTUt

Figure

G,.4

split

flow

heat exchanger

[38].

Thermal effectiveness referred to tubeside.

127

Heat Exchanger The rmo

ydrau

ic Fundamen ta

through the partition plates cannot be ruled out. Therefore, this difficulty is overcome by with multiple

shells with basic shell arrangements.

configuration with

shell passes, each one with

tube passes

(similar

Fig.

59),

equivalent

a series assembly of

shells each with

tube passes.

This

illustrated in Fig.

for

El.:.

Multiple

Shells

Series, Each with Two Tube Passes,

E,.:.

Thermal effectiveness charts

for 2E1.?, 3EI.?, 4EI.?, 5E1.?, and

6E1.2

are given in Figs.

61,

62,

63,

64,

and

65.

Thermal relation chart for TEMA

G,4.

Figure

gives details about (i) schematic of

G14

exchanger, (ii) temperature correction factor for

G,,

exchanger, and (iii) thermal effectiveness

for

G'+

exchanger (38).

ACKNOWLEDGEMENT

The author acknowledges Dr.

Shah, Technical Director of Research, Delphi-Harrison

Thermal Systems, for providing the thermal relation formulas for various cases of crossflow

arrangements and shell and tube heat exchangers.

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Heat Exchanger Thermal Design

FUNDAMENTALS

HEAT EXCHANGER DESIGN

METHODOLOGY

Heat exchanger design methodology, shown in Fig.

involves the following major design

considerations

[

11:

Process/design specifications

Thennohydraulic design

Flow-induced vibration in the case of shell and tube heat exchanger and individual fin-

tube and bare tube bank compact heat exchanger

Mechanical design

Cost and manufacturing considerations

Trade-off factors and system-based optimization

Most of these considerations are dependent on each other and should be considered simul-

taneously to arrive at the optimum exchanger design. Of these design considerations, items

and

are discussed in this chapter, flow-induced vibration and mechanical design are discussed

in Chapters

and

11,

respectively, agd item

was

discussed earlier. The remaining item,

trade-off factors and system-based optimization, is not within the scope of this book.

1.1

ProcesdDesign Specifications

Process or design specifications include all the necessary information to design and optimize

the exchanger for a specific application. It includes the following information

[

11:

(1)

problem

specification,

(2)

exchanger construction type,

(3)

flow arrangement,

(4)

construction materials,

(5)

desigdoperatiodmanufacturing

constraints,

(6)

construction code, and

(7)

safety and pro-

tection. Items

and

are discussed next, and the others have been covered elsewhere.

I29

Problem specification

Select Construction type,

flow orrongement,surfoce

selection,fluid allocation,

size and shape

consideration and

consider limitations,if any

Thermo physical fluid

properties

Thermo hydraulic design

Heat transfer and

Modify the

variables

vibration of shellside

the case of shell and

Mechanical

Trade

off

factors

Manufacturing

Considerations and cost

Optimum

estimates

solution

Figure

Heat exchanger design methodology.

130

Heat Exchanger Thermal Design

I31

Problem Specification

The problem specification involves the process parameters, operating conditions, and environ-

ment in which the heat exchanger is going to be operated. Typical details pertaining to problem

specification include design parameters such as inlet temperatures and pressures, flow rates

(including composition for mixtures), vapor quality, heat duty, allowable pressure drops, and

fluctuations in the process parameters; overall size, layout, weight, etc.; and corrosiveness and

fouling characteristics of fluid. Other factors that must be considered are

Climatic conditions-minimum ambient, frost, snow, hail, and humidity

Operating environment-maritime, desert, tropical, seismic, cyclonic, and dust

Site layout-proximity to buildings or other cooling equipment, prevailing wind directions,

duct allowances, length of pipe runs, and access

Exchanger Construction

Based on the problem specifications and experience, the exchanger construction type and flow

arrangement are first selected. Selection of the construction type depends upon the following

parameters

[

11:

Fluids (gas, liquids, or condensing/evaporating) used on each side of a two-fluid exchanger

Operating pressures and temperatures

Fouling

Whether leakage or contamination of one fluid to the other is allowed or not

Cost and available heat exchanger manufacturing technology

Surface Selection

Compact

Heat Exchanger.

Factors that influence the surface selection include the operating

pressures, fouling, maintenance requirements, erosion, fabricability

cost, etc.

Shell and Tube Heat Exchanger.

For shell and tube exchangers, the criteria for selecting core

geometry or configurations are the desired heat-transfer performance within specified pressure

drops, fouling, corrosion, maintenance, repair, cleanability by mechanical means, minimal op-

erational problems (flow-induced vibrations), safety, and cost; additionally, the allocation

fluids on the shell side and the tube side is an important consideration.

1.2

Thermohydraulic Design

Heat exchanger thermohydraulic design involves quantitative heat transfer and pressure drop

evaluation or exchanger sizing. Basic thermohydraulic design methods and inputs to these

analyses are as follows.

