Mechanical Constraints on Thermal Design of Shell and Tube Exchangers

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-HEA-512

Mechanical Constraints on Thermal
Design of Shell and Tube Exchangers

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Process Engineering Guide:

Mechanical Constraints on
Thermal Design of Shell and
Tube Exchangers

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

STANDARD DIMENSIONS

3

4.1
4.2
4.3
4.4

Shell Diameters
Tube Lengths
Tube Diameters
Tube Wall Thicknesses

3
4
4
4

5

CLEARANCES

5

5.1
5.2
5.3
5.4
5.5
5.6

Tube Pitch
Pass Partition Lane Widths
Minimum 'U' Bend Clearance
Tube-to-Baffle Clearance
Baffle-to-Shell Clearance
Bundle-to-Shell Clearance

5
6
6
6
6
7


6

TUBESHEET THICKNESS

8

7

END ZONE LENGTHS

12

8

TUBE COUNTS

14

8.1
8.2
8.3
8.4
8.5

Program Correlations
Use of Tube count Tables
Graphical Layout
Use of Computer Programs
Tie Rods

15
15
15
15
15

TABLES

1

2

3

HEAT EXCHANGER SHELLS - GEOMETRICAL DATA
FOR INLET & OUTLET BRANCHES: PIPE WITH ANSI
150 FLANGE

13

HEAT EXCHANGER SHELLS - GEOMETRICAL DATA
FOR INLET & OUTLET BRANCHES: PIPE WITH ANSI
300 FLANGE

14

TEMA TIE ROD STANDARDS

16


FIGURES

1

DEFINITION OF TUBE PITCH, LIGAMENT THICKNESS
& PASS PARTITION LANE WIDTH

5

DEFINITION OF PASS PARTITION LANE WIDTH FOR
U-TUBES

7

BUNDLE TO SHELL CLEARANCES FOR DIFFERENT
BUNDLE TYPES

8

ESTIMATED TUBESHEET THICKNESS FOR FIXED TUBE
CONSTRUCTION

10

ESTIMATED TUBESHEET THICKNESS FOR U-TUBE
CONSTRUCTION

11

6

END ZONE

13

7

EXAMPLE OF OPTU3 GRAPHICAL OUTPUT

16

2

3

4

5

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

17


0

INTRODUCTION/PURPOSE

The thermal design of a shell and tube exchanger is influenced by mechanical
requirements such as materials thicknesses and clearances necessary for
fabrication purposes.
Although these are ultimately determined by the Mechanical Designer, with
reference to the appropriate codes, it is helpful if the thermal designer is aware of
the constraints, to avoid unnecessary recycling in the design process.
1

SCOPE

This document gives guidelines on dimensions to be used in the thermal design
of shell and tube heat exchangers.
Dimensions given in this guide are only for use in thermal calculations.
2

FIELD OF APPLICATION

This Guide applies to the process engineering community in GBH Enterprises
worldwide.
3

DEFINITIONS

For the purposes of this Guide, the following definitions apply:
HTFS

Heat Transfer and Fluid Flow Service. A cooperative research
organization, with headquarters at Harwell, UK, involved in
research into the fundamentals of heat transfer and two phase flow
and the production of design guides and computer programs for the
design of industrial heat exchange equipment

HTRI

Heat Transfer Research Incorporated. A cooperative research
organization, based in the USA, involved in research into heat
transfer in industrial sized equipment, and the production of design
guides and computer programs for the design of such equipment.

TEMA

Tubular Exchanger Manufacturers Association. An organization of
(US) heat exchanger manufacturers. Their publication "Standards
of the Tubular Exchanger Manufacturers Association" is a widely
accepted industry standard.


