Air Cooled Heat Exchanger Design

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.


The calculation of the capitalized cost of fan power is very complex; it should
have been supplied among the economic data of the plant, but often it is missing.
If so, then a figure found by multiplying the power cost by the hours running per
year and by the number of year’s payoff will suffice.
When power saving methods of control (i.e. auto-variable pitch fans or variable
speed drives) are used, then the annual power absorption is considerably
reduced. (see Clause 6). The calculation of this reduction is difficult and
uncertain, though a manufacturer might give an estimate for a large unit. An
annual power consumption of 40% the full power consumption may be assumed
if better data are not available - so multiply the capitalized cost of a kWh by
0.4 when power saving control is to be used.
The cost of electric supply (cabling, switching etc.) is not small, and this cost may
also be added to the offers, in terms of the cost of each motor of a given power.
This parameter is also probably unknown; if so, then this factor might be ignored,
on the grounds that lower powered designs will use more fans, although each is
lower powered. In certain circumstances there may be additional constraints on
electricity supply. These could be either the need to run in an additional supply if
the additional demand exceeds a certain value, or the need for a new switch
house if more than a certain number of additional drives are required. Either of
these cases can result in a step change in the installed cost of the air cooled heat
exchanger. Such constraints are most likely to occur when considering
extensions to existing plant.

6 CONTROL
Ref. [2] is an excellent guide to the control of air cooled heat exchangers, and
should be referred to by the engineer wishing to study the control in detail. It is
not intended to duplicate this reference here, but some of the key points are
given.
ACHEs invariably form parts of a system involving other types of equipment. A
review of the control requirements as a whole is therefore necessary before
ACHE controls are considered. This may show that no ACHE controls are
required, or only coarse controls suitable for start up or extremes of climate.
Alternatively, the review may indicate that precise control of the ACHE
is desirable. The commonly used methods for controlling an ACHE are:
(a)

Bypassing of process fluid.

(b)

Auto-variable pitch fans.


(c)

Louvers.

(d)

On-off control of selected fans.

(e)

Two speed motor control of some or all fans.

(f)

Variable speed motors for some or all fans.

(g)

Steam Coils.

(h)

Switching from countercurrent to co-current flow in multi-pass units.

(j)

Controlled air recirculation.

Table 1, which is extracted from Ref. [2], sets out the general attributes of the
various options. Ref. [2] should be consulted for further information.
TABLE 1

ATTRIBUTES AND APPLICATIONS OF COMMON METHODS
OF ACHE CONTROL


Notes:
(a)
Methods (4), (5) and (6) are primarily used with fixed pitch fans.
(b)

Methods (6) and (9) have more complicated mechanisms which can be
expected to produce higher maintenance costs.

(c)

Control is less coarse with high numbers of fans on large ACHEs.

(d)

Cost-optimize by using the minimum number of fans appropriate to the
particular application.

(e)

Cost-optimize by applying manual louvers if possible. Avoid hunting of
AVP fans and auto louvers.

(f)

Applications should be for start up and shut down conditions. Otherwise
choose a different ACHE arrangement.

One particular control requirement which should not be overlooked is that known
as "winterization". This is the protecting of the performance of the exchanger
from the adverse effects of low temperatures, which could cause the process
fluid to freeze, crystallize or become very viscous. The engineer should
remember that even if calculations show that the bulk fluid leaving the exchanger
is above the temperature which could cause severe problems, the fluid in the
bottom row of tubes may well be below this temperature. A detailed row by row
examination of the predictions should highlight this.
Another aspect of ACHE operation related to control is the performance of the
unit under conditions of fan failure. Because of natural convection, an air cooled
exchanger will continue to dissipate heat, albeit at lower than design rate, even
without the fans running. Induced draught units perform better than do forced
draught units in this respect, as the plenum chamber and fan ring form a chimney
above the bundle.
The behavior of an ACHE under natural draught conditions may be estimated
using commercially available computer programs.


