Air Cooled Heat Exchanger Design Guide

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-HEA-513

Air Cooled Heat Exchanger Design

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
good faith, but it is for the User to satisfy itself of the suitability of the information
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:

Air Cooled Heat Exchanger
Design

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

SUITABILITY FOR AIR COOLING

4

4.1
4.2

Options Available For Cooling
Choice of Cooling System

4
9

5

SPECIFICATION OF AN AIR COOLED HEAT
EXCHANGER

16

Description and Terminology
General
Thermal Duty and Design Margins
Process Pressure Drop
Design Ambient Conditions
Process Physical Properties
Mechanical Design Constraints
Arrangement
Air Side Fouling
Economic Factors in Design

16
19
19
20
21
25
26
33
33
34

5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10


6

CONTROL

35

7

PRESSURE RELIEF

37

8

ASSESSMENT OF OFFERS

37

8.1
8.2
8.3
8.4

General
Manual Checking Of Designs
Computer Assessment
Bid Comparison

37
37
39
40

9

FOULING AND CORROSION

40

9.1
9.2

Fouling
Corrosion

40
41

10

OPERATION AND MAINTENANCE

42

10.1
10.2
10.3
10.4

Performance Testing
Air-Side Cleaning
Mechanical Maintenance
Tubeside Access

42
45
48
48

11

REFERENCES

50


APPENDICES

A

PRELIMINARY ESTIMATION OF ACHE SIZE AND COST

51

TABLES
1

ATTRIBUTES AND APPLICATIONS OF COMMON METHODS OF
ACHE CONTROL
36

2

AIR COOLED HEAT EXCHANGER FAULT FINDING CHART

43

3

SUGGESTED FILM RESISTANCE FOR USE IN PRELIMINARY
EXCHANGER SIZING

52

FIGURES

1

DIRECT CONTACT CONDENSER

5

2

USE OF RAW WATER ON A "ONCE THROUGH" BASIS

5

3

INDIRECT COOLING WITH RAW WATER VIA SECONDARY
COOLANT

6

COOLING WATER CIRCUIT WITH AN EVAPORATIVE
COOLING TOWER

7

5

DRY COOLING TOWER

8

6

INDIRECT AIR COOLING VIA A SECONDARY COOLANT

8

7

COSTS OF AIR COOLED HEAT EXCHANGERS

11

4


8

AIR FLOW NEAR AN AIR COOLED HEAT EXCHANGER

12

9

INFLUENCE OF LOCATION ON AIR RECIRCULATION

14

10

TYPICAL AIR COOLED HEAT EXCHANGER

16

11

BUNDLES, BAYS AND UNITS

18

15

TYPICAL TEMPERATURE VARIATION THROUGHOUT
A HOT SUMMER'S DAY

25

16

TYPES OF FINNED TUBING

29

17

HEADER TYPES

32

18

CURVES FOR COST FUNCTION "C"

53

19

CURVES FOR AREA FUNCTION "K"

53

20

NON-LINEAR TEMPERATURE ENTHALPY CURVES

55

21

CORRECTION FACTOR FOR SMALL EXCHANGERS

55

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

57


0

INTRODUCTION/PURPOSE

This Guide was prepared for GBH Enterprises.

1

SCOPE

This document is intended to provide a guide to the process engineer who may
be involved in the specification or operation of Air Cooled Heat Exchangers
(ACHEs).
It is concerned with such matters as choice of exchanger, specification of duty,
location, and assessment of tenders, control and maintenance.
It does not aim to give detailed information on the thermal design or rating of
ACHEs.
It is assumed that readers of the Guide have some general knowledge of heat
transfer. However, for the benefit of those readers who are unfamiliar with air
cooled heat exchangers, sub clause 5.1 gives a simple description and some of
the more common terminology used to describe these items. It may be beneficial
to read sub clause 5.1 as a precursor to this Guide.

2

FIELD OF APPLICATION

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

3

DEFINITIONS

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

Air Cooled Heat Exchanger. A heat exchanger designed for the
cooling and/or condensation of fluids by means of atmospheric air
flowing over the outside of a bank of tubes through which the fluid
to be cooled flows.


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.

HTFS

Heat Transfer and Fluid Flow Service. A cooperative research
organization, with headquarters in the 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.

4

SUITABILITY FOR AIR COOLING

Although this Guide is principally concerned with air cooled heat exchangers,
they are only one of several possible ways of rejecting heat to the environment.
Before deciding on the use of air cooling, the alternatives should be considered
and their relative merits assessed. Moreover, heat rejected to the environment is
wasted. Full benefit should be taken of the work on Process Integration to reduce
this waste heat as far as practicable. See Refs. [14] and [15].
4.1

Options Available For Cooling

4.1.1 General
The principal possibilities for process plant heat rejection are:
(a)

Direct contact cooling.

(b)

Direct cooling in a heat exchanger, using sea or river water on a "once
through" basis.

(c)

Indirect cooling using a secondary coolant, with sea or river water as the
ultimate heat sink.

(d)

Cooling water from an evaporative cooling tower.

(e)

Cooling water from a "Dry Cooling Tower".

(f)

Cooling water from an air cooled heat exchanger.


(g)

Direct cooling in an air cooled heat exchanger.

Although this Guide is mainly concerned with air cooled heat exchangers, the
relative merits of the other systems need to be considered.

4.1.2 Direct Contact Cooling
This process is normally limited to condensation duties, where there is a ready
supply of suitable water (river or sea), where it is not required to recover the
condensate, and where discharge of the resulting water/condensate mixture is
allowed. Condensation usually takes place in a spray or tray tower. If the
condensation is under reduced pressure a steam jet ejector or vacuum pump is
used to exhaust any non-condensables, with a barometric leg to discharge the
condensate. A typical system is shown in Figure 1.
This approach, where appropriate, is likely to be one of the cheapest, as the
equipment is little more than an empty shell, and does not suffer badly from
fouling when low quality water has to be used. For more information on direct
contact condensers see Ref. [1] and GBHE-PEG-HEA-508.
FIGURE 1

DIRECT CONTACT CONDENSER


4.1.3 Use of Raw Water On A "Once Through" Basis
For cases where there is a ready supply of river or sea water, but where direct
contact between the process fluid and the water is not possible, the use of such
water on a "once through" basis in a heat exchanger offers the simplest and
often cheapest solution. The heat sink is generally coolest when direct cooling of
this type is used. Figure 2 shows a typical arrangement.
FIGURE 2

USE OF RAW WATER ON A "ONCE THROUGH" BASIS

However, sea water is corrosive and river water may be also, and either may
give rise to severe fouling problems from scaling, sedimentation and
microorganisms. The effective treatment of the large volumes of raw water
involved, to reduce the fouling tendency, is often impracticable.
4.1.4 Indirect Cooling With A Secondary Coolant
An indirect system, as shown in Figure 3, can be used where one or more of the
following conditions apply:
(a)

If the raw cooling water is particularly corrosive.