4

STANDARD DIMENSIONS

In the past, it has been the practice to have a range of standard shell diameters
and tube lengths, based on imperial units. These standards can now be regarded
as obsolete in most cases, and the Engineer can in general specify what is really
wanted. Some Projects or Works may still require the use of standard
dimensions, for reasons such as to reduce the range of spares required. These
should be agreed before commencement of the design.
4.1

Shell Diameters

There is no need to use the Tubular Exchanger Manufacturers Association
(TEMA) standard shell internal diameters, except for carbon steel shells of
diameter less than or equal to 18", where shells can be made from standard
pipework.
For all other materials, and for larger diameter carbon steel shells, the shells are
formed by rolling and welding, so any diameter can be made. However, for small
diameter shells (less than 8") in other materials, it may be worth investigating the
availability of standard pipe.
4.2

Tube Lengths

There is no need to use standard tube lengths in multiples of feet. If a tube length
of 3 m is required, then that length should be specified rather than 3.048 m (10').
Above 12 m, tubing becomes more expensive, but is available; exchangers with
tube lengths up to 75' (22.86 m) have been built for special situations. However,
such tubing may not be available in stock.
For U-tube exchangers in particular, the requirement for standard tube lengths
makes little sense, as before bending the various tubes will be of different
lengths, depending on whether they are for the outside or inside of the bundle.
4.3

Tube Diameters

Heat exchanger tubing is specified on an outside diameter (O.D.) basis. Tubes
are available in either Imperial or Metric sizes, although Metric sizes in the UK
generally command a premium.


Imperial sizes are typically:
-

0.625”, 0.75", 1.0", 1.25" and 1.5" O.D. Of these 0.75" and 1.0” are
preferred, as they are most commonly available.

Typical metric sizes are:
-

16, 20 and 25 mm O.D. But intermediate sizes are available.

The availability of tubing in some exotic materials (e.g. Hastelloy) is limited, and
sometimes only unusual sizes may be readily obtained (e.g. 26.2 mm O.D.).
Vessels Section should be consulted before the thermal design is finalized.
4.4

Tube Wall Thicknesses

Tube wall thicknesses for imperial sized tubes are normally expressed in terms of
Standard Wire Gauge (swg) or Birmingham Wire Gauge (bwg). Usually the even
numbered gauges are preferred. For metric tubing, the equivalent metric size will
be used. It is exceedingly unlikely that the tube thickness will be determined by
the simple pressure containment criteria (i.e. hoop stress); because of the small
diameters used, even very thin walled tube would stand very high pressures. The
decision on what thickness to use is generally determined more by weld details
and longitudinal stresses.
Beware of unthinkingly specifying corrosion allowances of 2-3 mm for the tubes,
as these can result in excessive tube thicknesses. It is normal practice to have
no corrosion allowance for tubes (but see below). If in doubt, Metallurgical
specialist should be consulted.
For carbon steel, the minimum thickness used is normally 14 swg (0.080”, 2.00
mm); in the North West, particularly on cooling water duties where water
corrosion may be expected, 12 swg (0.104”, 2.65 mm) or even 10 swg (0.128”,
3.15 mm) are commonly specified. For stainless steels, typical thicknesses range
from 12 swg (0.104”, 2.65 mm) to 16 swg (0.064”, 1.6 mm). For the more exotic
materials, which will be specified for their low corrosion, thinner tubing is
often used to reduce the cost. Thicknesses down to 22 swg (0.028”, 0.71 mm)
have been used for Hastelloy and tantalum.
Note:
The metric dimensions given above are the 150 metric preferred series
equivalents of the swg sizes, not direct conversions).


5

CLEARANCES

Minimum clearances between the components of the exchanger have to be
maintained either to allow for appropriate weld details, or to accommodate
manufacturing tolerances.
5.1

Tube Pitch

The minimum tube pitch, defined as the distance between the centers of
neighboring tubes, depends on the method of tube to tubesheet attachment.
TEMA recommends a minimum pitch of 1.25 times the tube outside diameter.
However, for tube sizes of less than 1” this would in general only allow an
expanded tube-tubesheet joint. The GBHE standard for all process
exchangers is to use welded tube-tubesheet joints.
The minimum thickness of metal in the tubesheet between adjacent tubes,
known as the 'ligament' (equal to the tube pitch minus the tube O.D.)
The minimum ligament is necessary in part to avoid overlap of the welds for
neighboring tubes (Figure 1). A 6 mm ligament allows a medium integrity weld.
This is suitable for most cases, including exchangers handling toxic or flammable
materials.


