Kuppan T. Heat Exchanger Design Handbook

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254

Chapter

the floating tube sheet moves back and forth. The lantern ring is packed on both sides, and

provided with vent or weep holes

that leakage through either should be to the outside.

Number of tube passes is limited to one or two. The tube bundle is removable.

Floating Head, Pull Through Floating Head

In the floating head (T head), pull through head exchanger, a separate head or cover is bolted

to the floating tube sheet within the shell. In this design, the tube bundle can be removed

without dismantling the joints at the floating end. Due to the floating head bonnet flange and

bolt circle, many tubes are omitted from the tube bundle at the tube bundle periphery and

hence it accommodates the smallest number of tubes for a given shell diameter. This results

in the largest bundle-to-shell clearance or a significant bundle-to-shell bypass stream

overcome the reduction in thermal performance, sealing devices are normally required and

the

shell diameter is somewhat increased to accommodate a required amount of surface area.

ideal application for the T head design is as the kettle reboiler, in which there

ample space

on the shell side and the flow bypass stream

is of no concern.

Floating Head with Backing Device

In the floating head

head) with backing device, the floating head cover (instead of being

bolted directly to the floating tube sheet as in the pull through type) is bolted to a split backing

ring. The shell cover over the floating head has

diameter larger than the shell.

a result,

the bundle to shell clearance is reasonable and sealing strips are generally

not

required. The

tube bundle is not removable. Both ends of the heat exchanger must be disassembled for

cleaning and maintenance.

11.4

Differential Thermal Expansion

Means should be identified to accommodate the thermal expansion or contraction between the

shell and the tube bundle due to high mean metal temperature differentials between the shell

and the tube bundle. This is particularly true for fixed tube-sheet exchangers. Differential

thermal expansion is overcome by the following means in the shell and tube heat exchangers:

U-Tube Design.

The U-bend design allows each tube to expand and contract independently.

Fixed Tube-Sheet Design.

For fixed tube-sheet exchangers, when the difference between shell

and tube mean metal temperatures becomes large (greater than approximately

50°C

for carbon

steel), the tube-sheet thickness and tube end loads become excessive [29]. When a thermal

expansion problems exists, an expansion joint is incorporated in the shell. This permits the

shell to expand and contract.

Flouting Head Designs.

The floating head exchangers solve the expansion problem by having

one stationary tube sheet, and one floating tube sheet that

free to accommodate the thermal

expansion of the tube bundle.

TEMA CLASSIFICATION OF HEAT EXCHANGERS

TEMA has set up mechanical standards for three classes of shell and tube heat exchangers:

and

[3].

Class

heat exchangers specify design and fabrication of unfired shell and tube

heat exchangers for the generally severe requirements of petroleum and related processing

applications, class

for the generally moderate requirements of commercial and general

pro-

cess applications, and class B for chemical process service. Salient features of TEMA standards

are discussed in Chapter 11. Table

shows some major differences between TEMA classes

and B.

255

Shell

and

Tube Heat Exchanger Design

Table

Major Differences Between TEMA Classes R, C, and B [3]

Item Description Classes Re marks

RCB 1.5

Corrosion allowances

R,C

and in

RCB

2.5

Tube pitch

R,C,B

1.25

or 1.2 times tube

RCB 4.7

Tie rods size

R,C

vs.

{

RCB 5.1

Floating head covers

R,C

Depth vs. tube area

RCB

5.3

Lantern ring design

C,R

Maximum pressure limits

RCB 6.2

Gasket materials

R,C

Pressure limits

RCB 6.3

RCB 6.32

Minimum width of gaskets

Gasket surface flatness

C,B

R,C

Full-face gasket

Tolerances

RCB 6.5

Pass partition gasket

R,C

Confined (or unconfined)

RCB 7.13

Minimum tube-sheet thickness

R,C,B

Function of tube diameter

RCB 7.44

Tube hole grooving

C,R

only above

300

psi

RCB 7.51

Tubehube-sheet

joints

C,R

Length

expansion

RCB 7.6

RCB 7.8

Pass partition grooves

Clad tube-sheets

R,C

C,R

Pressure limit for C, B

I63

RCB 9.11

Channels and bonnets

R,C

Minimum thickness

RCB 9.22

Channel cover grooves

W,B

Pass partition, minor

RCB 11.1

End-flange minimum bolt size

W,B

IL!