(b)

If it is important that the process cooling water be clean.

(c)

If the risk of leakage of water into the process is unacceptable.


FIGURE 3

INDIRECT COOLING WITH RAW WATER VIA
SECONDARY COOLANT

The secondary coolant may be either clean water, dosed with suitable chemicals
to prevent corrosion or, where the mixing of water and process fluid cannot be
tolerated, some other suitable fluid. It is usually cheapest to cool the circulated
fluid in a plate-type exchanger, which can use plates of a corrosion resistant
material, such as titanium, and can be easily cleaned.
This system may be particularly appropriate where there are several separate
cooling duties and the only available water is corrosive or fouling. By providing a
central supply of clean noncorrosive fluid, cooled in one exchanger designed to
handle the raw water, the process exchangers may all be fabricated in less
expensive materials.
This system has the disadvantage that the secondary coolant has to be run at a
temperature above that of the raw water, in order to provide a driving force for
the cooler, so that the available temperature driving force in the process coolers
is reduced.


A paper exercise was carried out by the author in 2001 to assess the relative
benefits of an indirect system against a conventional cooling water system. The
study showed that there was little overall change in the plant capital for the two
cases, the lower temperature driving force for the indirect system being offset by
the lower fouling resistances that could be used. Un-quantified benefits of the
indirect system would be reduced need for cleaning, and the possibility of using
more compact forms of exchanger. The major disadvantage was the high cost of
the interchanger needed between the closed circuit and the ultimate sink.
However, if the closed circuit enabled the cooling tower to be dispensed with,
using raw water instead, substantial savings could be made. It is emphasized
that each case should be analyzed on its own merits.

4.1.5 Cooling Water From An Evaporative Cooling Tower
This is the most common form of process cooling recommended by GBH
Enterprises. The evaporative cooling tower of Figure 4 may be fan-blown or use
natural draft generated in a concrete shell - or even both. Natural draft towers are
more usual for larger applications; fan blown towers are the norm in certain
geographic regions. For small applications, a packaged system is often
attractive. (However, there may be problems in controlling the water
quality. Consult a Water Technologist for further advice.)

FIGURE 4

COOLING WATER CIRCUIT WITH AN EVAPORATIVE
COOLING TOWER


The fan-blown option, because the towers are relatively low and the mass
transfer efficiency high, may produce an unpleasant plume, especially in winter.
One of the principal problems of evaporative cooling systems is the quality of
water. The cooling tower can be an ideal environment for the growth of
microorganisms, and the tower itself acts as an efficient scrubber for dust laden
air. Severe fouling and/or corrosion problems can result if an adequate water
treatment program is not maintained, or if the heat exchangers are not carefully
designed or correctly operated.
An evaporative system requires a supply of make-up water, the minimum
acceptable quality of which depends on the nature of the water treatment
program used. In general, modern non-chromate systems require a purer makeup water than do the earlier chromate based treatments. The system, in general,
also requires a blowdown which, because of the treatment chemicals added, may
be subject to environmental constraints. For further information see consult a
Water Technologist.
4.1.6 Cooling Water From A "Dry Cooling Tower"
The "Dry Cooling Tower" of Figure 5 replaces the evaporative cooling pack of an
ordinary cooling tower by radiator elements, with the water in closed tubes. There
is a saving of water, there is no plume and clean water is used for process
cooling. However, dry cooling is more expensive than evaporative cooling, as the
heat transfer coefficient to the air is low, and the temperature approach is to the
dry-bulb temperature rather than the lower wet-bulb value. No example of a dry
cooling tower is known in a process plant, although they have been used in
thermal power stations.


FIGURE 5

DRY COOLING TOWER

4.1.7 Cooling Water From An Air Cooled Heat Exchanger
As an alternative to the natural draft "Dry Cooling Tower" shown in Figure 5, a
conventional air cooled heat exchanger can be used to cool a secondary fluid,
usually water, which itself cools the process. See Figure 6.


FIGURE 6

INDIRECT AIR COOLING VIA A SECONDARY COOLANT

This may be chosen for various reasons:
(a)

If the direct air cooler has to be made of expensive material, there may be
an economic case for using an indirect system.

(b)

Low pressure gases tend to require a high ratio of pressure drop to
absolute pressure when cooled or condensed in an air cooled heat
exchanger, which may be expensive in compressor power, and a directcontact exchanger with an indirect air cooled heat exchanger may be
economic.

(c)

Freezing or control problems might be eased by adopting an indirect
system.

An indirect system using recirculated condensate with a jet condenser (the
"Heller" system) has been extensively used in thermal power stations.
4.1.8 Direct Cooling In An Air Cooled Heat Exchanger
Straight forward air cooling is the most common alternative to a cooling tower
system for process cooling. It is particularly attractive when supplies of suitable
water for evaporative cooling are not readily available, or there are severe
environmental restrictions on discharge of cooling tower blowdown.


If it is possible for all cooling duties to be done using air cooling, the capital and
running costs of an evaporative cooling system and all the associated fouling and
corrosion problems are removed. Against this, the capital costs of the actual
process exchangers are higher for air cooling, and the coolers require
considerable space within the plant structure and generally require more
maintenance than shell and tube units.

4.2

Choice of Cooling System

4.2.1 Economic Factors
In order to choose correctly between the available cooling systems, it is
necessary to estimate the cost of the various options, not only as a cooling
system, but also in their effect on the overall plant performance and efficiency.
For example, a water cooled refrigerant condenser will, in general, condense the
refrigerant at a lower temperature, and hence pressure, than will an air cooled
condenser. The compression ratio of the water cooled system will be lower,
which may lead to significant savings in refrigerant compressor power and cost.
Thus, the choice of system may be governed by more complex considerations
than the simple cost and power consumption of the cooling system itself.
The accurate estimation of the advantages of the available cooling systems will
always be a lengthy and time consuming process, and will be difficult to justify for
any but the largest plants. The engineer will have to make the choice in many
cases without the benefit of such a study, so some general "rules of thumb" may
be helpful. As with all such rules, they should be qualified by common sense and
discretion:
(a)

Should water be available near the plant battery limits, in sufficient
quantity to ensure the cooling of every part of the unit, then use it in
preference to air cooling, either directly or with an indirect system.

(b)

If it will be necessary to use town's mains water, or other highly treated
water, for the make-up of evaporative towers, then choose air cooling.

(c)

If the average level of heat rejection is 20°C or less above air design
ambient temperature, choose water cooling. If 30°C or more choose air
cooling. If 20-30°C, there is unlikely to be a strong economic case either
way.


