Selection of Heat Exchanger Types

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-HEA-506

Selection of Heat Exchanger Types

Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
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for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.

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Process Engineering Guide:

Selection of Heat Exchanger
Types

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

BACKGROUND

4

5

FACTORS INFLUENCING SELECTION

4

5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10

Type of Duty
Temperatures and Pressures
Materials of Construction
Fouling
Safety and Reliability
Repairs
Design Methods
Dimensions and Weight
Cost
GBHE Experience

4
4
5
5
5
5
6
6
6
7


6

TYPES OF EXCHANGER

9

6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
6.12
6.13
6.14

Shell and Tube Exchangers
Cylindrical Graphite Block Heat Exchangers
Cubic Graphite Block Heat Exchangers
Air Cooled Heat Exchangers
Gasketed Plate and Frame
Spiral Plate
Tube in Duct
Plate-fin
Printed Circuit Heat Exchanger (PCHE)
Scraped Surface/Wiped Film Exchangers
Welded or Brazed Plate
Double Pipe
Electric Heaters
Fired Process Heaters

9
17
19
20
22
24
25
27
29
31
32
35
37
38

ADVANTAGES AND DISADVANTAGES OF DIFFERENT
SHELL AND TUBE DESIGNS

14

TABLE

(1)

FIGURES
1

ESTIMATED MAIN PLANT ITEM COSTS

7

2

ESTIMATED INSTALLED COSTS

8

3

TEMA HEAT EXCHANGER NOMENCLATURE

11

4

F ‘CORRECTION FACTORS' : TEMA E SHELL WITH EVEN
NUMBER OF PASSE

13

SHELL AND TUBE HEAT EXCHANGER HEAD TYPES

15

5


6

GENERAL ARRANGEMENT OF A CYLINDRICAL GRAPHITE
BLOCK HEAT EXCHANGER

17

EXPLODED VIEW OF A CUBIC GRAPHITE BLOCK
HEAT EXCHANGER

19

8

TYPICAL AIR COOLED HEAT EXCHANGER

21

9

GENERAL VIEW OF ONE END OF A 3-STREAM
PLATE-FIN HEAT EXCHANGER

27

10

TYPICAL PCHE PLATE

29

11

VICARB ‘COMPABLOC' EXCHANGER

33

12

‘BROWN FINTUBE' MULTITUBE HEAT EXCHANGER

36

13

FIRED HEATER : SCHEMATICS AND NOMENCLATURE

39

7

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

42


0

INTRODUCTION/PURPOSE

This Guide is one of a series on heat transfer produced for GBH Enterprises.
Although the majority of heat exchangers used on chemical plant are shell and
tube units, there are many other types available. The engineer may have little
knowledge of the attributes of these other types so is not able to make a
reasonable selection. This Guide aims to assist in this task.

1

SCOPE

This Guide describes the factors which influence the choice of heat exchanger
and introduces the various types, giving their advantages and disadvantages.
It does not give hard and fast rules which will automatically lead to the selection
of the 'best' exchanger for a given duty; often there is no one correct solution.
Rather it seeks to give the engineer the information on which a rational decision
can be made.

2

FIELD OF APPLICATION

This Guide applies to process engineers in GBH Enterprises worldwide, who
may be involved in the specification or design of heat exchangers.

3

DEFINITIONS

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

Heat Transfer and Fluid Flow Service. A cooperative research
organization, based in the U.K., 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.


PCHE

Printed Circuit Heat Exchanger. A design of compact exchanger
manufactured from chemically etched plates which are joined by
diffusion bonding.

TEMA

The Tubular Exchanger Manufacturers Association. An association
of US manufacturers of shell and tube exchangers whose purpose
is to draw up standards for their manufacture. See Ref. [2].

With the exception of terms used as proper nouns or titles, those terms with initial
capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.

4

BACKGROUND

Increased profits are more likely to come through the development of new
products or processes than from the selection of better heat exchangers.
Because of this, the order of emphasis in selection is:
(a) Safety and reliability.
(b) Performance.
(c) Cost.
Attempts have been made in the past to develop selection methods for heat
exchangers, either in the form of flowcharts or scoring techniques. These
approaches often implicitly assume that there is a single 'correct' solution for
each problem. This is rarely the case. An initial screening can be done to reject
designs which are unsuitable for reasons of materials, operating conditions or
safety, for example, but the engineer will often be left with a range of designs to
consider. The final decision will be based on engineering judgment.
Probably 90% or more of heat exchangers in the industry are of the shell and
tube type. Although not necessarily the best design, shell and tube exchangers
can give satisfactory performance on most duties and their design, operation and
maintenance are well understood. As a result they are likely to be the standard
against which any alternative will be judged. Some of the hurdles which have to
be overcome before a different type of exchanger is installed are given in
Ref. [1].


This Guide assists in the selection process by discussing the various types of
exchanger using a range of factors which should be considered before finalizing
the choice. For some types, more detailed discussions are given to assist in
choosing between options within the general type.

5

FACTORS INFLUENCING SELECTION

5.1

Type of Duty

Duties can be classified as:
(a) Heating of liquids.
(b) Evaporation and boiling.
(c) Heating of gases and vapors.
(d) Cooling of gases and vapors.
(e) Condensation.
(f) Cooling of liquids.
(g) Heating and cooling of slurries.
Certain types of equipment are less suitable than others for some of these duties.
For example plate and frame exchangers are not particularly suitable for handling
gases.
Often an exchanger is required to perform more than one of these duties for the
hot or cold fluid, or both. An example is the production of superheated vapor from
subcooled liquid. Sometimes it may be possible to do this in one exchanger. At
other times separate exchangers may be required for each stage; different types
of exchanger may be appropriate for the differing duties.
5.2

Temperatures and Pressures

Mechanical and materials constraints may limit the operating pressures and
temperatures that can be handled by the equipment.
5.3 Materials of Construction
Materials of construction are selected for their combination of corrosion
resistance and mechanical strength. For the case of a heat exchanger, there is
the additional requirement of reasonable thermal conductivity, although with the
exception of plastics, this is not usually of great importance.