7

PRESSURE RELIEF

It is sometimes necessary to provide pressure relief for one stream of a heat
exchanger if excessive heat transfer from the other stream could lead to build-up
in pressure, particularly if the exchanger can be shut in between isolation valves.
For an air cooled heat exchanger, where there is only one stream which can be
isolated, this is not usually a problem, as the process stream will generally be at
a higher temperature than the surrounding air. There is, however, one set of
circumstances where relief may be required. This is in the event of an external
fire, resulting either in flames impinging directly on the exchanger, or the
temperature of the air sucked into the unit being raised by the fire.
Appendix D of Part C of Process SHE Guide No. 8 discusses fire relief for Air
Cooled Heat Exchangers. It should be consulted for further information. Note that
the problem may be reduced in some cases by suitable siting of the exchanger
away from potential fire sources.

8

ASSESSMENT OF OFFERS

8.1

General

Although an ACHE manufacturer may give a performance guarantee, it may
prove impossible in practice to prove any shortfall, as explained in sub clause
5.3. Moreover, any liability on the part of the manufacturer will be limited to
correcting the exchanger design, and will not cover consequential losses. It is the
responsibility of the purchasing engineer to ensure that the ACHE purchased is
satisfactory for the required duty.
8.2

Manual Checking Of Designs

GBHE has developed some simple hand calculations to perform comparative
checks on competitive designs, which may be used for preliminary screening.
However, he emphasizes that any comparison of this nature can only be
approximate, especially where there are considerable differences between the
number of tube rows, fin heights etc. of the different designs. For a good
comparison, a design check using a computer is necessary.


8.2.1 Mean Temperature Difference
Different designs, using different air flows, will result in different mean
temperature differences. However, for the same number of tube passes,
the mean temperature difference should vary monotonically with air flow. If
designs are offered with different numbers of tube rows and passes, the
quoted mean temperature difference can be corrected to a common fixed
number of passes using ACHE pass correction diagrams. Such diagrams
are available in SectionD1.2.2 of Ref. [12] or sheet AM11 of Ref. [11].
Plot the corrected mean temperature difference against air flow. This will
show up any questionable point. Obtain a modified MTD for each design
for subsequent use.
8.2.2 Heat Transfer Rate
Calculate an effective surface defined as:

Choose one design as a reference and calculate for each design a measure of
heat transfer rate given by:


Using this technique, all the measures of heat transfer rate should reduce
to approximately the same value. Designs producing relatively high values
should be treated with reserve.
8.2.3 Process Pressure Drop
A pressure drop parameter is defined as:-

The diameter/length ratio will often be constant for a set of designs, and then it
may be omitted. A relatively low value of this pressure drop parameter will
indicate a relatively optimistic claim for pressure drop.


8.2.4 Fan Power
It is not possible to suggest a simple method to compare the airside
pressure drop of ACHE bundles. Without this, there is little point in making
detailed comparisons. In cases where tenderers have used similar fintube
matrices, some comparison can be made. The motor power should be
checked at the fan power absorbed at the minimum air temperature
expected; the design power absorbed will be increased by the ratio of
design air temperature to minimum ambient temperature, both expressed
in Kelvin.
8.2.5 Noise Claims
All fans will produce about the same sound power if run at the same tip
speed and shaft power. "Low noise" fans are those that can be run at
relatively low speed to produce a given duty. A broad blade such as the
Stork or Hudson will make more noise in the 250-500Hz region; a narrow
bladed type such as Moore or Axial Italiana will tend to be noisier in the
1000-2000Hz region. This latter is unfavorable for dB"A", but favorable for
distant community noise.
Should the fan speeds and powers vary significantly they may reduce to a
common basis by subtracting:30 × log tip speed (m/s) + log fan power (kW)
from the stated noise level or power (SPL or PWL, dB). Should any
reduced claim be low, it should be mistrusted.

8.3

Computer Assessment
8.3.1 Introduction
The manual comparison described in 8.2 will not show if all the tenderers
have cheated equally. The air cooled heat exchanger market is highly
competitive, and manufacturers are tempted to design very tightly,
knowing that in many cases the actual performance will be hard to check.
This emphasizes the need for the process engineer to do proper checks
himself, using a computer code. Such a check also allows the engineer to
study the performance of the unit in more detail. Items which should be
considered may include the performance under turndown conditions,
including the effect this might have on fan performance if louvers are used


for air flow control, and the performance of the unit under natural
convection conditions, to model the effect of power failure.
The only good check on cooler thermal design is by using an advanced
computer code to analyze the offers. This may not be practical in many
cases; or, perhaps, only the apparently best one or two offers might be
checked.
The preferred computer program for rating Air Cooled Heat Exchangers is
the HTFS program.
8.3.2 Process Flow Distribution
The computer programs used for ACHE rating are based on the
assumption of good flow distribution between different tubes within the
same pass of an exchanger. In practice there may be some
maldistribution from a variety of causes, which will in general reduce the
performance of the unit. This should be remembered when assessing the
likely performance. Sub clause 10.1.3 should be consulted for a fuller
discussion of this subject.
8.4