Source:

Ref.

30.

SHELL AND TUBE HEAT EXCHANGER SELECTION

The fixed tube-sheet exchanger is usually the cheapest. If, however an expansion joint has to

be used, then a U-tube exchanger may prove cheaper. If the bundle has to

removable, a

U-tube will be the cheapest.

TEMA SYSTEM

FOR

DESCRIBING HEAT EXCHANGER

TYPES

Major components

shell and tube heat exchanger are the front head, shell section, and rear

head. Each of these components is available in a number of standard designs.

TEMA stan-

dards, they are identified by an alphabetic character. A heat exchanger unit is designated using

the designations of front head, shell, and rear head. It consist of three alphabetic characters,

such as AES, AKT, AJW,

BEM,

AEP, and

CFU.

Seven major types of shells, five types

front heads, and eight types of rear heads are shown in Fig. 24. In addition to these types,

special types of shells and heads are also available depending upon the applications and cus-

tomer needs.

14.1

Shell

Type

Seven types of shells are standardized by TEMA

[3].

They are:

One-pass shell

Two-pass shell with longitudinal baffle

Split flow

Double split flow

Divided flow

Kettle type reboiler

Cross flow

Chapter

FRONT-END RUR-END

STATIONARY HEAD

TYPES

SHEU

TYPES

HEAD

TYPES

FIXED TUBESHEET

ONE PASS SHELL

LIKE "A" STATIONARY

HEAD

CHANNEL

AND REMOVABLE COVER

FIXED TUBESHEET

PASS SHELL

"B'

STATIONARY HEAD

WITH LONGITUDINAL BAFFLE

-~~

FIXED TUBESHEFT

1,i-i5

"N"

STATIONARY HEAD

BONNET (INTEGRAL COVER)

XJTSIDE PACKED FLOATING

HEN

DOUBLE SPLIT FLOW

CHANNEL INTEGRAL WITH TUBE

SHEET AND REMOVABLE COVER

FLOATING HEAD

WITH BACKING DEVICE

DIVIDED FLOW

CHANNEL INTEGRAL WITH TUBE

SHEET AND REMOVABLE COVER

KETTLE TYPE REBOILER

U-TUBE BUNDLE

EXTERNALLY SEALED

SPKIAL

HIGH

PRESSURE CLOSURI

CROSS FLOW

FLOATING TUBESHEET

Figure

TEMA designation for shells and heads. (Copyright, Tubular Exchanger Manufacturers

Association,

1988.)

brief description of each type is provided next.

TEMA

Shell

The

shell is the most common due to its cheapness, simplicity, and ease

manufacture. It

has one shell pass, with the shell-side fluid entry and exit nozzles attached at the two opposite

ends of the shell. The tube side may have a single pass

multiple passes. The tubes are

----

257

Shell and

Tube

Heat Exchanger Design

supported by transverse baffles. This shell is the most common for single-phase shell fluid

applications. Multiple passes on the tube side reduce the exchanger effectiveness or

LMTD

correction factor

over a single pass arrangement. If F is too low, two

shells in series may

be used to increase the effective temperature difference and thermal effectiveness. Possible

(Fig.

25).

flow arrangements are

El-),

El-*,

and

EISN

TEMA

Shell

The

shell with two passes on the shell side is commonly used with two passes on the tube

side as shown in Fig.

that the flow arrangement

countercurrent, resulting in

factor

1.0.

This is achieved by the use of

shell having a longitudinal baffle on the shell side.