For most types of exchanger design, the choice of material is not an overriding
process consideration, although it may influence the economics. Even if the
exchanger cannot be fabricated in the cheapest suitable material, there is usually
a more expensive choice which may still give an acceptable overall cost. Thus
plate and frame exchangers, which use thin sheets of material with no corrosion
allowance, are not fabricated in carbon steel, but the item cost of a stainless steel
plate and frame unit is often comparable with that of a carbon steel shell and
tube exchanger designed for the same duty, and the installed cost is usually
lower than that for the shell and tube unit.
However, certain types of exchanger may only be available in a limited range of
materials because of the methods of fabrication. An example of this is the platefin exchanger. Although this is available in stainless steel in small sizes,
developed for the aerospace industries, larger units have until recently only been
fabricated in aluminium, using either salt bath or vacuum brazing. A number of
companies have announced a new range of units in stainless steel and nickel
alloys, but, like the aluminium units, these are of brazed construction, which will
limit their application in corrosive environments.
5.4

Fouling

Many fluids handled on chemical plants have a tendency to foul the heat transfer
surfaces. Fouling is more likely in some types of exchanger than others. For
instance, particulate fouling tends to occur in zones where the velocity is low,
such as round the baffles in shell and tube exchangers.
Fouling may reach an acceptable asymptotic value, but often it is desirable to be
able to clean the heat transfer surfaces. Some designs can readily be cleaned
mechanically. For others this is not possible and chemical cleaning may be
necessary. If the very small passages in a PCHE are blocked, even chemical
cleaning may not be successful.
5.5

Safety and Reliability

Safety and reliability is the most important factor to be considered in selection.
On nonhazardous duties it may be acceptable to have some risk of equipment
failure, which would not be acceptable for hazardous duties. For example, with a
plate and frame heat exchanger, there is always a possibility of gasket failure,
although with correct installation and maintenance this should be low. Such a
failure may be an acceptable inconvenience for an exchanger handling
water; it would be unacceptable if the fluid were liquid chlorine.


5.6

Repairs

Corrosion or fatigue may put a limit on the acceptable working life of an
exchanger. Unforeseen plant upsets may lead to premature failure. Some
designs of exchanger can be repaired in an acceptable and economic fashion.
For example, in many types of shell and tube exchanger individual tubes which
have failed may be replaced, or if this is not possible, plugged with little
deterioration in overall performance. Other designs of exchanger may not
facilitate repairs, and a replacement unit may be required. The likelihood and
consequences of failure should be assessed at the selection stage.
5.7

Design Methods

See GBHE-PEG-HEA-502 for information on computer programs for the thermal
design of heat exchangers.
Well established and proven methods exist for the thermal and mechanical
design of shell and tube exchangers. This is not so for many of the other types,
particularly those which can be classified as 'proprietary' designs. These often
use correlations for the thermal design which have not been published, and for
some types the mechanical design does not conform to any recognized code.
When purchasing such units, the engineer has to rely to some extent on the
manufacturer. It is true that manufacturers will generally offer a thermal and
mechanical guarantee, but their liabilities only cover replacing the unit if it fails to
perform. The consequential loss to a company may far outweigh the value of the
item. Where the mechanical design does not conform to an established code, the
GBH Enterprises mechanical engineer may insist on the unit or parts of a
prototype being pressure tested to a much higher pressure than would be normal
for an established design.
5.8

Dimensions and Weight

The installed cost of an item is significantly greater than the item cost, typically by
a factor of about 3. This covers the need to provide pipework, instruments,
foundations, lagging etc. Many of these are related to either the dimensions or
the weight of the item, or both. Considerable savings can often be made in the
total cost of an exchanger by the use of a compact design, even if the item cost
exceeds that of a more conventional exchanger. An extreme case of this is
equipment to be used offshore; the cost of providing 1 m2 of platform space was
quoted in 2009 at about $180,000 !


5.9

Cost

As indicated in 5.8, it is the total installed cost which is of ultimate concern. In
addition, there may be extra running costs associated with the exchanger, such
as power requirements for pumping.
Some information on costs, including installation costs, of certain types of
exchanger OGJ Nelson Cost Index. Data are available for:
(a)

Shell and tube exchangers with a variety of materials of construction,
including graphite tubes.

(b)

Air cooled heat exchangers with a variety of tube materials.

(c)

Plate heat exchangers with stainless steel, titanium or Hastelloy plates.

(d)

Cubic graphite block exchangers with phenolic resin impregnation.

(e)

Cylindrical 'polyblock' graphite exchangers with phenolic resin and PTFE
impregnation.

However, the Nelson Index data has to be used with care, as the cost information
is not always up to date; occasionally some serious discrepancies can occur.
More reliable information, including data on types of exchanger not covered by
Nelson Index, can be obtained by consulting manufacturers.( Ultimately, the only
totally reliable data for main plant item costs are manufacturers' quotations.)
The estimation of installed cost can be even more difficult, especially if the object
is to get a reasonable estimate of the difference in total cost between two
different types of exchanger for the same duty.
Figure 1 shows some data for main plant item costs obtained from NELSON
Index and are for illustrative purposes only. Figure 2 shows the estimated
installed costs for the same data.
Estimates of cost are frequently given on the basis of exchanger area. This can
be misleading when comparing different types unless the different heat transfer
coefficients of different designs are taken into account. What really matters is the
cost for the required duty. Some so-called 'Compact' exchangers often score by
giving higher heat transfer coefficients than the equivalent shell and tube units for
the same pressure drop, rather than by packing more surface into a given
volume.

5.10

GBH Enterprises

Where information is available, examples of the use of different types of
exchanger are given.
FIGURE 1

ESTIMATED MAIN PLANT ITEM COSTS
(Data are for illustrative purposes only!)


FIGURE 2

ESTIMATED INSTALLED COSTS
(Data are for illustrative purposes only!)


6

TYPES OF EXCHANGER

6.1

Shell and Tube Exchangers

For additional information on shell and tube exchangers and their applications,
see GBHE-PEG-HEA-507, GBHE-PEG-HEA-508, GBHE-PEG-HEA-515,
GBHE-PEG-HEA-516 and Ref. [2].
(a)

Type of duty
Suitable for all types of duty; single phase gas and liquid, boiling,
condensation. Slurries should not be handled on the shell side because of
the risk of deposit build-up in the dead zones. Materials which become
viscous on cooling, if cooled on the shell side, may give problems with
severe bypassing.

(b)

Operating limitations
Can be designed for almost any combination of temperature and pressure.

(c)

Materials of construction
Can be fabricated in most materials. The tubes are generally metallic, but
specialist manufacturers offer units with tubes of graphite, plastic or silicon
carbide.