It is worth reemphasizing that the only reliable method of choosing is by making a
serious and expensive costed study of the options. In assessing the difference
between systems, it is necessary to include the difference in piping, erection and
electrical costs, as well as the capital costs. In many cases this is not a
practicable proposition, as much of the required information may not be available
at the time the decision has to be made.
In performing these comparisons it will be necessary to make an estimate of the
cost of air cooled heat exchangers. Manufacturers will normally be prepared to
provide budget costs of ACHEs if the duties are well enough defined.
Alternatively, the engineer could perform a preliminary design, and obtain a cost
estimate, by using Figure 7. However, for rough preliminary costing, the method
described in Appendix A may be used. This bypasses the step of designing the
exchanger, going straight from duty to an estimate of cost and plot area.
4.2.2 Process Considerations
There are some occasions when consideration should be given to factors other
than the straight economic choice of an ACHE, for process reasons.
Ambient air temperatures vary more than cooling tower water temperatures. If
the product being cooled is adversely affected by low temperatures - the most
common being freezing/crystallization, hydrate formation, cooling below the pour
point, or wax deposition, then it is usually possible to use an ACHE with special
precautions, such as recirculation of warm air from the bundle outlet to the air
inlet, to attemper the ambient air. Such solutions are expensive, clumsy and not
too reliable. Steam coils mounted below the main bundle may be a better option,
although they are wasteful of energy. Alternatively, an indirect cooling system
may be cheaper and easier to operate. See also Clause 6 on Control.
It is more economic to cool hot streams with air, and cooler streams with water. It
is therefore sometimes suggested that to cool a product from, say, 100°C to 40°C
air cooling be used from 100°C to say 55°C and a water cooled trim cooler from
55°C to 40°C. This is rarely justified. The extra pressure drop of the trim cooler
and its associated piping may lead to a less economic air cooler duty; and the
additional cost of the water supply, trim cooler and pipework are usually more
than the reduction in ACHE cost. Of course, if the process demands cooling
to a particularly low temperature, then the use of a trim water cooler will permit
reaching a lower temperature, possibly reducing or avoiding a refrigeration load.
In this case, look at the possibility of cooling by water only, unless the process
inlet temperature is so high that it could lead to problems on the water side, such
as boiling or excessive fouling.

A system occasionally used, particularly in desert locations, uses a water spray
and drift eliminators to reduce the air inlet temperature close to the wet bulb
temperature. If sufficient water is available, then an indirect system is almost
certainly cheaper. However, consider annual water consumption carefully. In this
respect, the spray system will usually have a greater hourly water consumption,
but will not be used continuously.
FIGURE 7

COSTS OF AIR COOLED HEAT EXCHANGERS
Index Base: 2010 = 220


4.2.3 Layout
ACHEs are bulky, and produce noise and warm air. Their siting should be
considered at an early stage of plant design.
The total plot area can be estimated by the method given in Appendix A, or other
methods. It is probable that no convenient area is available at grade for the
coolers, and that they will have to be mounted above other equipment. Pipe
tracks are often convenient. It is usual to find a place for ACHEs without great
difficulty, but remember that high mounted ACHEs will not benefit from any
ground attenuation of noise when community noise calculations are made.
Finding a grade position for the ACHEs might be worth more than 15 dB in the
noise calculations.
A check on possible air recirculation within banks and between banks should be
made. This check will owe more to art than to science, but some guidance may
be helpful.
The airflow pattern into an ACHE shows a high velocity near the edge of the inlet
(see Figure 8). This is associated with a low air pressure, and there is a risk that
the warm air from the outlet will be sucked into the inlet. This is particularly true
of forced draft units. As a general rule, some warm air recirculation will occur with
all long forced draft ACHE banks in a quartering wind (induced draft should avoid
this form of air recirculation). Should the air inlet be restricted, for instance by
neighboring buildings or too low a fan height, this effect will be increased.


FIGURE 8

AIR FLOW NEAR AN AIR COOLED HEAT EXCHANGER

If there is more than one ACHE bank on a site, air recirculation between banks is
possible. The following recommendations represent the ideal:
(a)

f the banks are close to each other, then sheet the space between them to
prevent down-flow of air. Otherwise separate the banks by 15 m if on the
same level, or by 30 m if on differing levels. This will prevent recirculation
in "no wind" conditions, but the plume from one bank may be blown to
another in a turbulent wind.

(b)

Downwind of large buildings, where downdraughts are possible, the very
turbulent air indicates separation of banks by 60 m. The longitudinal axis
of the bank should be across the airflow from the building.

(c)

As far as possible, avoid close proximity to sources of stray heat, such as
furnaces. Also avoid placing ACHE fans above the exhaust of a
mechanical draft evaporative cooler.

(d)

"A" or "V" frame air cooled heat exchangers in a cross wind may suffer
from reverse flow through the upwind and downwind banks respectively.

These points are illustrated in Figure 9.


In practice, these ideal requirements are unlikely to be met. If they cannot be, the
possible increase in air temperature into the coolers should be estimated, and
the design air temperature to the ACHE adjusted accordingly. For critical duties
in difficult locations, wind tunnel studies may be necessary to determine the
influence of neighboring structures on the performance. However, such tests are
difficult and expensive to conduct, and it may be worth reconsidering the decision
to use air cooling.
4.2.4 Site Conditions
Various site conditions may force the choice of air or water cooling.
(a)

Environmental conditions may forbid the use of cooling towers or
mechanical draft evaporative towers, by imposing excessively stringent
constraints on plumes or discharge of the blowdown.

(b)

If there is a shortage of suitable make-up water, water cooling may be
impracticable.

(c)

An excessively stringent noise requirement may force water cooling, (see
4.2.5).

When, as is the case in dry tropical climates, there is a large difference between
wet and dry bulb temperature, water cooling will be especially favorable.
unfortunately, water is often in short supply in such climates.


FIGURE 9

INFLUENCE OF LOCATION ON AIR RECIRCULATION


4.2.5 Noise
Noise specifications fall into two classes:
(a)

Limitations near the ACHE to protect the hearing of operators.

(b)

Limitation at points remote from the plant, to protect the amenity of
neighboring communities.

The actual specification of maximum permitted noise levels will vary from case to
case, and is subject to control by the planning authorities. Machinery Section
should be consulted for further information.