FIGURE 1

DEFINITION OF TUBE PITCH, LIGAMENT THICKNESS & PASS
PARTITION LANE WIDTH

For critical duties, where leakage may be catastrophic due to interaction of the
hot and cold streams (e.g. chlorine/water), a 10 mm or 0.375" ligament is
necessary for a high integrity weld, to avoid overlap of the welds from
neighboring tubes. (Some manufacturers may be able to produce high integrity
welds with a 6 mm ligament, using an orbital welding machine, but this
should not be assumed without consulting Vessels Section).
Explosion welding usually also requires a 10 mm ligament; the stresses induced
in the tubesheet during the welding process may cause excessive distortion with
smaller ligaments.

5.2

Pass Partition Lane Widths

A greater ligament is necessary between the outer tubes in neighboring passes
than between the tubes in a pass, to allow for the pass partition plate. This
ligament is known as the pass partition lane width (Figure 1).
The default values assumed by the thermal design programs are too small to
allow for reasonable plate thickness and a welded tube-tubesheet joint (HTFS
use 0.625" in OPTU3, 0.625" - 0.75" in TASC3; HTRI use between 0.625” and
0.875” depending on shell diameter).
A reasonable clearance is 1" or 25 mm.
5.3

Minimum 'U' Bend Clearance

The pass partition lane width for U-tube bundles is determined by the tightness to
which the tubing can be bent. The minimum radius achievable for carbon steel is
generally 1.5 times the tube diameter, giving a centre-centre distance of 3D, and
a pass partition lane width of 2D (Figure 2). Note that this only applies to the
pass partition lane normal to the plane of the Ubend; the other lanes will be
governed by the need for weld clearance as above.
For some materials, for example nickel alloys, it may not be possible to achieve
such tight bends. Further information is available in the standard B163 of the
American Society for the Testing of Materials (ASTM). Vessels section should be
consulted for advice.
5.4

Tube-to-Baffle Clearance

For a maximum unsupported tube length of 36", TEMA recommends a clearance
of 1/32" (0.8 mm). However, this may lead to an excessive tube-to-baffle
leakage, resulting in a low cross flow fraction. A clearance of 0.4 mm, the TEMA
standard for unsupported lengths above 36", is the normal GBHE standard for all
baffle pitches.
5.5

Baffle-to-Shell Clearance

The size of this clearance is governed by the allowable tolerance on circularity of
the shell, particularly if rolled. The values in TEMA, which are the default values
used by the standard exchanger programs, are reasonable.


Note that the quoted tube-to-baffle and baffle-to-shell clearances are those for a
clean exchanger. If the shell side suffers from fouling in service, these clearances
may become blocked by the dirt deposits. When checking designs, calculations
should also be done with zero clearance, as this will give the worst case for
vibration and pressure drop. In some cases, the pressure drop may rise by a
factor of 2 to 4 times the clean value. Some of the computer programs used for
thermal rating interpret a zero clearance in the input as a request for the
default value. In these cases, it is necessary to input a small value, e.g. 0.1 mm.
FIGURE 2

DEFINITION OF PASS PARTITION LANE WIDTH FOR U-TUBES


5.6

Bundle-to-Shell Clearance

The value used for this has an effect on the tube count (see below) and also on
the 'C' stream leakage (bypassing round the outside of the bundle). A high
clearance, with a consequent high leakage, is undesirable as it will in general
seriously affect the exchanger performance.
The minimum value obtainable depends on the type of exchanger. The minimum
clearance is obtained with a fixed tubesheet or U-tube bundle, and the maximum
with a pull through floating head. The values used by the HTRI program ST-4,
which are shown in Figure 3, are reasonable for most purposes.
FIGURE 3