(d)

Fouling
Can operate reasonably on moderately fouling duties. Can usually be
cleaned mechanically on the tube side. Prone to sedimentation fouling on
the shell side, especially in the dead zones around baffles, but can be
designed for mechanical cleaning on shell side if a removable bundle is
used.

(e)

Safety and reliability
Generally good. Areas to watch are tube-tubesheet joints; corrosion,
especially in the dead zones around baffles; tube vibration. Good design
should avoid these problems.


(f)

Inspection and repairs
With a removable bundle, all parts can be inspected visually. With a fixed
tubesheet design, only the tube side can be inspected visually, but
techniques such as ultrasonic thickness measurements can be useful to
give a measure of the condition of the tubes. Except for U-tube designs,
replacement of individual tubes is possible.

(g)

Dimensions and weight
Shell and tube exchangers have a low surface/volume ratio. The basic
surface/volume ratio of the bundle is typically 50 to 120 m2/m3; the overall
figure is reduced further by the volume of the headers, allowance for the
flanges etc., and also, if a removable bundle is used, by the space
required for bundle removal. They are thus relatively large and heavy.
Units have been fabricated with over 5000 m2 of heat transfer surface,
with diameters of over 4 m and lengths of over 20 m.

(h)

Design methods
Well established computer-based methods are available for the thermal
design and rating of shell and tube exchangers (see GBHE-PEG-HEA502). The mechanical design is covered by established codes such as BS
5500, ASME Boiler and Pressure Vessel Code: Section VIII: Division I,
etc.

(j)

Cost
See Figures 1 and 2.

(k)

GBH Enterprises
Shell and tubes exchangers are widely recommended for most client
plants, and if properly designed and operated give years of trouble free
performance.

6.1.1 Types of Shell and Tube Exchanger
Shell and tube exchangers are usually classified with reference to the
TEMA designations, with a three letter code describing front end head
type, shell type and rear end head type. See Ref. [2] for more information.

Figure 3, which shows the various types diagrammatically, is taken from
Ref. [2].
Note:
Type P is sometimes referred to as “Outside packed stuffing box”, type S
as “Split ring floating head” and type W as “Outside packed lantern ring”.
6.1.1.1

Shell Types

6.1.1.1.1

Single pass

The simplest form of shell and tube exchanger is the single pass fixed
tubesheet design, or TEMA type -EL, -EM or -EN.
The tube to tubesheet joints on a shell and tube exchanger are subject to
forces due to differential expansion between the shell and tubes. If these
forces would be too high for a simple fixed tubesheet design, some
provision has to be made to reduce them. This is done either by providing
a bellows in the shell or using one of the other rear end head types shown
in Figure 3.
The use of bellows should be avoided where possible. Not only are
bellows expensive, sometimes adding significantly to the cost of the
exchanger, but they represent a weak point in the shell. They are sensitive
to corrosion, overload (including an excessive number of cycles) and poor
welding. They provide a stagnant region where, because solids can settle
out or conditions become modified for other reasons, corrosion is more
likely. Repair is often impossible and replacement can require major
dismantling of the exchanger.
When there is a hazardous fluid on the shell side of an exchanger, bellows
pose a particular hazard, as failure could lead to a major release. If it is
impossible to avoid their use, special care should be taken in design and
fabrication, and regular inspections will be necessary during operation.
Consult a vessels engineer for advice.
The cost of bellows increases rapidly with pressure to an upper practical
limit of about 35 bar.
If bellows are specified for an exchanger which is lagged and out of doors,
there is a possibility of rainwater corrosion unless steps are taken to
prevent water from getting below the lagging.

As a rough guide, bellows are likely to be required for a fixed tube sheet
design when the temperature difference exceeds about 50°C if the shell is
hotter than the tubes, and 30°C if the tubes are hotter than the shell.
These figures are only a guide; the actual stresses will be calculated by
the mechanical designer, who will determine whether bellows are
required.


FIGURE 3

TEMA HEAT EXCHANGER NOMENCLATURE


6.1.1.1.2

Multiple tube side passes

The U-tube exchanger obviously has to have more than a single pass on
the tube side. Multiple pass exchangers are also used with the other types
of rear end head for one of two reasons:
(a)

Where a single pass exchanger would be of an excessive length.

(b)

Where the tube side flow rate is much less than the shell side. The
use of multiple passes allows the designer to achieve a reasonable
tube side velocity and hence heat transfer coefficient without an
excessive shell side pressure drop.

Unless one or both of the fluids is essentially isothermal, for example a
single component fluid either boiling or condensing, there is a penalty to
pay for multi-pass design. Because some of the tube side passes are in
co-current flow to the shell side fluid, the effectiveness of the exchanger is
less than that of a pure counter-current design with the same area and
heat transfer coefficient. This is often accounted for in single phase flow
by the 'F' correction factor to the log mean temperature difference. Refs.
[3] or [4] show how to calculate this correction.
Some sources recommend the avoidance of designs where the 'F'
correction factor falls below some arbitrary value, often 0.75. This can be
misleading and potentially dangerous, as can be seen from Figure 4,
which shows the value of the 'F' factor for a TEMA E shell with an even
number of passes. The parameters P and R are functions of the terminal
temperatures as shown in the Figure. The regions to avoid are where
small changes in P or R result in large changes in F, because this implies
the exchanger performance is very sensitive to small
changes in temperature. This corresponds to regions where the curves
become steep. It can be seen that although F = 0.75 may be acceptable
for R = 1.0, for R = 10 even temperatures which result in F = 0.9 are
unsafe.
In extreme cases, a duty which can be performed in a counter-current
design cannot be achieved in a mixed flow design regardless of length,
and multiple shells in series are required.