Bid Comparison

Having checked the bids for thermal and mechanical acceptability, the engineer
may find that more than one design will meet the required process duty, and that
the cost differential is not significant. The decision on which unit to purchase will
then depend on other factors, many of which may be subjective. However, if a
power penalty, as suggested in sub clause 5.10 of this Guide, has been
specified, then it is morally incumbent on the purchaser to apply it rigorously;
and it is in the long and short term interest of GBHE to do so.

9

FOULING AND CORROSION

9.1

Fouling

Only airside fouling is considered here; tubeside fouling problems are generally
not specific to air cooled exchangers. Over a decade ago fouling was described
by Taborek as the major unsolved problem in heat transfer. Since then many
millions of pounds have been spent on fouling in shell and tube exchangers, but
very little on air side fouling of ACHE fintubes. Little progress has been made on
tube fouling; there is virtually no information available on process ACHE airside
fouling.

Fouling and corrosion of ACHE finned surfaces are inextricably mixed.
Aluminium corrosion products will act as a filter, and attract fouling; fouling in
chemical plants is often corrosive.
Five major types of fouling can be identified:
(a)

Airborne dust may accumulate, particularly on the leading edge of the
lowest and second lowest rows. If not removed, this form of fouling may
tend to accrete, and after a few years may be impossible to remove.

(b)

The air may contain organic material, particularly some forms of seeds,
which may be trapped on and in the fintube matrix.

(c)

Some plant operations - stripping insulation, unloading catalyst etc., may
produce particles which are carried into the ACHE.

(d)

If there is a liquid leak near an ACHE, sticky oil may be carried onto the
ACHE tubes. The matrix may then start acting as an air filter. This may
produce a baked-on fouling, especially if the tube is hot, and may prove
very difficult to remove.

(e)

Units in coastal areas may become fouled with salt derived from airborne
sea spray which has evaporated off in the exchanger. The use of spray
water to maintain the efficiency may also lead to similar problems if the
water contains a high proportion of dissolved solids.

Light fouling causes only a slight decrease in heat transfer coefficient for a
constant air flowrate, but leads to a significantly increased pressure drop. As the
axial flow fans used in ACHEs have relatively flat characteristics, the effect of the
increased flow resistance is to move the fan operating point back up the
characteristic curve, resulting in a significant reduction in air flowrate. In extreme
cases, air flows of less than 50% of the design have been measured, and
reductions of 10 to 20% are common. This in turn results in both a poorer heat
transfer coefficient and a reduction in the mean temperature difference. It is
these effects, rather than the direct thermal resistance of the fouling layer, that
are mainly responsible for the fall off in ACHE performance when fouled.
Work by HTRI on tubes with very heavy fouling showed both a decrease in heat
transfer coefficient and an increase in pressure drop.


Experience shows that fouling is mainly confined to the first two tube rows in the
air flow path, presumably because these rows are acting as air filters. This is
confirmed in work by Covrad, on tube-in-fin matrices.
There seems to be a general belief in GBHE that low air velocities lead to lighter
fouling but the reverse is probably the case. Control of temperature by the
restriction of air flow can aggravate fouling since the lower air velocity provides a
better opportunity for particles of dust to settle out. The fouling associated with a
variable pitch fan is often heavier than that associated with neighboring fixed
pitch fans.