Any reasonable hearing protection specification can be met at reasonable cost,
using normal designs and standard fans, although hearing protection devices
may have to be specified for personnel working in the vicinity of the unit.
Community noise specifications can be very difficult to meet. A tight noise
specification, coupled with the requirements of E494 relating to fans, (see also
sub clause 5.7.5), can lead to a practically impossible task for the ACHE
designer, and certainly will result in very expensive designs. Great attention
should be given to the alternate cooling methods - evaporative or dry cooling
towers. Should the use of ACHEs be inevitable, it is difficult to recommend any
general rules, for each case will be different. A noise expert and an ACHE expert
should be consulted from the earliest possible stage, and a flexible attitude to fan
requirements and to ACHE siting taken.
Planning authorities sometimes impose a more stringent noise specification at
night time than during daytime. As ambient air temperatures are usually lower at
night, it may be possible to run the fans at slower speed during the night time. As
noise increases with the fifth or sixth power of the tip speed, this can give a
marked reduction in noise.
4.2.6 Ambient Conditions
The size and hence cost of an air cooled heat exchanger is sensitive to the
assumed design air inlet temperature, especially when it is required to cool the
process to a relatively low temperature. Ambient air dry bulb temperatures vary
significantly over short time periods and in the height of summer can reach 2530°C for short periods, even in the UK. For overseas locations, significantly
higher figures may be regularly attained. In contrast, the wet bulb temperature,
which controls the re-cool temperature of a wet cooling tower, does not vary so
much, as the relative humidity is generally lower in warmer weather. In selecting
the maximum design inlet air temperature, it is the engineer's responsibility to
consider the frequency with which the chosen temperature may be exceeded,
and to assess the level of risk involved in under-designing against the cost of a
too conservative design. This is discussed in more detail in sub clause 5.5.
The minimum design temperature is important in considering control and
winterization requirements (see below).


5

SPECIFICATION OF AN AIR COOLED HEAT EXCHANGER

5.1

Description And Terminology

This sub clause is intended to give a brief description of typical Air Cooled Heat
Exchangers and to explain the terminology for the benefit of those who are not
familiar with the items.
An Air Cooled Heat Exchanger (ACHE) is a device for cooling and/or condensing
a fluid, usually called the Process Fluid, using atmospheric air as the heat sink.
The process fluid flows through the tubeside of one or more bundles of tubes; the
air flows in cross flow over the outside of the tubes, assisted by a fan or fans. An
example familiar to everyone is the motor car radiator. In principle, there are
many ways in which an ACHE could be arranged; this Guide in general is
confined to the sorts of design that are found in the chemical and petrochemical
industry.
Figure 10 shows the major parts of a typical air cooled heat exchanger.


FIGURE 10 TYPICAL AIR COOLED HEAT EXCHANGER

Notes to Figure 10:
(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.
The central elements of an ACHE are the TUBES through which the process fluid
flows. Although plain tubes could be, and in certain rare circumstances are, used,
in almost all cases the tubes are finned on the outside. This is to counter the
relatively poor film heat transfer coefficient that occurs on the air side. Sub clause
5.7.3 describes the types of finned tube in common use. Tubes are typically from
2 to 12 m long.
The tubes are grouped in BUNDLES, typically 1-2 m wide. Within the bundle, the
tubes are arranged in horizontal rows, with a tube spacing marginally greater
than the fin o.d. A bundle will usually contain between 3 and 6 rows of tubes, with
successive rows staggered to give a triangular tube pitch.


The tubes are fixed into HEADERS, which serve the same function as those in a
shell and tube exchanger but, because of the shape of the bundle, ACHE
headers are long and narrow. Different forms of header are used, depending on
the duty. See sub clause 5.7.8 and Figure 17 for information on header types. An
ACHE bundle can have either single pass process flow, with the process fluid
inlet connected to the header at one end and the outlet to the other, or a
multi-pass arrangement, with pass partition plates dividing up the header(s).
Unlike shell and tube exchangers, it is common for the different passes to have
significantly differing numbers of tubes. A typical arrangement for an air cooled
condenser where sub cooling is required, for example, is to have several rows of
tubes in parallel performing the condensing part of the duty, followed by a single
row of tubes for the sub cooling duty, resulting in an increased liquid velocity
in this stage. Not all the tubes in one row need be in the same pass.
Bundles are usually mounted horizontally, but for condensers there may be a
slight slope to assist in drainage.
A large ACHE will require several bundles to provide the surface. Bundles are
grouped into BAYS, each bay containing one or more (typically 2-3) bundles in
parallel. The complete UNIT may contain several bays.
Air for cooling is assisted through the bundle by FANS. Axial flow fans, giving a
large volumetric flow for a very low pressure drop (of the order of 1-2 inches
water gauge) are used. On large units these fans are often 3-4 m in diameter;
diameters of 7 m are not unknown. The width of a bay, the chosen tube length
and the fan diameter are loosely interrelated. In order to ensure reasonable air
distribution across the unit, it is desirable to divide each bay up into roughly
square sections between the headers, each section being served by one fan (see
Figure 11). It is normal to have between one and three fans for each bay. On
small units the fans may be driven by a directly coupled electric motor, but it is
more usual for them to be driven through a gearbox or belt drive. See sub clause
5.7.7.
The fans are mounted within a FAN RING and connected to the bundle by a
PLENUM chamber. This may be a simple rectilinear box, as shown in Figure 10,
or may be shaped to reduce the pressure drop associated with the change in
flow from the circular fan ring to the rectangular bundle. The fan and plenum may
be mounted above the bundles, as shown in Figure 10, giving an INDUCED
DRAUGHT arrangement, or below it, giving FORCED DRAUGHT (see sub
clause 5.8). It is also possible to arrange pairs of bundles in an "A" or "V"
formation (see Figure 9(e)), but this is not common in the process industries.


Air flow through the bundle can be controlled by mounting LOUVRES across the
inlet or exit from the bundle. It is more usual, however, to control air flow, if
desired, either by using variable pitch fan blades, variable speed drives or
switching off some fans (see Clause 6). In certain cases, especially in locations
with extremely cold winters, STEAM COILS may be mounted below the bundle,
warming the inlet air somewhat, to prevent over-cooling of the process fluid.
The inlet and exit headers on each bundle will have at least one connection for
the process fluid; on wide bundles there may be several, to aid flow distribution.
The several inlets or outlets will be connected by MANIFOLDS. See sub clause
8.3.2 for a discussion of distribution problems.
The complete ACHE installation will include a support framework to mount it
clear of other equipment, to avoid restricting the air flow, and walkways, stairs
etc. for access to the bundle and fans.

FIGURE 11 BUNDLES, BAYS AND UNITS


5.2

General

Unlike shell and tube exchangers, where the thermal and mechanical design are
frequently done "in-house", it is not usual within GBH Enterprises to design an
air cooled heat exchanger. The normal approach is to specify the required duty,
and place the thermal and mechanical design out to tender with selected ACHE
manufacturers. In order to obtain an acceptable design, the manufacturer needs
to know not only the process conditions, but also any constraints that GBH
Enterprises wish to place on the design. These will include layout constraints,
noise specifications, preferred fans and drive systems, control requirements and
economic factors.