The use of the two pass TEMA F shell (see Figure 3) in theory can get
round this problem for an exchanger with two tube side passes, by
maintaining pure counter-current flow. However, the performance of the F
shell design in practice is rarely as good as theory. Leakage of fluid and
thermal leakage across the longitudinal baffle can seriously affect the
performance. Sealing devices to prevent the physical leakage are often
damaged if the exchanger is dismantled. Because of this, the GBH
Enterprises strongly discourage the use of F shells, unless the
longitudinal baffle is welded to the shell. If this is done, it prevents bundle
removal but still does not overcome thermal leakage. For special duties
designs have been produced with a double longitudinal baffle with an
insulating air gap between the baffles, but such solutions require careful
consideration of the mechanical design; Vessels Section should be
consulted. Multiple shells in series may prove more economic.
A 4-pass F shell design can be fabricated with a welded longitudinal baffle
and removable bundle. This is equivalent to two 2-pass E shells in series,
and has a 30 to 40% cost advantage over the separate shells. A check for
the effect of thermal leakage should be performed.
The use of multiple tube passes other than two pass U-tubes for tube side
condensation or boiling can present design problems because of phase
separation in the headers. This results in different flowrates and
compositions in different tubes of the same pass, which cannot be allowed
for with the design programs used. Condensers can be designed in which
the condensate is removed at the end of each pass (inter-pass luting).
This gets round the design problem, in that each tube of a given pass will
have the same composition. However, the flowrate through each pass will
be different, so the normal programs cannot be used directly; each pass
will require to be designed in isolation and the results merged together.
Moreover, there may be problems with flashing in the pipework where the
condensate streams from the various passes are mixed.
The split flow arrangements of types G and H are usually only found in
horizontal thermosyphon reboilers. Here, some leakage around the
longitudinal baffle will have only a minor effect on the performance.
The type J shell, with one shell side inlet and two outlets, is used for cases
where there is a low allowable pressure drop, since half the flow is flowing
through half the length of the shell. It is effectively two E shells back-toback.


Note:
With a single pass on the tube side, half the exchanger will be in cocurrent flow and half in counter-current, so its performance will not be as
good as a pure counter-current exchanger.
The kettle type K is a special case of the shell and tube exchanger with an
oversized shell to allow for vapor disengagement when used as a boiler.
The bundle can be a fixed tubesheet design, in which case the shell will
taper at both ends rather than just the one as shown in Figure 3. More
usually, the bundle will be a type U or T (see 6.1.1.2). See GBHE-PEGHEA-507 for more information on reboilers.
FIGURE 4

F ‘CORRECTION FACTORS' : TEMA E SHELL WITH EVEN
NUMBER OF PASSES


6.1.1.2

Head Types

Rear end head types P, S, T, U and W offer different approaches to the
problem of differential expansion. Table 1 summarizes some of the
advantages and disadvantages of the different types. It is worth noting that
the pull-through floating head type T requires a large clearance around the
bundle to permit withdrawal. This can lead to excessive bypassing of the
shell side fluid, with a resulting poor performance. The split ring floating
head design type S is preferred in this respect, but is more expensive.


TABLE 1

ADVANTAGES AND DISADVANTAGES OF DIFFERENT SHELL
AND TUBE DESIGNS
Increasing cost  


The method of sealing of the type P and W rear end heads is prone to
leakage. These designs should be avoided for high pressure or hazardous
duties. Their use is normally restricted to cooling water duties. Single pass
floating head designs of type S or T usually also involve a sealed gland as
in Figure 5 (a). An alternative to this where hazardous materials are
involved is the use of an internal bellows as shown in Figure 5 (b).
Although this bellows may fail, unlike a full shell bellows it is relatively
cheap, can be replaced relatively easily if necessary, and any leakage
arising from failure will be contained within the shell. The last point
assumes that the shell and tube side fluids are compatible. This type of
bellows is especially relevant to high pressure interchangers.
FIGURE 5

SHELL AND TUBE HEAT EXCHANGER HEAD TYPES


Head types A, C, L, N and P have removable covers, which enables
access to the tube side without having to disconnect the tube side
pipework. This can be an advantage if frequent mechanical cleaning of the
tube side is required. However, the design does involve extra flanges and
gaskets, with potential for leaks, and will generally be more expensive
than the integral cover of types B and M.
Types C and N are used to reduce the number of main flanges on the
exchanger, particularly when handling hazardous materials and/or subject
to severe mechanical duties (e.g. high temperature cyclic duties). They
have the disadvantage that it is difficult to access the tubesheet face for
repairs, particularly for replacement of the outer tubes; it may be
necessary to specify a larger than normal bundle-to-shell clearance for
this purpose, which may lead to excessive bundle bypassing. As the
clearance is only required on the tube side, this can be avoided by using a
larger diameter for the header than the main shell, as shown in Figure 5
(c).
.
6.1.2

Fluid Allocation

The choice as to which fluid to put on the shell side and which on the tube
side is influenced by several factors. Often these are in conflict, with some
factors suggesting the hot fluid on the shell side and some on the tube
side. In this case some compromise is required and it may be necessary
to perform designs for both alternatives, and select the cheaper.


6.1.2.1

Materials of Construction

If one of the fluids requires the use of a corrosion resistant, and hence
probably expensive, material of construction, whereas the other can be
contained in a cheaper material, it is usually cheaper to put the corrosive
fluid on the tube side. This will require the tubes and headers to be made
of the corrosion resistant material, whereas the shell and baffles can be in
the cheaper material. The tubesheet will either be made of the expensive
material in solid form or from cheaper material such as carbon steel clad
with the more expensive material on the header side, whichever is
cheaper.
If the corrosive material is placed on the shell side, only the headers can
be made of the cheaper material. Cladding the shell side of the tubesheet
is not usually a practical proposition. It may be economic to consider lining
the shell with a corrosion resistant material.
6.1.2.2

Fouling

Fouling is generally less of a problem on the tube side, and cleaning of the
tube side is easier than the shell side. If it is required to clean the shell
side mechanically, it will probably be necessary to use a square tube
pitch, which implies a larger shell for the same surface area.
6.1.2.3

Pressures

It is usually preferable to put the fluid which requires the higher design
pressure on the tube side as the required shell thickness is then less. This
becomes more important at higher pressures.
6.1.2.4

Flowrates

It is usually easier to produce a reasonable design for exchangers where
the fluid with the higher volumetric flowrate is on the shell side. For this
reason, gases are usually handled on the shell side.
6.1.2.5

Pressure Drops

For a given flowrate and film coefficient, the pressure drop of the shell side
fluid is generally less than the tube side.


6.1.2.6

Enhancement of Heat Transfer

When the shell side heat transfer coefficient is controlling, the effective
coefficient can sometimes be enhanced by the use of low fin tubing with
relatively small pressure drop penalty. Tube side enhancement devices,
such as twisted tapes or looped wire inserts, usually impose a larger
penalty on pressure drop, but can be very useful when dealing with high
viscosity fluids or laminar flow applications.
6.1.2.7

Cooling of Viscous Fluids

If a viscous fluid is cooled on the shell side of an exchanger, the fluid in
the bundle becomes colder than that in the bypass lanes, and
consequently more viscous. This can result in a significant increase in
bypassing, possibly leading to the situation where essentially all the flow is
bypassing the bundle. The computer programs used for design are not at
present capable of modeling this phenomenon.
6.2 Cylindrical Graphite Block Heat Exchangers
These units are sometimes known as 'polyblock' exchangers. See Figure 6. For
general information on graphite heat exchangers.