BUNDLE TO SHELL CLEARANCES FOR DIFFERENT BUNDLE
TYPES


6

TUBESHEET THICKNESSES

An estimate of the tubesheet thickness is necessary to determine the length of
tube available for heat transfer, and to calculate the baffle pitch and end zone
lengths.
The precise calculation of the necessary tube sheet thickness is a complex
procedure beyond the scope of this Guide. Consideration should be given to both
bending and shear forces, and a knowledge is required of the mean metal
temperatures for both shell and tube, as well as the pressures, under normal
operation and upset conditions.
The tubesheet thickness for fixed tubesheets can be estimated using Figure 4.
These data were calculated using BS 5500 (1991), assuming all carbon steel
construction, with the tubeplate welded to both shell and channel and the tube
metal temperature 20°C hotter than the shell. Departure from these assumptions
will introduce error.
For U-tube exchangers, the formula for bending is usually the dominant
consideration. BS 5500 gives the following equation:

where:
t

is the tubesheet thickness

C

is a constant (take as 0.66)

Do

is the outer tube limit

EP

is the differential pressure across the tubesheet

]

is a constant (take as 2)

o

is the ligament efficiency = ligament/tube pitch

f

is the design stress (take as yield).


Note:
For carbon steel: f will normally range from 150 N/mm2 at ambient to 120 N/mm2
at 250° C. Figure 5 is based on this formula. (The figures are for guidance only)


FIGURE 4

ESTIMATED TUBESHEET THICKNESS FOR FIXED TUBE
CONSTRUCTION


FIGURE 5

ESTIMATED TUBESHEET THICKNESS FOR U-TUBE
CONSTRUCTION


7

END ZONE LENGTHS

The central baffle pitch can usually be specified simply from hydraulic
considerations to give an acceptable velocity and pressure drop. This does not
apply to the inlet and outlet end spaces, which are influenced not only by the
diameter of the nozzles, but also by the mechanical details of the exchanger.
Tables 1 and 2 give values of minimum end space for a range of nozzle
sizes. These data are intended as a guide for use in thermal design, and as
approximations for branch positioning where no exact dimensions have been
established by detailed mechanical design. Where exact dimensions do exist,
they should be used in preference to these Tables.
The assumptions made in deriving the Tables were:
(a) The branch is a pipe.
(b) Pipe wall thickness is 10 mm in all cases, with dimensions being calculated
on pipe nominal bore.
(c) The tubesheet thickness, including tube standout, is estimated at 50 mm for
Class 150 pressure rating and 75 mm for Class 300 rating. A 15 mm deep spigot
is assumed on the tubesheet back face for the tubesheet to shell weld.
(d) The pipe flange edge is aligned with the back face of the tubeplate flange
extension, as illustrated in Figure 6. The data are principally for tubeplates which
are welded directly onto the exchanger shell. Where the tubeplate is clamped
between a pair of flanges, an additional allowance is required for the flange.
Typical flange thicknesses are between 50!mm and 150 mm for design pressures
between 10 bar and 100 bar.
Notes:
1 Tables 1 and 2 are based on PIPE nozzle walls.
2 The data shown in Tables 1 and 2 will normally be adequate for most thermal
design/approximate orientation purposes, but should be used with caution if:
(a) tubesheet thickness is likely to differ significantly from given values,
(b) detailed dimensional data (e.g. exact end baffle pitch for a construction
drawing) is required.


FIGURE 6

END ZONE


TABLE 1

HEAT EXCHANGER SHELLS - GEOMETRICAL DATA FOR
INLET & OUTLET BRANCHES: PIPE WITH ANSI 150 FLANGE
(50 mm Tubesheet assumed)


TABLE 2

HEAT EXCHANGER SHELLS - GEOMETRICAL DATA FOR
INLET & OUTLET BRANCHES: PIPE WITH ANSI 300 FLANGE
(75 mm Tubesheet assumed)


8

TUBE COUNTS

The number of tubes which can be fitted into a given shell size is determined by
many factors, based on required clearances, as described above. Relevant
factors are:
(a) Tube diameter.
(b) Tube pitch.
(c) Layout angle.
(d) Bundle-shell clearance.
(e) Number of tube-side passes and pass arrangement.
(f) Minimum pass partition lane width.
(g) Whether or not the exchanger layout is symmetrical about the axes.
(h) Whether or not the tubes in different passes are to be aligned on the same
overall grid.
(j) The presence or absence of an impingement plate.
(k) The diameters of the shell side nozzles.
(l) The requirements for tie rods.