5.3

Thermal Duty And Design Margins

See GBHE-PEG-HEA-504 for guidance on design margins for heat exchangers.
The thermal duty will usually be specified by the process engineer, who should
also be responsible for deciding on an appropriate design margin over the
flowsheet duty. The information should be recorded on the standard GBH
Enterprises Engineering Data Sheet.
A design margin may be specified for several reasons:
(a)

The section of plant may be required to run at instantaneous rates above
the normal plant throughput as part of the normal plant operation.
Designing for this condition does not represent a true design margin, as
the higher rate represents normal conditions.

(b)

The engineer may wish to make provision for future plant uprating. If it is
probable that the plant will be uprated at some future date, there may be a
case for increasing the design throughput, with a corresponding increase
in heat load. However, the heat transfer coefficient under the initial
operating conditions will be lower than the design figure because of the
lower velocities; the performance under the initial operating conditions
should be checked to determine the expected design margin. It may be
preferable to make provision for increasing the size of the ACHE at some
later date, by adding further bundles in parallel with the original ones.

(c)

It is probable that an air cooled heat exchanger on a critical duty will be
condensing and/or cooling a complex mix of products. The physical
properties of the mixture may be uncertain, and plant measurements of
actual flowrates and compositions may be unreliable. Hence, the
possibility of enforcing any thermal guarantee is remote. The manufacturer
is under great pressure to design as cheap a unit as possible. Further, the
heat transfer data used by the manufacturer to design the cooler are, at
best, subject to some uncertainty. It is generally advisable, for a critical
duty, to provide some form of safety margin to allow for uncertainties in
the design methods.

A thermal design margin (safety factor) may be provided in several different
ways, which have their own advantages and disadvantages. It is important that
the engineer understands the implications of these. The engineer should be wary
of disclosing design margins to a manufacturer, as the latter may be tempted to
design with negative margins himself, knowing that in many cases, actual
performance checks under design conditions may be difficult or impossible.

Because of this, it may be necessary to produce a separate data sheet which is
sent to the manufacturer, on which certain items have been removed or altered.
This sheet should be included, suitably annotated, in the plant manual, along with
the correct data sheets, so that the true situation is recorded:
(1)

The provision of excess surface:
If the extra surface is provided by increasing the number of tubes per
pass, this may prove unsatisfactory. It will result in a more expensive unit
but because of the lower process side velocity, and hence coefficient,
there may be little effective increase in performance. It is better to provide
the extra area by increasing the exchanger length. It is not possible
to use this approach without declaring it to the manufacturer.

(2)

Increasing the design ambient air temperature:
Sometimes a higher air temperature is specified for critical services than
for others. This suffers from the disadvantage that the actual margin on
performance at normal air temperatures will depend on the product
temperature. A refrigerant condenser might have 25% margin; for a
reactor cooler/condenser, with a higher outlet temperature, it could be
only 5%. The specification of design ambient temperature is discussed in
sub clause 5.5. It should be used to ensure that a critical unit is designed
to meet its duty on warm days, but it is not recommended to use this
parameter to control design margins at other ambient conditions.

(3)

Increasing the design process throughput:
As a means of providing a design margin, this suffers from the same
disadvantage as increasing the number of tubes, namely that under
normal conditions the tubeside performance will be poorer than design, so
the margin may be less than expected. If this approach is used, and the
higher throughput is not actually likely to occur, the allowable pressure
drop supplied to the manufacturer should be increased above the actual
value by the square law, in order to avoid undue constraints. As the unit
will end up being designed for a flowrate above that at which the plant will
run, it will not be possible to do performance checks at design conditions.

(4)

Increasing the design fouling resistance:
This reduces the overall heat transfer coefficient, resulting in a larger
surface area being selected for the ACHE. The manufacturer will seek to
minimize the area, within the constraints of allowable pressure drop; the
film coefficients used by the manufacturer will not be affected by the
"safety margin" as is the case for using an increased throughput.
The approach is useful when dealing with a manufacturer, as it means that
the safety margin does not have to be revealed. However, it is good


practice to disclose the actual safety margins in the final documentation,
so the expected fouling resistance should be recorded in the final
revisions of the data sheets.
(5)

Reducing the design process outlet temperature:
In many ways this is the most satisfactory form of safety margin, and it
does allow the final unit to be checked against design conditions.
However, it suffers from the same drawback as does raising the design air
temperature, in that the margin will appear greater for units with a low
outlet temperature.

5.4

Process Pressure Drop

As a general rule, high heat transfer coefficients tend to be associated with high
pressure gradients. In some cases the section of the plant upstream of the ACHE
is required, for process reasons, to run at a higher pressure than the
downstream, and any pressure drop not absorbed by the exchanger will be taken
by a control valve. An example of this might be where the product from a
pressure reactor is to be cooled before storage at atmospheric pressure. In
these cases the pressure drop can be regarded as "free" and it will usually pay
the engineer to design the unit to absorb as much of the available pressure drop
as possible, consistent with the requirements for control. However, in general,
pressure drop has to be provided by a pump or compressor. The cost of pressure
drop may be considerable, especially with less dense fluids, as the power
absorbed is proportional to the volumetric throughput times the pressure
drop. However, a large pressure drop with viscous fluids, by improving the
process side heat transfer coefficient and hence reducing the exchanger capital
cost, may more than outweigh the cost of the pressure drop.
For low pressure condensation duties, particularly vacuum condensers, it is
usually necessary to limit the pressure drop, as the condensing temperature, and
hence the driving force, falls with reducing pressure.
Fouling resistances specified frequently take no account of the effect of fouling
layer thickness on pressure drop. As the pressure drop for a single phase fluid
through a pipe varies inversely with the fifth power of the diameter, any
significant fouling layer can have a noticeable effect.


The effect of pressure drop on ACHE cost is so complex, especially with viscous
products, that it is not possible to suggest simple rules. Comparison of the
estimated exchanger and pressure drop costs, together with common sense,
should show if there is a serious problem. If so, the only solution is to make
several designs at varying pressure drop, with a computer, and compare the
resultant overall costs. (see also sub clause 8.3)

5.5

Design Ambient Conditions
5.5.1 Dry Bulb Air Temperature
The specified ambient temperature is an important parameter affecting
plant costs and operability. A rigorous examination of the effect of ambient
design temperature on plant economics will be so expensive and time
consuming as to be impracticable. The best that can be hoped for is a
crude optimization of the largest units, perhaps so inaccurate as to be
misleading.
In general, the effect of too low a design air temperature will be a
turndown of the plant on hot days. The true cost of such turndown
depends on market conditions at that time and hence is almost impossible
to forecast. The engineer will, therefore, have to make a judgment, based
on no sound data. The following data are given as a guide:
(a)

Lenient Design (Non-critical duties):
The chosen temperature is exceeded for approximately 450 hours
per year. (5% frequency).