FIGURE 6

GENERAL ARRANGEMENT OF A CYLINDRICAL GRAPHITE
BLOCK HEAT EXCHANGER


(a)

Type of duty
Suitable for all types of duty; single phase gas and liquid, boiling,
condensation.

(b)

The design pressure is normally limited to 5 to 6 bar on the process side
and 8 bar on the service side, although some units can be designed for up
to 16 bar operation. The maximum operating temperature depends on the
type of impregnation used: 165 to 185°C for phenolic resin, 230°C for
PTFE, 400°C for carbon. The use in high temperatures, particularly where
there is a risk of thermal shock, needs careful design; the manufacturers
should be consulted for advice.

(c)

Graphite has very good chemical resistance to a wide range of corrosive
chemicals. The corrosion resistance of the graphite used in exchangers is
generally limited by that of the impregnant used. PTFE or carbon
impregnation offer better corrosion resistance, but tend to be mechanically
weaker. If both fluids are corrosive it is possible to line the shell with,
for example, PTFE.

(d)

Fouling
Mechanical cleaning is possible, but there is a risk of damage to the
blocks.

(e)

Graphite is a brittle material, which can be damaged by mechanical or
thermal shock. Experience has been mixed, some plants having many
years of satisfactory performance, whereas others have had repeated
failures. Some chemicals may leach the impregnants from the graphite,
resulting in the graphite becoming porous over a period of time.

(f)

Repairs
The units can be dismantled for inspection. If the graphite is damaged, the
damaged blocks have to be replaced, but undamaged ones can be
reused.


(g)

The surface/volume ratio of the blocks is typically 30 to 70 m 2 /m3 , which
makes this design even bulkier than a shell and tube exchanger. Block
sizes range from 200 to 1800 mm in diameter, and heights from 100 to
700 mm, depending on diameter. Exchangers are built up from multiple
blocks, up to 25 being possible in one shell, depending on block size.
Total heat transfer areas approaching 1000 m2 can be obtained
with exchangers using the largest blocks. The units are usually mounted
vertically, but some recent designs allow horizontal mounting.

(h)

Design methods
The performance of a cylindrical graphite block exchanger can usually be
simulated using methods developed for shell and tube units, with some
adjustment for the 'shell side' coefficient. The mechanical design of the
steel shell, which forms the ultimate pressure envelope, is covered by
established codes such as BS 5500.

(j)

Cost

(k)

GBH Enterprises experience
Contact Us.

6.3

Cubic Graphite Block Heat Exchangers

For general information on graphite heat exchangers . An exploded view of a
typical cubic graphite block exchanger is shown in Figure 7.


FIGURE 7

(a)

EXPLODED VIEW OF A CUBIC GRAPHITE BLOCK
HEAT EXCHANGER

Type of duty
Used on all types of duty. Common in the pharmaceuticals industry, where
the relatively small maximum size is not a problem, and the resistance of
graphite to a wide variety of chemicals is an advantage in multi-purpose
plants.

(b)

The design pressure is normally limited to about 6 bar. The maximum
operating temperatures are as for cylindrical graphite units.

(c)

As for cylindrical graphite units.


(d)

Fouling
As for cylindrical graphite units.

(e)

Generally as for cylindrical graphite units. However, unlike the cylindrical
units, there is no outer steel shell to act as a containment in the event of a
major failure of the graphite.

(f)

Repairs
Damaged blocks cannot be repaired and have to be replaced.

(g)

Units are available with up to 90 m2 of heat transfer surface. The surface
to volume ratio is relatively low, typically 50 m2/m3.

(h)

Design methods
The thermal performance of cubic graphite block exchangers can be
estimated with methods developed for shell and tube units, but with some
difficulty. The mechanical design is proprietary.

(j)

Cost

(k)

Contact us.


6.4

Air Cooled Heat Exchangers

A typical air cooled process heat exchanger is shown in Figure 8. For more
detailed information on air cooled heat exchangers and a discussion on the
relative merits of air and water cooling see GBHE-PEG-HEA-513.
(a)

Type of duty
Cooling of gases and liquids, condensation.

(b)

Air cooled exchangers tend to become uneconomic compared with water
cooled units when the required exit temperature is less than 20 to 30°C
above ambient. There is no practical pressure limitation.

(c)

Tubes and headers can be fabricated in most materials. Fins are usually
aluminium, although galvanized steel is sometimes used in corrosive
environments, but with a considerable loss in fin efficiency.

(d)

Fouling
With the correct choice of header box design, easy access to the tube side
for mechanical cleaning is possible. See GBHE-PEG-HEA-513 for more
information on fouling and cleaning of the air side.


FIGURE 8

TYPICAL AIR COOLED HEAT EXCHANGER

Notes:
(1) The supports for the fan and motor have been omitted for clarity.
(2) One fan and plenum have been omitted to show the tubing.
(e)

Serious corrosion of aluminium fins has occurred in the past on sites with
a polluted atmosphere, but current hygiene standards have largely
eliminated this problem. The major problems are around air side cleaning
and fan drives. Noise emitted from the fans and drives can be a problem.

(f)

Repairs
Individual tubes can be plugged. Replacement of individual tubes is not
generally possible without major dismantling. Complete re-tubing is
possible.


(g)

Typical air cooled heat exchangers using 1" tubes on a 2 to 2.5" pitch
have a low process (inside) surface/volume ratio of about 30 to 35 m2/m3.
Moreover, as the thickness of the bundle is usually low, the plot area
requirements are large. Large units are built up from individual bundles
with multiple fans. There is thus no limit to the ultimate size of an
installation. However, there may be distribution problems with the manifold
of large installations. See GBHE-PEG-HEA-513 for more details.

(h)

Design methods
It is usual to ask a manufacturer to perform thermal design. Commercially
available computer programs exist, for checking designs (see
PEG.HEA.005), which experience suggests are reasonably accurate.

(j)

Cost
The capital cost of air cooled heat exchangers is high, principally because
of the high installation costs. In addition, there is a continued running cost
for the fans. However, this has to be compared with the capital and
operating cost of cooling tower installations. An optimization of capital
against running cost should be performed at the design stage. See
Appendix A of PEG.HEA.203 for a preliminary costing method for air
cooled heat exchangers.