The entry and exit nozzles are located at the same end. The amount of heat transferred is more

than in an

shell but at the cost of an increased pressure drop. Although ideally this is a

desirable flow arrangement, it is rarely used because of many problems associated with the

shell-side longitudinal baffle. They include the following:

There will be a conduction heat transfer through the longitudinal baffle because of the

temperature gradient between the shell-side passes.

If the longitudinal baffle is not continuously welded to the shell,

if seals are not provided

Counter

flow

-----+

Parallel

flow

-1

shell

flow

arrangement.

Flow

arrangement

second

pass

first

pass

._-__

Parallel

flow

T'QQ

Temperature distnbution

Figure

TEMA

E-shell

flow

arrangement and temperature distribution. (a)

El.,;

(b)

E,.?;

and

(c)

(temperature distribution not given).

258

Chapter

SECTION-XX

LONGITUDINAL BAFFLES

Figure

TEMA

shell.

effectively between the longitudinal baffle and the shell, there will

fluid leakage from

the high pressure to low pressure side.

Both the factors will reduce the mean temperature difference and the exchanger effective-

ness more than the gain by achieving the pure counterflow arrangement. The welded baffle

construction has a drawback: It does not permit the withdrawal of the tube bundle for inspec-

tion or cleaning. Hence, if one needs to increase the mean temperature difference, multiple

shells

series are preferred over the F shell.

TEMA

Shell or Split Flow Exchanger

In this exchanger, there is one central inlet and one central outlet nozzle with a longitudinal

baffle. The shell fluid enters at the center of the exchanger and divides into two streams.

Hence,

is also known as a split flow unit. Possible flow arrangements

(GI.I,

Gl.?,

and

G,.J

are shown in Fig.

27.

shells are quite popular with heat exchanger designers for several

reasons. One important reason is their ability to produce “temperature correction factors” com-

parable to those

an F shell with only a fraction of the shell-side pressure

loss

the latter

type

11.

TEMA H Shell or Double Split Flow Exchanger

This is similar to the

shell, but with two inlet nozzles and two outlet nozzles and two

horizontal baffles resulting in a double split flow unit. It is used when the available pressure

drop is very limited. Possible pass arrangements are and

HI-?

(Fig.

28).

The H shell ap-

proaches the crossflow arrangement of the

shell, and it usually has low shell-side pressure

drop compared to E,

and

shells

[2].

TEMA

Shell or Divided Flow Exchanger

The divided flow

shell has two inlets and one outlet or one inlet and two outlet nozzles (i.e.,

a single nozzle at the midpoint of shell and two nozzles near the tube ends). With a single

inlet nozzle at the middle, the shell fluid enters at the center of the exchanger and divides into

two streams. These streams flow in longitudinal directions along the exchanger length and exit

from two nozzles, one at each end of the exchanger. The possible pass arrangements are one

shell pass, and one, two, four,

(even) or infinite tube passes.

shell pass arrangements with

temperature distribution are shown in Fig.

[32].

With a TEMA

shell, the shell-side velocity

will be one-half that

the TEMA

shell and hence the pressure drop will be approximately

one-eighth that of a comparable

shell. Due to this reason, it is used for low pressure-drop

applications like condensing

vacuum

[2].

For a condensing shell fluid, the

shell is used

with two inlets for the gas phase and one central outlet for the condensates and leftover gases.

SHELL

FLUID

,-LONGITUDINAL

BAFlrLE

TUBES

ON SHELL

INLET

SIDE

tU8ES

SHELL

L-m-

SHELL

FLUID

LONGK

UDlNAL

MFFLE

L-

IT'

Figure

shell

flow

arrangement

and

temperature distribution. (a) G,.,;(b)

G,..;

and

(c)

G,,

(for

temperature distribution refer to Ref.

(38)

of Chapter

2).

259

260

Chapter

Figure

shell flow arrangement.