9.2

Corrosion
9.2.1 Introduction
As with fouling, only air side corrosion is considered here. As explained in
sub clause 5.7.2, in general it will be the fin material which corrodes
preferentially when aluminium fins are used. Corrosion problems are
dependent on location. If the exchanger is located such that it receives
clean air, corrosion is not likely to be a problem. On the other hand, on a
chemical works there is likely to be chemical contamination of the
atmosphere. SOx, NOx, HCl and chlorine are common trace contaminants
on many sites, and in the presence of water can give rise to serious
corrosion problems with aluminium fins. GBH Enterprises recommends
the use galvanized mild steel finned tube in customer plants. Modern
hygiene standards have resulted in a significant improvement in the
atmospheric quality within the works, and unprotected aluminium finned
tubes have given acceptable service in more recent years.
9.2.2 Protective Coatings
Protection of aluminium fintube was considered necessary for ACHEs, at
and was adopted in other plants. This was invariably by dipping the tube
after finning in a polyurethane paint bath, a double dipping with shaking
being specified. This was applied to the bottom two rows of the bundles.
GBHE specifically requests this treatment. In general, this treatment has
proved satisfactory, but a badly applied coating can lead to the coating
peeling from the fins, causing severe flow restrictions.


Recently, electrostatic coating of fintubes using epoxy urea formaldehyde
paint has been proposed. The coating is thin, tough and very adherent,
but it is notably more expensive than is the polyurethane dip process. This
technique has been used on an air cooler on the sodium nitrite plant’s at,
and remained fairly robust for the first few years. After five years in
operation, it began to fail at the fin tips.
Coating a fintube will make it easier to clean, but GBHE has an unproved
suspicion that the coated (insulating) surface can acquire an electrostatic
charge, and attract dust; thus it will require cleaning more frequently. The
impression that coated tubes are always fouled is strong.
9.2.3 Sacrificial Tubes
It is possible to provide one or two rows of dummy tubes below the actual
bundle to stop the fouling and corrosion of the lower tube rows. These will
be removed from the ACHE for tube cleaning and be cleaned themselves.
Although this is sometimes advocated, it is hard to justify. Apart from the
cost of the sacrificial tubes, the fan power will have to be increased to
allow for the extra pressure drop caused by them. If the atmospheric
conditions are such that this approach is necessary, the whole concept of
using air cooling should be questioned.
10

OPERATION AND MAINTENANCE

10.1

Performance Testing

10.1.1 General
During the life of an ACHE there may be occasions when it is
necessary to assess its actual performance. This could be as part
of the acceptance trials, or subsequently when it is suspected of
falling short, or when plant uprating is being considered. Ref. [6]
gives a detailed discussion of the requirements for testing and
advice on the methods to adopt. Sub clause10.1.2 indicates some
of the problems that may arise. For further information, see Refs.
[6] and [16]. Table 2, which is extracted from Ref. [16], indicates
some of the possible faults which may be found, together with
remedies.


The performance of a shell and tube exchanger can, in principle, be
assessed simply, by measuring the flowrates, compositions and
bulk temperatures of the two streams and comparing the results
with the predictions of one of the standard computer programs.
Obviously this requires the provision of suitable instrumentation, but
this should not present an insuperable problem, especially if the
need for testing was identified at the design stage.
The problem is by no means as easy for an air cooled heat
exchanger. Obtaining measurements for the tube side is
comparable to that for a shell and tube unit. (However,
beware of flow maldistribution, particularly in large units). The
difficulty arises in obtaining reliable and accurate measurements of
the air side conditions, especially for forced draught units.
TABLE 2

AIR COOLED HEAT EXCHANGER FAULT FINDING CHART.
Some of the symptoms, faults and solutions. (Note: this list is by no
means exhaustive.)


10.1.2

Process Flow Distribution
The effects of maldistribution can have a significant effect on the
performance of a heat exchanger. This is particularly the case with
condensers, where, in extreme cases, maldistribution can result in reverse
flow in some tubes, leading to inert blanketing with consequent fall-off in
performance. This problem can be aggravated in air cooled heat
exchangers because of their particular geometries.
Several basic types of maldistribution are possible in an ACHE system:

(a)

Single phase maldistribution in the inlet and exit manifolds:
In all but the smallest ACHEs there are likely to be several inlet and outlet
nozzles. Individual bundles may have several nozzles, and the unit may
be made up of several bundles. Because of the effects of pressure drop
down the pipework comprising the manifolds, there will be differing
pressures at each nozzle.