(k)

Contact us.

6.5

Gasketed Plate and Frame

(a)

Type of duty
Where suitable, the plate and frame exchanger is likely to be the most
economic choice. Very good for liquids. Their use for condensing and
boiling duties is low at present, but the manufacturers are moving into
these applications. Not well suited for gases. Not very suitable for liquids
where the flowrates of the two streams differ widely. For an economic
design, a reasonable allowable pressure drop (say > 0.25 bar) is generally
desirable, but they should not be ruled out for lower pressure drop cases.


(b)

Their use is normally limited by availability of suitable gaskets. Historically,
the normal maximum design pressure was 10 bar, but more recent
designs have raised this to 25 bar. The normal operating temperature
range with elastomeric gaskets is minus 25 to plus 160°C; in special
circumstances it can be as high as 250°C.

(c)

Plates can be made from any material which can be pressed, but are not
available in carbon steel. Alfa-Laval and Vicarb also offer plates in
graphite. Gaskets are normally elastomeric, with Viton as the most
resistant readily available. Selection of a suitable gasket material is often
more of a problem than plate material. Some applications using CAF
(compressed asbestos fibre) gaskets are known, but these frequently
pose problems with achieving and maintaining a seal.

(d)

Fouling
Intrinsically less prone to many types of fouling than are shell and tube
units. Typical design fouling resistances should be ½ to ¼ of those for
shell and tube units. There are no dead spaces where deposits may
accumulate. However, fibrous solids tend to collect at the contact points
between successive plates, and build up rapidly. Either the tube side
of a shell and tube unit or a spiral plate exchanger is better for fibrous
materials. Can be easily disassembled for cleaning if required.

(e)

The plates are very thin, typically under 1 mm, so no significant corrosion
can be tolerated. The weak points are usually the gaskets, although with
proper assembly these should give no problems within their design limit.

Note:
The gasket design prevents cross contamination of the fluids even if the gasket
should leak; all leaks are to atmosphere. Cross contamination can only occur
from corrosion failure of the plates.
It is recommended practice to surround the plate stack with a thin carbon steel
shroud, open at the base, to deflect any leaks to ground and avoid spraying
personnel. Not recommended for toxic or flammable liquefied gases.

(f)

Repairs
Units can be readily disassembled for inspection and repair, giving
complete access to all the heat transfer surface. Individual plates can
readily be replaced. Re-gasketing may be required from time to time.
Modern plate designs normally have some form of snap-on gasket
attachment, which makes re-gasketing easy. Older designs require the
gaskets to be glued to the plates; removal of the old gasket and cleaning
of the locating channels for the replacement gaskets can be difficult. Many
manufacturers offer a re-gasketing service. In some cases, a spare plate
stack may be advantageous, allowing rapid turn-round.

(g)

Typical surface/volume ratios for plate heat exchangers are 50 to 120 m2
/m3. This at first sight seems low, considering that plate exchangers are
considered to be compact. However, the volume here is the total volume
occupied by the plate exchanger, including the header plates, frame and
allowance for disassembly. The heat transfer coefficients obtainable within
a plate exchanger are generally higher than those for a shell and tube unit
with the same pressure drop, a factor of three not being unreasonable for
a liquid-liquid duty. Thus the plate exchanger is considerably more
compact than the shell and tube for a given duty.

(h)

Design methods
Manufacturers have their own thermal design methods, which are specific
to the detailed dimensions of their plate designs. Computer programs are
commercially available to do preliminary estimations (see GBHE-PEGHEA-513 05), but should not be relied on for final design. Because of the
ease with which the size of a plate and frame exchanger can be modified,
by the addition or subtraction of plates, reliance on manufacturers
presents no major problem. Mechanical design is proprietary.

(j)

Cost
Very competitive, especially in exotic materials of construction. Typically, a
stainless steel plate and frame exchanger is comparable in item cost with
a carbon steel shell and tube unit for the same duty; the installation costs
of the plate and frame will be less. The use of expensive gasket materials,
such as Viton, can have a significant effect on the cost;


In one instance it almost doubled the price of an exchanger with titanium
plates. The costs of spare gaskets should be included in any cost
comparison.
6.6

Spiral Plate

(a)

Type of duty
Can be used for any type of duty; condensing, boiling or single phase,
including slurries.

(b)

The normal maximum design pressure is 15 bar. There are no particular
limits on design temperature, but avoid excessive temperature cycling.

(c)

Can be fabricated in any material.

(d)

Fouling
The smooth flow path with absence of dead zones makes the spiral
exchanger particularly suited to handling slurries, including fibrous
materials. Other forms of fouling, such as scaling, may be expected to be
comparable with the tube side of shell and tube exchangers, although the
manufacturers claim that the curved flowpath produces a scrubbing action
that keeps the surfaces clean, enabling lower fouling resistances to be
used.
Normal designs, with spiral flow both sides, give access to both sides for
mechanical cleaning. The spiral flow/cross flow variant can only be
cleaned mechanically on the cross flow side.

(e)

Unlike the gasketed plate and frame exchanger, the spiral plate only has
two main gaskets. Moreover, these are arranged between conventional
flanges which can be bolted up to a high degree, and use conventional
gasket materials. The unit should thus be as reliable as a shell and tube
exchanger, and should be suitable for handling toxic or flammable
liquefied gases.


(f)

Repairs
If the heat transfer surface fails through local corrosion, unless this occurs
at the edge of the plates, it is almost impossible to repair the exchanger.
The main block of the exchanger will have to be replaced, although it may
be possible to reuse the cover plates.

(g)

Spiral exchangers are comparable in volume to shell and tube units with
the same surface. However, they are generally more squat in shape, and
require less space for servicing. The standard design, with spiral flow on
both sides, operates in pure counter-current flow, so will have a higher
effectiveness than a shell and tube unit with mixed flow. Their squat
shape and circular cross section makes them suitable for direct mounting
on distillation columns as condensers or reboilers. See GBHE-PEG-HEA516 for more information on the use of spiral exchangers as refluxing
condensers.