(a)

and

(b)

TEMA

Shell or Kettle Type Reboiler

The

shell is used for partially vaporizing the shell fluid. It is used as a kettle reboiler in the

process industry and as a flooded chiller in the refrigeration industry

[2].

Usually, it consists

of a horizontal bundle

heated U tubes

floating head placed in an oversized shell

(Fig.

30).

The tube bundle is free to move and it is removable. Its diameter is about

50-70%

of the

shell diameter. The large empty space above the tube bundle acts as a vapor disengaging space.

The liquid to be vaporized enters at the bottom, near the tube sheet, and covers the tube bundle;

the vapor occupies the upper space in the shell, and the

dry

vapor exits from the top nozzle(s),

while a weir (the vertical unperforated baffle shown in Fig.

30)

helps to maintain the liquid

level over the tube bundle. The bottom nozzle in this space is used

drain the excess liquid.

TEM

Shell

The

shell (Fig.

31)

is characterized by pure shell-side crossflow.

transverse baffles are

used in the

shell; however, support plates are used to suppress the flow-induced vibrations.

It has nozzles in the middle as in the

shell. The shell-side fluid is divided into many sub-

streams, and each substream flows over the tube bundle and leaves through the bottom nozzle.

Flow maldistributions on the shell side can be a problem unless a proper provision has been

made to feed the fluid uniformly at the inlet. Uniform flow distribution can be achieved by a

Figure

TEMA

shell

flow

arrangement and temperature distribution,

(a)

JI.,;

(b)

J1.:;

and

(c)

JI.+

(From Ref.

32.)

261

Shell

and

Tube

Heat

Exchanger

Design

Figure

TEMA

K-shell.

(Copyright,

Tubular

Exchanger Manufacturers Association,

1988.)

bathtub nozzle, multiple nozzles, or keeping a clear lane along the length of the shell near the

nozzle inlet

[2].

The tube side can be single pass, or two passes, either parallel crossflow or

counter-crossflow. For a given set of conditions, the

shell has the lowest shell-side pressure

drop compared to all other shell types (except the

shell). Hence, it is used for gas heating

and cooling applications and for condensing under vacuum.

Comparison of Various

TEMA

Shells

In general,

and

type shells are suited to single-phase fluids because of the many different

baffle arrangements possible and the relatively long flow path

[7].

When the shell-side pressure

drop is a limiting factor,

and H shells can be used. The

and H shells are not used for

shell-side single-phase applications, since there is no edge over the

shells. They are

used as horizontal thermosyphon reboilers, condensers, and in other phase-change applications.

The longitudinal baffle serves to prevent flashing out of the lighter components of the shell

fluid, helps flushing out noncondensables, provides increased mixing, and helps flow distribu-

tion

[2].

Velocity head and pressure drop for various TEMA shells are given in Table

3 [33].

Figure

TEMA

shell.

262

Chapter

Table

Shellside Velocity Head

and

Pressure Drop

for

Some

TEMA

Shells

[33]

~~ ~ ~ ~ ~~~

TEMA shell type Velocity head Pressure drop

shell

a8V'L

shell

H shell

vi2

aV'W8

shell

vi2

aV'W8

Source:

Ref.

33.

14.2

Front and Rear Head Designs

Head designs can

vary

from plain standard castings to fabricated assemblies with many special

features. Two

the major considerations in the choice of heads are

[4]

(1)

accessibility to the

tubes, and

(2)

piping convenience. Where fouling conditions are encountered or where frequent

access for inspection is desired, a head or cover plate that can be easily removed is an obvious

choice. In this head, connections are located

the sides, not on the ends of removable heads.

Typical open end heads used for this purpose are called channels. They are fabricated from

cylindrical shells and fitted with easily removable cover plates

that the tubes can be cleaned

without disturbing piping.

Designations for Head Types

Front Head.