Basic Thermohydraulic Design Methods

discussed in Chapter

the

E-NTU,

P-NTUt,

LMTD, or

y-P

method can be used for solving

the thermal design problem.

Thermophysical Properties

For heat-transfer and pressure-drop analysis, the following thermophysical properties

the

fluids are needed: dynamic viscosity

density, specific heat

cp,

surface tension, and thermal

conductivity

For the conduction wall, thermal conductivity is needed.

Surface Geometrical Properties

For heat transfer and pressure drop analyses, at least the following surface geometrical proper-

ties are needed on each side

a two-fluid compact heat exchanger:

I32

Chapter

Heat-transfer area,

which includes both primary and secondary surface area if any

Core frontal area,

Afr

Minimum free flow area,

Hydraulic diameter,

Flow length,

Fin thickness and fin conduction length

Core volume,

and core dimensions

L,,

and

These quantities are computed from the basic dimensions of the core and heat-transfer surface.

On the shell side of a shell and tube heat exchanger, various leakage and bypass

flow

areas

are also needed. The procedure to compute these quantities is presented elsewhere.

Surface Characteristics

Surface characteristics for heat transfer,j, and

flow

friction,f, are key inputs for the exchanger

heat transfer and pressure drop analysis, respectively. Experimental results for a variety

exchanger surfaces are presented in Kays and London

[2],

and correlations in Shah and Bhatti

[3].

More references on

and

factors are furnished in Chapter

Heat Exchanger Specification Sheet

Heat exchanger specification sheet for shell and tube heat exchangers is shown in Fig.

and

for plate-fin heat exchangers in Fig.

DESIGN PROCEDURE

The design of heat exchangers requires the consideration of the factors just outlined. It is

unlikely that any two designers will arrive at exactly the same design for a given set of condi-

tions, as the design process involves many judgments while carrying out the design

[4].

Gollin

[4]

describes the step-by-step design procedure for a heat exchanger. Important steps in the

heat exchanger design procedure include the following:

Assess the heat transfer mechanisms involved.

Select the heat exchanger class.

Determine the construction details and select the surface geometry.

Determine size and layout parameters keeping in mind the constraints imposed by the

purc haserklient

Perform preliminary thermal design known as approximate sizing.

Perform the detailed design.

Check the design.

Optimize the design.

Perform the check for flow-induced vibration in the case of shell and tube heat exchanger,

individually finned

tube,

and bare tube bank compact heat exchangers.

10.

Perform mechanical design.

11.

Estimate cost and finalize the design as per trade considerations.

THERMOHYDRAULIC DESIGN PROGRAM STRUCTURE

One of the pioneers in developing computer software for thermal and mechanical design of

shell and tube exchangers and air coolers

M/s

B-Jac International, Inc., Midlothian,

VA.

insight into the logic of their programs for the design of shell and tube exchangers, and air

coolers is discussed next.

133

Heat

Exchanger Thermal Design

Table

B-Jac HETRAN Technical Specifications

Specification Description

TEMA exchanger types

Front head:

C, N

Shell: E,F,G,H,J,K,X

Rear head: L,M,N,P,S,T,U,W

Special types

Vapor belts

Double tubesheets

Tube types

Plain

Integral low finned tubes

Baffle types

Single segmental

Double segmental

Triple segmental

Full supports

tubes in window

Strip baffles

Rod baffles

Baffle cuts

Horizontal, vertical, rotated

Tube patterns

Triangular

Rotated triangular

Square

Rotated square

Tube passes

(odd

even)

Pass layout types

Quadrant, mixed, ribbon

3.1

Thermal Design Program for Shell and Tube Heat Exchanger

The salient features of the B-Jac HETRAN program for the thermal design of shell and

tube

exchanger are given in Tables

and Table

Additionally, the following features are available:

Table

B-Jac HETRAN Output

Design

summary

comprehensive overview of the most important variables: shell- and

tube-side temperatures, flow rates, coefficients, velocities, and pres-

sure

drops,

plus a concise description of key heat exchanger dimen-

sions and geometry

Performance evaluation Required area

and distribution of resistances in clean, specified fouling,

and maximum fouling conditions

Coefficients

Film coefficients and their components

MTD and flux

LMTD, correction factors, heat fluxes, and heat

flux

limitations

Pressure drop

Clean and dirty pressure drops, velocity and pressure drop distribution

from inlet to outlet

Shell-side flow

Stream analysis flow fractions and

pV’

analysis (per

TEMA)

Construction of tube bundle Baffle

and tube layout specifications

Vibration analysis

Fluid elastic critical velocity and frequency matching for acoustic, natu-

ral, and vortex shedding at inlet, bundle, outlet, and user-specified

spans

Recap of design cases

Concise

summary

of alternative solutions that have been explored and

their costs