(h)

Design methods
Spiral plate exchangers are normally considered as proprietary items and
are designed by the manufacturers. Commercially available programs can
be used to rate a spiral plate exchanger used as a thermosyphon reboiler,
but checking calculations for all other duties have to be done by hand. The
recommended approach is to use the 'hydraulic mean diameter' concept
combined with conventional correlations for heat transfer and pressure
drop in pipes. It is debatable whether any enhancement to allow for the
curved flow path should be included. Such enhancement occurs in flow in
curved tubes due to the secondary circulation set up in the flow.
Secondary circulation in the flat rectangular passages of a spiral plate
exchanger is likely to be much less.

(j)

Cost
The costs of spiral exchangers are generally between those of shell and
tube and gasketed plate types.

(k)

Contact us


6.7

Tube in Duct
These units usually consist of a rectangular duct across which are
arranged several rows of tubes in a direction normal to the flow in the
duct. As the fluid in the duct is normally a gas, the tubes are usually finned
to counter the poor gas-side coefficient. For low to medium temperature
operation spiral wound finned tubing similar to that used for air cooled
heat exchangers is generally used. For high temperature duties stud fins
are often used because of their greater robustness. The number of rows of
tubes is determined by the duty, and can range from a single row to more
than 30 rows. For duties where a gas is heated by steam and more than
one pressure level is available, it is common practice to use low pressure
steam in the first banks and higher pressure in later ones.
(a)

Type of duty
Normally used for the heating or cooling of low pressure gases in
the duct. Examples include the preheating of combustion air for
furnaces, heat recovery from furnace combustion gases, e.g.
furnace convection banks, and the heating of air for driers.

(b)

Because the ducts are generally of rectangular construction, and
large dimension, it is difficult to design them to withstand any
significant pressure differential with the atmosphere. Typical
pressure limits are 0.1 to 0.2 bar. Temperature limits are governed
by the choice of materials. There are no practical pressure
limitations on the tube side.

(c)

Ducts can be made from any suitable material; lining may be used
if the gas is corrosive. Tube material is governed by the nature of
the tube side fluid. Fins will normally be of aluminium for noncorrosive duties up to medium temperatures, say gas temperatures
up to 150 to 200°C. At higher temperatures carbon steel fins are
often used, and for corrosive gases more exotic materials will be
necessary.


Note:
The efficiency of fins made from these materials will be lower than that for
aluminium fins of the same dimension.
(d)

Fouling
Finned surfaces are always prone to fouling by dusts. If the gas is
dirty, it will be necessary to clean the outside of the tubes. Some
designs of tube in duct exchanger allow individual banks of one or
two rows of tubes to be withdrawn from the duct for cleaning. In
other cases, access for cleaning lances may be provided. In the
convective sections of boiler plant permanent cleaning nozzles are
sometimes employed (so-called soot blasters). For air heaters,
some form of dust filter is often provided in the duct before the tube
banks.

(e)

No information.

(f)

Repairs
Maintenance is easier if the exchanger is designed so that
individual banks can readily be removed. If severe corrosion is
expected, the holding of a spare bank could be a worthwhile
investment. Depending on the method of fabrication, replacement
of an individual failed tube within a bank may not be easy without a
major dismantling of the bank.

(g)

No data.

(h)

Design methods
The calculation of the outside heat transfer coefficient and pressure
drop is a relatively simple procedure. Standard correlations have
been developed for cross flow over rectangular banks of plain and
finned tubes; the recommended methods are as given in the HTFS
Handbook (Ref. [5]). These methods are provided in commercially
available programs.


The tube side coefficient and pressure drop can be calculated by
the appropriate established methods for in-tube flow. There is as
yet no general purpose program which will combine the tube side
and outside calculations. Most commercially available programs
assume constant gas properties through the bundle. If the gas
temperature varies significantly it is necessary to perform several
runs to determine behavior through the bundle, and combine these
results with tube side values by hand.
(j)

Cost
No data.

(k)

Contact Us

6.8

Plate-fin

For general information on plate-fin exchangers see Refs. [6] and [7]. Figure 9
shows the constructional details of a typical plate-fin exchanger.


FIGURE 9

GENERAL VIEW OF ONE END OF A 3-STREAM PLATE-FIN
HEAT EXCHANGER


(a)

Type of duty
Widely used in the cryogenics field. Becoming used in petrochemical
applications. Can handle liquids, gases, condensing and boiling duties.
Multi-stream duties are normal; up to at least 11 separate streams can be
handled in one exchanger. A long thermal length and good countercurrent flow makes close approach temperatures possible; units have
been designed for cryogenic service with temperature differences of less
than 2°C.

(b)

The use of aluminium limits the upper temperature of such units to about
65 to 90°C. There is no lower temperature limit, units being used on liquid
helium production. Typical upper pressure limits are 70 bar.
Some manufacturers have announced units in brazed stainless steel or
nickel alloys which can operate at up to 650°C; the pressure limits of these
units are stated to be over 100 bar.

(c)

Although large plate-fin exchangers are no longer limited to aluminium,
they are generally fabricated by brazing. This introduces other materials,
which gives rise to the possibility of dissimilar metal (galvanic) corrosion. A
European manufacturer is currently attempting to develop an all stainless
steel plate-fin exchanger, without brazing metal. Rolls-Royce and
Associates, in conjunction with Alfa-Laval have developed an exchanger
of plate-fin type in titanium, using diffusion bonding and super-plastic
deformation techniques.

(d)

Fouling
Not suitable for fouling duties; mechanical cleaning is not possible, and
the small passages can readily become blocked.

(e)

No information.

(f)

Repairs
No information.


(g)

Very compact surface/volume ratio typically 1000 m2/m3. However, much
of that surface is secondary. In aluminium construction, with a high
thermal conductivity, the fin efficiency will be high. In materials such as
stainless steel, lower fin efficiencies can be expected. The dimensions of
exchangers are limited by the brazing furnaces used in their
manufacture. Maximum dimensions are typically 1200 x 1200 x 7000 mm,
allowing up to 10,000 m2 of heat transfer surface.

(h)

Design methods
Plate fin exchangers are generally designed by the manufacturer to a
user's specification. The HTFS programs MUSE, MULE and MUSC can
be used to rate units. See PEG.HEA.005 for information on computer
programs. Mechanical design is proprietary.
General codes are being developed by the Plate-fin Manufacturers'
Association, a grouping of the major manufacturers worldwide.

(j)

Cost
No information.