For the variable front head, the constants and their meanings are:

Channel with removable cover

B Bonnet with integral cover

Channel integral with the tube sheet and removable cover when the tube bundle is remov-

able

Channel integral with the tube sheet and having a removable cover but the tube bundle is

not removable

D Special high-pressure closure

Rear

Head.

Fixed tube sheet like

stationary head

Fixed tube sheet like

stationary head

Fixed tube sheet like

stationary head

Outside packed floating head

Floating head with backing device

Pull through floating head

U-tube bundle

Externally sealed floating tube sheet

Salient features of these heads are discussed and tabulated by Larowski et al.

[7].

SHELLSIDE CLEARANCES

Even though one of the major functions of the plate baffle is to induce crossflow over the tube

bundle, this objective is achieved partially. Various clearances on the shell side partially bypass

the fluid. Bypassing

defined as a leakage flow where fluid from the crossflow stream, in-

Shell

and Tube Heat Exchanger Design

263

tended to flow through the tube bundle, avoids flowing through it by passing through an

alternative, low-resistance flow path [34]. A major source of bypassing is the shell-side clear-

ances. Clearances are required for heat exchanger fabrication. Fabrication clearances and toler-

ances have been established by the Tubular Exchanger Manufacturers Association (TEMA)

[3], and these have become widely accepted around the world and are enforced through inspec-

tion during fabrication. Three clearances are normally associated with a plate baffle exchanger.

They are (1) tube to baffle hole, (2) baffle to shell, and (3) bundle to shell. Additionally, in a

multipass unit, the tube layout partitions may create open passages for the bypass of the cross-

flow stream. Since bypassing reduces heat transfer on the shell side, to achieve good shell-side

heat transfer, bypassing of the fluid must be reduced. These clearances and their effect on the

shell-side performance are discussed in detail next.

15.1

Tube-to-Baff le-Hole Clearance

The holes in the baffles must be slightly larger than the tube outer diameter to ensure easy

tube insertion. The resultant clearance is referred to as the tube-to-baffle-hole clearance. It

should be kept at

minimum

(1)

to reduce the flow-induced vibration and (2) to minimize the

A leakage stream (various shellside bypass streams are discussed next). As per TEMA

RCB-

4.2, where the maximum unsupported tube length is 36 in (914.4 mm) or less, or for tubes

larger than 1.25 in (31.8 mm) outer diameter, the clearance is

(0.80

mm); where the

unsupported tube length exceeds

in for tube outer diameter 1.25 in or smaller, the clearance

in (0.40 mm).

15.2 Shell-to-Baff le Clearance

The clearance between the shell inner diameter and baffle outer diameter is referred to as the

baffle-to-shell clearance. It should be kept to a minimum to minimize the

leakage stream.

TEMA recommend maximum clearance as per RCB-4.3 [3] varies from 0.125 in (3.175 mm)

for 6 in (152.4 mm) shell inner diameter to 0.315 in (7.94 mm) for

in (1524 mm) shell

inner diameter.

15.3

Shell-to-Bundle Clearance

Since the tube bundle does not fill the shell, there

a shell-to-bundle clearance. This allows

the so-called bypass

stream

flowing around the bundle. Sealing strips are used to block this

space and force the bypass stream to flow across the tubes. The use of sealing strips is recom-

mended every five to seven rows of tubes in the bypass stream direction.

15.4

Bypass Lanes

The lanes provided for the tubeside partition ribs is a source of bypass on the shell side. The

pass partition lanes (Fig. 32) are, however, placed perpendicular to the crossflow whenever

possible, or the bypass lanes are usually blocked by tie rods, which act similar to the sealing

strips. Hence, the effect of the bypass lanes on thermal performance is usually can be neglected.

16 DESIGN METHODOLOGY

16.1 Shell-Side

Flow

Pattern

Shell Fluid Bypassing and Leakage

In shell and tube exchangers with segmental plate baffles, the shell-side flow is very complex

due to a substantial portion of the fluid bypassing the tube bundle through various shell-side