(b)

Moderate Design (Normal duties):
The chosen temperature is exceeded for approximately 150 hours
per year. (1.7% frequency).

(c)

Very Safe Design (Critical duties only):
The chosen temperature is exceeded for only 30 hours per year.
(0.3% frequency).

Ideally, temperature frequency data should be obtained for the works
where the exchanger is to be installed. Failing this, the Meteorological
Offices maintain records for a number of locations throughout your
geographic region, but it should be remembered that weather conditions
can vary significantly over small distances, so these data may not be
representative.

The normal ambient air temperature within the plant will be higher than
that for the surroundings, due to heat escapes from other items of
equipment. The proximity of potential sources of warm air (e.g. furnaces)
should be considered when choosing the location of the air cooled heat
exchanger, and selecting the design temperature. As a guide, the in-plant
temperature may be 2-3°C over the local ambient temperature.
The minimum expected air temperature should be specified, as this not
only determines the performance of the unit on cold days, and shows up
any tendency for process freezing etc., but is also needed to determine
the maximum power drawn by the fans.
FIGURE 15 TYPICAL TEMPERATURE VARIATION THROUGHOUT A HOT
SUMMER'S DAY


5.5.2 Altitude
Although within Europe most plants are sited at an altitude not far
above sea level, this may not be the case for overseas locations.
The performance of a given air cooled heat exchanger will
be less at higher altitude due to the fall-off in air density, and hence
volumetric heat capacity. (At 1500 m the air density is
approximately 85% of that at sea level for the same ambient
temperature).

5.6

Process Physical Properties

Although manufacturers of air cooled heat exchangers will generally have access
to physical property data for the more common fluids encountered, they are
unlikely to have reliable data for many of the mixtures that are used within all
industries, especially where these exhibit non-ideal behavior. The best way of
supplying these data, especially for multi-component condensation, is in the form
of a "Physical Properties Profile", where the properties of the vapor and liquid
phases together with the heat load and weight fraction vapor are given for a
range of temperature values spanning the expected operating conditions. Such
data can be generated for most cases. See GBHE-PEG-HEA-500.

5.7

Mechanical Design Constraints
5.7.1 Standard Specifications
The specification generally used for the purchase of GBHE recommended
ACHEs, is largely concerned with the mechanical specification of the heat
exchanger.
The Process Engineer should discuss this with the manufacturer, based
on the use of "normal" ACHE bundles, with welded steel headers, and
round steel tubes and aluminium fins. For many duties, especially with low
pressure and clean fluids, other forms of ACHE are more efficient. If offers
for "different" ACHEs are required.


5.7.2 Materials Of Construction
Tube materials will normally be dictated by process considerations,
the choice being beyond the scope of this Guide.
Three fin materials are commonly used in fin tubes - aluminium, steel and
copper. The virtues and disadvantages of the three metals can be
summarized:
(a)

Aluminium:
is the most cost effective of the three, having good thermal
conductivity and reasonable cost per square meter. (The cost of
heat transfer surface is "per square meter", not "per ton").
Aluminium has adequate corrosion resistance for most ACHE
applications, though it is reasonable to have some reservations on
this question. The almost universal choice of aluminium fins in
process ACHEs involves the use of helical fins on round tubes. The
performance of aluminium fins is much better than that of steel fins,
and they are much cheaper than copper helical fins.

(b)

Steel fins:
often galvanized, are occasionally used in process plants. Steel,
galvanized, is much the same cost "per square meter" as
aluminium. However, it is rather a poor conductor, resulting in low
fin efficiencies. The result is that steel finned exchangers are much
more expensive than are aluminium finned. They are, in some
atmospheres, more resistant to corrosion. They are also much
stronger than are aluminium fins, but cost has limited their
use to some particularly corrosive services.
The efforts made to improve air quality at these sites has been
such that aluminium finned tubes are now acceptable, and there
now seems hardly any market for steel finned ACHEs on process
plants.

(c)

Copper:
is about the same cost/ton as aluminium, and over three times its
density. It is thus more expensive "per square meter" and little
advantage can be taken of its superior thermal conductivity in round
helical fin tubes, where fin thickness is dictated by manufacturing
considerations, resulting in very high fin efficiencies for aluminium
fins. Especially when tinned, copper offers superior corrosion
resistance to either of the other metals. The cost disadvantage of
copper in relation to aluminium is reversed if very thin fins can be


used, taking advantage of the good fabrication possibilities of
copper. Such ACHEs will be present on all sites, probably as diesel
or transformer coolers. However, copper finned ACHEs have
scarcely been used for process units.
Thus aluminium finning is the almost invariable choice for process ACHEs.
The choice of tube metal to which it is applied is determined by process
requirements; carbon steel is probably used in 90% of cases.
With applied helical fin aluminium/steel fintubes, the aluminium is of rather
high purity, usually 99.6%, though 99.5% is usually specified. This has an
electrolytic potential lower than that of carbon steel, or any other tube
metal commonly used in process plants. The aluminium therefore acts as
a sacrificial protection to the steel. The result is that external corrosion of
the tube is virtually unknown over the finned portion of fintubes. Some
manufacturers leave an unfinned part near the tubesheet. This will be
subject to corrosion if it is longer than 10 mm, and the provision of
protection of these parts (by e.g. galvanization or zinc spray) may be
considered.
If aluminium tubes are used with aluminium fins, it is necessary to check
that the tube is electropositive to the fins at the temper used for both. If
not, preferential pitting and failure of the tube may occur.
The corrosion to be avoided is a general corrosion of the fins. Unprotected
fins would have corroded rapidly in the atmosphere in certain plant
locations; certainly, with the lower rows of fintube protected, life of
aluminium surfaces will be similar to that of the plant. At less aggressive
site locations, including coastal sites with chlorine in the air, atmospheric
corrosion of the general finned surface is rarely important. As explained in
5.9, corrosion associated with fouling may be serious.
Should atmospheric corrosion occur, the corrosion product is bulky and
adherent, and very difficult to remove. It will cause an increased
resistance to airflow, and hence loss of performance. Generally, there will
be preferential corrosion close to the tube, which will cause further loss of
performance due to decreased fin thermal efficiency, and the fin may be
seriously weakened.