(k)

Contact Us


6.9

Printed Circuit Heat Exchanger (PCHE)

For general information on PCHEs see GBHE-PEG-HEA-516.
A PCHE is fabricated by diffusion bonding a stack of plates into which a complex
pattern of channels has been chemically etched. Plates are typically 1 to 2 mm
thick and the channels, which are semi-circular in cross section, are typically 0.5
to 1 mm deep. Headers are welded to the outside of the stack after bonding.
A typical plate for a PCHE, before bonding, is shown in Figure 10. Note the zigzag channels. The small channel size of a PCHE results in low Reynolds
numbers, producing laminar flow in some cases. In straight tubes, the growth of
the boundary layer results in a fall-off in heat transfer coefficient with distance,
although with the small channels typical of a PCHE the limiting coefficient may
still be quite high. By repeatedly tripping the flow, this effect of this fall-off is
minimized, giving a beneficial trade-off between pressure drop and coefficient.
FIGURE 10 TYPICAL PCHE PLATE (shown reduced size).

(a)

Type of duty
In principle, PCHEs should be applicable to all types of (clean) duty except
for slurries. GBH Enterprises experience to date is limited to single phase
liquids and gases, condensing steam (in one case with some wet-wall
desuperheating) and boiling CO2. The manufacturer has built many units
for boiling refrigerants, and has supplied units for duties such as molten
caustic soda. Multi-stream designs present no problems in fabrication,
using approaches similar to plate-fin units, although to date no examples
with more than three streams in one unit are known.


Note:
The PCHE cannot be conveniently configured to cope with moderate pressure
gas-gas duties unless the NTU is large.
(b)

The diffusion bonding process, which results effectively in a solid block of
metal with holes through it, coupled with the very small channel sizes,
makes the PCHE an extremely robust form of exchanger. They can be
designed for almost any combination of temperature and pressure.

(c)

In principle, they can be fabricated from any metal which is available in
sheet form, for which a suitable chemical etching fluid and mask material
can be found, and which can be diffusion bonded. In practice, the majority
of units fabricated to date have been in stainless steel. Excellent corrosion
resistance for the duty is required.

(d)

Fouling
Not suitable for most fouling duties, although the absence of dead spots
implies that if fouling can be surface shear controlled, a PCHE might still
be feasible. The units cannot be mechanically cleaned, and if passages
become blocked, the chances of chemical cleaning are remote. Work
within ICI suggests that particles of up to at least of the channel
dimensions will find their way through the exchanger, but in practice
filtration of the process stream to a much smaller size is recommended,
even if particulate fouling is not expected.

(e)

A large unit installed on the Nitric Acid Plant in Europe, suffered from
corrosion failure, which was originally ascribed to a minor fault in the
diffusion bonding process. The manufacturer's quality control procedures
have since been tightened. However, the replacement unit has since also
suffered a minor failure. After investigation, it has been concluded, that
this was due to incorrect estimation of the acid dew-point of the process
gases, leading to condensation in some passages. Stainless steel is an
inappropriate material of construction if condensation occurs. This failure
is not a valid reason to reject PCHEs for the right duties. Experience of the
use of PCHEs in the USA, particularly in the refrigeration industry, is
extensive, and shows the PCHE to be very reliable.


PCHEs are finding applications in the demanding conditions of the
offshore oil industry, where their extremely compact form makes them
particularly attractive for space and weight saving.
(f)

Repairs
Leakage can only be dealt with by welding up the ends of passages. The
reliability of units after welding is still under debate.

(g)

Very compact, surface/volume ratio typically 500 to 1000 m 2/m3. Unlike
the plate-fin exchanger, all the surface is primary, so there are no fin
efficiency considerations. Individual block sizes are relatively small,
because of the limits of brazing furnaces; larger units are made by welding
several blocks together.
The highly compact nature of the PCHE makes innovative space saving
designs more possible.
Note:
The approach area remains close to that of more conventional units; the
savings are all in the length of the exchanger.

(h)

Design methods
The design of PCHEs is proprietary. Part of the expertise lies in
determining the layout of the channels. The chemical etching process
allows far more flexibility than in other designs, and designs are limited by
the imagination of the designer. Some information on heat transfer in
small channels can be found in Ref. [8].

(j)

Cost
Can be very competitive for the right duty. The units are particularly
suitable for high pressure duties. They are unlikely to be competitive for
duties for which a plate and frame exchanger is suitable.


(k)

Contact Us

6.10

Scraped Surface/Wiped Film Exchangers

(a)

Type of duty
Limited to specialized applications. The two major areas of use are as
crystallizers and evaporators. Can handle highly viscous or heat sensitive
fluids (successfully used to concentrate egg white). Can evaporate fluids
to a solid product.

(b)

No significant limitations. The manufacturers claim evaporation rates of
between 150 and 250 kg/m2 per hour can be achieved.

(c)

Can be fabricated from most materials.

(d)

Fouling
The scrapped surface operation should reduce or prevent fouling in many
instances.

(e)

Because they involve moving parts, glands, bearings etc., they can be
expected to require more maintenance than conventional exchangers.

(f)

Repairs
Scraper blades will wear with time but can be replaced.

(g)

Evaporators with at least 13 m2 of surface have been produced, but units
occupy very large volumes for the heat transfer area.


(h)

Design methods
The applications for which scraped surface units are appropriate are those
in which thermal performance is unlikely to be calculable by normal
methods. It is normal practice to base the design on trials conducted either
at the manufacturer's premises or on a pilot unit leased from a
manufacturer. The cost of conducting trials can vary greatly between
manufacturers.

(j)

Cost
An expensive way to provide heat transfer surface, but may be the only
practical solution to some problems.

6.11

Welded or Brazed Plate
There are a variety of types of exchanger which come under the generic
heading of welded or brazed plate. They are all of a 'proprietary' nature.
6.11.1 Plate and Frame Type
One of the weaknesses of the conventional gasketed plate and frame
exchanger is the extensive gasket round the edge of each plate, which is
potentially a source of leakage.
Manufacturers have produced designs which reduce the length of
gasketing, whilst retaining some of the good features of the plate
exchanger. In one of these types, conventional plates are welded or
brazed in pairs, and the plate pairs then assembled in a conventional
frame. Hazardous chemicals can be handled between the brazed plates
without risk of leakage. It is true that the hazardous chemicals do pass
through the plates with the conventional gaskets, but only through the
ports which are completely surrounded by a circular gasket, where the risk
of leakage is less.
If all gaskets are to be avoided, the complete plate stack can be brazed.
This gives an exchanger where the risks of leakage are minimized.
However, this is at the expense of the flexibility and ease of cleaning of
the conventional plate and frame unit.


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