8.1

Program Correlations

Most shell and tube thermal programs have built-in correlations for tube count.
These can be useful guides in the early stages of a design. However, while these
may take account of some of the above factors, they can only be regarded as
approximations, and may predict tube counts significantly different from what can
be achieved. This is particularly the case for small diameter exchangers with
many tube passes.


8.2

Use of Tubecount Tables

Standard tube count tables are available in the literature. They do represent
tubecounts which can be achieved, based on assumed, (not always stated)
values of the above variables, but may not be applicable to the GBHE standards
for tube pitch etc.
8.3 Graphical Layout
The traditional way to determine the tubecount for an exchanger is to draw out
the tubesheet with the correct clearances. While this does give a tubecount
which can be achieved, it is a time consuming process, especially for a multipass exchanger. Moreover, some skill is needed to produce the optimum
arrangement.
8.4

Use of computer programs

Computer programs are available for estimating the tubecounts in a shell and
tube exchanger.
Typically the programs allow for different pass layouts for multi-pass exchangers,
and the user can input the various clearances desired. Note that the default
values for tube pitch, clearances etc. may differ from those given in Clause 5.
Clause 5 values should be used in the program input.
The programs attempt to fit the maximum number of tubes into a shell within the
constraints of the clearances, while keeping the numbers of tubes in different
passes to near the same value. The program can either be run to estimate the
number of tubes in a given shell diameter, or, using 'WRAP' options, the
programs will provide the shell diameters and estimate the tubecounts for the two
'standard' shells which contain more and fewer tubes than the desired tubecount.
There are graphical output options, which will produce a scale drawing of the
tubesheet. An example of a graphical output is given in Figure 7.
Programs do not cover every possibility. For instance, one program offers a
maximum of six alternative pass layouts for multi-pass arrangements, and certain
combinations of baffle orientation and pass arrangement are excluded. With a
certain amount of ingenuity, most of these constraints can be overcome, but
there will be cases where there is no substitute for hand layout.


8.5

Tie Rods

The transverse baffles or support plates in a shell and tube exchanger are
located and supported by a set of tie rods and spacers. The rods are fastened to
the inside face of the stationary tubesheet. Where possible, some of the rods
may be located in the pass-partition lanes, where they perform the additional
duty of seal rods, reducing flow bypassing in the lanes. The other rods will have
to be located in the main parts of the bundle, occupying positions
which would otherwise contain tubes, and reducing the total tube-count.
Some programs do not make allowance for tie rods. The user will have to decide
whether tie rods can be located in the free areas or whether it will be necessary
to remove tubes to allow for them.
Table 3 gives the TEMA recommendations for the number and diameter of tie
rods.
TABLE 3

TEMA TIE ROD STANDARDS


FIGURE 7

EXAMPLE OF OPTU3 GRAPHICAL OUTPUT


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
BRITISH STANDARDS
BS 5500 Specification for unfired fusion welded pressure vessels
(referred to in 5.1 and Clause 6)
AMERICAN STANDARDS
ASTM B163 Specification for seamless nickel and nickel-alloy condenser
and heat-exchanger tubes (referred to in 5.3)
ENGINEERING SPECIFICATIONS
E488 Specification for Welded Tube/Tubeplate Joints in Ferritic and
Austenitic Steel Heat Exchangers (referred to in 5.1)
EDS.VES.01.06 Requirements for Shell and Tube Heat Exchangers
(referred to in Clause 1 and Clause 6)
OTHER DOCUMENTS
TEMA Standards of the Tubular Exchanger Manufacturers Association
(referred to in Clause 3, 4.1, 5.1, 5.4, 5.5 and 8.5)


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Mechanical Constraints on Thermal Design of Shell and Tube Exchangers