5.7.3 Fintube Type
5.7.3.1 Introduction
There are many different varieties of finned tubing available. (See, for
example, sheet AE2 of Ref. [11]). The types commonly found within
normal process ACHEs are shown diagrammatically in Figure 16.
(a)

"G" Fins:
This is the recommended form of tubing for process duties most typically
recommended by GBH Enterprises. These finned tubes are
manufactured by opening up a groove in the base tube, tension winding a
strip of fin material into the grove, and then peening the base tube so that
the fin is securely held. The resultant tubing is robust, with little likelihood
of the fin coming away from the base tube. It is sometimes suggested that
water can enter the crack between tube and fin and cause a thermal
resistance at this point. Some tubes submitted to the British Non-ferrous
Research Association for long term marine and industrial corrosion tests
indeed show corrosion at this point; however, when tested for heat
transfer, they showed a small increase in heat transfer coefficient
compared to new tubes. There is a suspicion that preferential corrosion
may occur near the base of "G" fins, which would lead to a weakening of
the fins and a loss of performance. There is no known evidence to support
this suggestion, but it remains a nagging doubt.

The remaining types of finned tubing are not generally recommended for process
duties, but are described below for completeness.
(b)

Edge footed or "I" Fin:
A strip of metal is tension wound onto the outside of the tube to give a
continuous spiral. This fin-tube interface is not recommended, and will
rarely be found on process ACHEs, although such tubing may be found on
steam heated process air heaters, which can be considered to be a type
of ACHE. As the fins are not positively located onto the base tube, relative
movement of fins tends to occur, and continuous contact between the fin
base and the tube cannot be guaranteed. In the extreme, if the fin should
break or become detached at one end, the complete fin spiral can end up
at one end of the tube, leaving a bare tube.


(c)

"L" Fin:
This is similar in construction to the "I" fin, except that the strip from which
the fin is made has an "L" foot formed in it before the strip is tension
wound onto the tube, to give more or less continuous cover of aluminium
over the tube. Although this construction does give an improved heat
transfer area between the tube and the fin and more positive location, its
use is not recommended. A particularly damaging form of corrosion occurs
when a bundle is wetted, possibly during construction or shut-down. Water
between the fins can infiltrate the space between tube and fin by capillary
action. A galvanic cell is set up between aluminium and steel, and
aluminium oxide corrosion product is formed. This makes an effective
insulating blanket between tube and fin.
Although only indirectly concerned with corrosion, there is another point to
avoid with "L"!fins. The base of the fin will not be truly flat, and there will
only be a relatively small proportion of the base of the fin in contact with
the tube. In the case of fins made with McElroy machines, this proportion
might only be 20-30%. The result is that any interface thermal resistance
will be multiplied by this ratio, when related to the whole outside surface
of the tube. Such a resistance will be present if mill scale is not removed
from the tube before finning, and can be appreciable. Values as high as
0.0008 W/m2.K (based on bare tube area) have been measured with "L"
fin tubes in new condition. If the mill scale is removed, then the tube is
very liable to corrosion before the finning is applied. Some McElroy
machines have a sand blast incorporated, thus avoiding these troubles.
Careful inspection of tubing is necessary before "L" fins are applied.

(d)

"LL" Fins:
These fins, which are like "L" fins but with the flange extended to be under
the neighboring fin, are sometimes specified. These are intended to give
better cover of the base tube with the aluminium. Since there is no risk of
corrosion of the base tube, there seems little point in paying extra for this
type of tube. They have the disadvantages of simple "L" fins.

(e)

"E" Fins:
Fintube can be formed by an extruding operation, rather like an
exaggerated thread rolling process. If an aluminium tube is threaded over
a steel tube, and fins formed on the aluminium, then a "muff" fin or "E" fin
is formed. Many advantages are claimed for these fintubes, especially that
the continuous cover of the steel prevents corrosion. However, no
external corrosion of the steel will occur in any case, because of the
galvanic protection afforded by ordinary aluminium fins, so this advantage
can be dismissed. The fins are, however, stronger than are "G" fins, so


resist damage from being walked on or from cleaning better than do other
aluminium fins.
(f)

Elliptical tubing:
The specification of rectangular steel fins galvanized onto elliptical tubing
was based on the normal design of exchanger offered by GEA.
GEA claimed as the advantage for this type of tubing that the airside
pressure drop characteristics are superior to those of round tube. Recent
experience of trying to re-tube exchangers dating from that period has
shown that the elliptical tubing is expensive and hard to obtain. Moreover,
the manufacturing process for the finned tube, which involved rolling round
tube to an elliptical cross section, threading the fins on and re-rolling the
ends to a circular cross section for welding into the tubesheet, was prone
to cause cracking of the tube ends. GEA appear no longer to offer it as
their standard.

FIGURE 16 TYPES OF FINNED TUBING


5.7.3.2 Tubing Dimensions
Although different dimensions may be used, the commonest form of
aluminium finned tubing has a base tube outside diameter of one inch. Fin
heights are usually either 0.5 or 0.625 inches, with a typical fin thickness
of 0.4 to 0.5 mm. The usual fin density is 11 fins/inch (433/m). However, in
particularly dirty environments it may be advisable to reduce this to
8!(315/m) or even 7 fins/inch (275/m), at least for the lower rows.
5.7.3.3 Temperature Limitations
Specification of the type of fin, still holds. The temperature limits for the
various types of fin should preferably refer to the tube metal temperature,
rather than the fluid temperature. However, any proposal which is based
on metal, rather than fluid temperature, may be considered carefully.

5.7.4 Airside Design Clearances
The clearance between the ACHE and grade, of one fan diameter, is
reasonable for large grade-mounted exchangers, but for pipe track
mounted or smaller ACHEs, this may be reduced to 0.75 × fan diameter.
Work by CMB Russell showed that obstructions in the fan discharge are
more damaging than those in the inlet, so the provisions of S.14.1.5 and
S.14.1.6 should apply also to the fan discharge (particularly for induced
draught and roof-type exchangers).
5.7.5 Noise
The noise levels as specified, although they could be met, may result in
rather expensive fans. The specification of noise levels near the ACHE to
protect operator hearing is straightforward. Should there be a community
noise requirement, then the noise specialist will specify a limit on sound
power (PWL) and will then suggest that a recognized method be used
to measure the PWL on site, probably the OCMA NWG specifications. In
practice, the noise level due to the fans away from the near field of an
ACHE bank is often below the background noise level. In these conditions,
the measurement of ACHE PWL is impracticable; guarantees cannot be
enforced. Insist that the specialist translate the allowable sound power into
a sound pressure level (SPL) near the fan. The allowable SPL near the
fan can be calculated from the PWL with a loss of accuracy of only a
decibel or so, and can be guaranteed and measured.
It is probable that a lower noise level will be required at night than during
the day. As ambient temperatures drop at night, the fan speed can be
reduced with a reduction in noise level, provided that variable speed fan
control is used. This advantage does not apply to variable pitch control,
the noise being almost independent of blade pitch. The reduction in noise
can be very dramatic: the sound power level for a given fan varies typically
with the speed raised to the power 5 or 6.
5.7.6 Fan Characteristics
It is most unwise to operate a fan at a point near the stall region, and
some requirements to avoid this are necessary. Fans meeting these
requirements will be operating at a very poor efficiency when at the design
point with clean fin surface. The requirement may affect the thermal
design adversely, especially if there are severe noise limitations.
The effect of stall is much more severe with broad chord fans, than is the
case with the narrow chord.

5.7.7 Fan Drives
Wedge V-Belts and gearboxes are both disliked on site, owing to their
maintenance difficulties. Toothed "timing" belts, however, although they
are specifically excluded, have shown good performance on many duties.
It seems reasonable to recommend them for drive motors up to 30!kW.
5.7.8 Header Types
Figure 17 shows diagrammatically some of the header types used in
ACHEs. Should it be essential to avoid tubeside leakage of an ACHE,
then a manifold type of header may be used. This permits radiography of
tube and manifold welds; the tube may be left unfinned to permit ultrasonic
inspection to the first, say, 200 mm of the tube from the manifold, to check
against erosion (but see 5.7.2). Headers between passes may be avoided
by the use of U-bends.
Tube fixing will be by welding when leakage is feared, and, although
welding and inspection are possible when plug headers are used, both are
more difficult than is the case when cover plate or "D" type headers are
used. Equally, inspection of tubes and tube ends for damage, corrosion or
erosion is more difficult with plug headers. Although plugs resist leakage
better than will rectangular joints, cover plate or "D" type headers will
normally be the choice when manifold headers are unacceptable, and
precautions against leakage are necessary. A dummy tubesheet may be
used to prevent the spread to atmosphere of any leakage that might occur
at the tube ends.
See also sub clause 10.4 for further comments on header types.


FIGURE 17 HEADER TYPES


5.8 Arrangement
5.8.1 Introduction
The manufacturer needs to be informed of the available space where the
exchanger is to be located, and also what provisions are to be made for
access.
The process engineer may have a preference for a forced or induced
draught unit. There are no hard and fast rules governing which type of unit
should be used. The major relative advantages of the two types are
outlined in 5.8.2 and 5.8.3.
5.8.2 Forced Draught Units
(a)

They are usually cheaper.

(b)

The required power is lower than for an induced draught unit.

(c)

The fans are closer to the ground and thus are easier to support
and maintain.

(d)

The fan and drive are not exposed to the hot exit air.

5.8.3 Induced Draught Units
(a)

The bottom rows of tubes, which are those most prone to fouling,
are more accessible forcleaning.

(b)

The plenum chamber protects the bundle from harsh weather
conditions, (e.g. hail stones), and prevents people from walking on
it.

(c)

There is less likelihood of air recirculation because of the higher
momentum.

(d)

If the process fluid is a liquid, leaks from the bundle should not fall
onto the fan. (But spray could be thrown over a wider area than
with forced draught units).


It has been accepted in the past that it is easier to achieve good air distribution in
an induced draught unit. However, work by Russell and Berryman of HTFS on ¼
scale models have suggested that the reverse is in fact the case, but that the
overall effect on performance is not great in either case.
Particularly if the ACHE is a large unit, with multiple bundles, the arrangement of
the manifolds connecting the units to the remainder of the plant could cause
maldistribution problems. This is discussed more fully in sub clause 10.1.2.

5.9 Air Side Fouling
When specifying ACHEs for a plant, it should be first decided if fin fouling and/or
corrosion is likely to be a serious problem. If not, then it is recommended that no
particular arrangements to ease cleaning should be specified at the design stage.
All sites without severe fouling, report that they are well able to cope with the
cleaning problems.
If serious fouling is expected, then the choice of direct air cooling for the plant
should be seriously questioned. Is water really not available for evaporative
cooling? If not, could not an indirect water ACHE followed by a process shell and
tube exchanger be used? (It is simple to provide a water cooler that will not
corrode and can be easily cleaned). Remember that the recommendations for
precautions to be taken on a site with fouling problems will be very expensive,
particularly when coupled with noise limitations, and this will modify the economic
choice of cooling systems.
Should direct air cooling be considered the correct choice, then the following
should be added to the specifications:
(a)

Induced draught ACHEs should be used in all cases where design
temperature does not prevent this.

(b)

Particular attention should be paid to giving good access to the bundles
for cleaning, including access inside the plenum hoods.

(c)

The fin pitch in the lower two rows should be limited, perhaps to 275
fins/meter (7!fins/inch).

(d)

The tube pitch should be such as to give at least Ý ins (9 mm) between
the fin tips.


(e)

The fan selection should allow for a suitable margin to avoid stalling when
fouled. If allied to a tight noise specification, this will lead to an
exceptionally expensive design of ACHE, owing to the limitation of fan
static pressure, leading to a very low face velocity of the air.

In addition, one of the following may be specified:
(1)

Protection by electrostatically applied coating. This is expensive, and will
ordinarily be applied to the bottom two tube rows of the bundle only. It is
unproven in service, but is expected to overcome the disadvantages of
polyurethane coatings. It may have disadvantages of its own, and may be
stripped when cleaning the bundle.

(2)

The use of galvanized steel fintubes (GEA ACHEs). This solution is
expensive, and seems to have been discarded throughout GBHE; but
there may be some atmospheres too corrosive to aluminium, where
galvanized steel is satisfactory.

(3)

Sacrificial dummy tube rows may be provided before the tube bundle. It
might be more effective, cheaper and less wasteful of power to provide a
simple air filter of the plate type.

5.10

Economic Factors In Design

Any ACHE design is a compromise between high fan power and a smaller and
cheaper exchanger, and low fan power with a larger exchanger - thus a balance
between capital and running cost has to be struck. If it is hoped to optimize these
parameters the manufacturer needs information on the relative value to the
project of capital and operating costs. There are many ways of performing such
comparisons, but the simplest, which is generally adequate for this purpose, is to
tell the ACHE manufacturers by how much their offer will be penalized for
each of kW of fan power installed. (i.e. 1 kW is equivalent to $USD x of capital.).
It is essential to impress on the tenderer that the offers will in fact be penalized
as indicated, and to do so. Unless this is done, past experience will convince the
tenders that good intentions will last no longer than the arrival of the lowest cost
quotation, and that they might as well use as much fan power as they think they
can get away with. You will not then have properly optimized designs offered.


Air Cooled Heat Exchanger Design Guide

Air Cooled Heat Exchanger Design Guide

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