(b)

Two phase maldistribution in the inlet manifold:
If the feed to an ACHE is a two phase mixture, phase separation will occur
to some extent where ever the flow is split, resulting in a different phase
ratio to each inlet nozzle. At present it is not possible to predict the phase
split for a dividing two phase junction. The only safe approach if it is
essential to feed a two phase mixture to the ACHE is to perform
a phase separation, divide each phase separately, and then recombine
the separate phases before feeding to the exchanger, but this may prove
prohibitively expensive. The effects of phase separation can be reduced,
but probably not eliminated, by ensuring symmetry between all flow paths
as far as possible.

(c)

Maldistribution caused by the inlet and outlet headers:
Unlike the headers of a shell and tube exchanger, those of an ACHE are
long and narrow. There can thus be a significant pressure drop between
the nozzles and the individual tubes. There are additional pressure gains
and losses associated with the flow splitting in the inlet header, and
recombining in the exit header. These effects lead to differing flows
down different tubes.


(d)

Phase separation in return headers:
The two phase fluid leaving the first pass of a multi-pass condenser will
tend to separate under the influence of gravity. If the second pass consists
of more than one tube row, there will be a tendency for the liquid to enter
the lower rows, while the vapor flows down the upper rows. under the
influence of gravity. If the second pass consists of more than one tube
row, there will be a tendency for the liquid to enter the lower rows, while
the vapor flows down the upper rows.

In general, all forms of maldistribution can be expected to lead to a poorer than
expected heat transfer performance, and often a higher pressure drop. If the
process fluid is liable to crystallize or become very viscous at low temperatures,
excessive cooling in tubes with a lower than average flowrate could be a
particular problem.
Section 4.1.5 of Ref. [6] gives a more detailed discussion of the effects of
maldistribution on exchanger performance. Ref. [8] should be consulted for
methods of calculating flow maldistribution.
10.1.3

Air Side Measurements

The exit air temperature is not constant across the face of the exchanger, either
on the macro or micro scale. On the macro scale the exit air temperature will be
greater at the process inlet end of the exchanger. In some of the more complex
pass arrangements, with passes side by side rather than underneath one
another, there will be different zones within the exchanger. On the micro scale,
the air temperature leaving the "jet" between neighboring tubes will differ from
that in the wake behind the tube. Obtaining a mean exit temperature requires an
averaging process of uncertain accuracy.
Similarly, the air velocity leaving the bundle varies, both from maldistribution due
to the use of circular fans in rectangular bundles, and because the air leaves in
the form of jets from between neighboring tubes. The majority of methods for
measuring velocity (e.g. pitot tubes or vane anemometers) depend on the air
momentum, which varies as the square of the velocity, so variations in velocity
can lead to serious errors.
To obtain reliable air side data requires good instrumentation, which is not readily
available in most works. There is also some skill in obtaining the best results
from its use. Because of this, there is a case for obtaining specialist assistance.
HTFS run a contract research organization called "Air Cooled Heat Exchanger
Advisory Service", who are fully equipped to perform such measurements on a
confidential basis.

For typical performance testing of an exchanger with 10 fans and 10 bundles the
setting up and on-site testing would take about 1½ days. A further 2 days would
be required for analysis and preparation of a written report. The total cost
is thus about $USD 6,000. For a greater number of exchangers on the same site,
the cost per exchanger would be less.
Note that commercially available computer programs, have provision for allowing
a limited air flow variation across the face of the bundle. If reliable measurements
have been obtained, the measured flow distribution should be used when running
performance checks on the computer.
10.2
10.2.1

Air-Side Cleaning
General
Cleaning of ACHEs is not general in the oil industry, and many refiners do
not clean an air cooler externally during the life of the plant. There is,
however, a lively bundle re-tubing industry. Possibly because chemical
works tend to be dirtier than refineries, GBHE has always been more
concerned with ACHE cleaning than most users. Inspection for evidence
of fouling is difficult, especially with forced draught units. Fortunately,
most of the fouling occurs on the bottom row, but only the bottom of this
can readily be seen. Some inspection of the top of the bottom row and the
bottom of the second row may be possible using a mirror and lamp, but
this is usually rather restricted. Similar inspection of the upper rows is
possible, but rarely reveals any appreciable fouling.
Although fouling will result in a reduced air flow rate, this is often not easy
to detect directly. Unlike centrifugal fans, the power vs. flowrate curves for
the axial flow fans used in air cooled heat exchangers tend to be flat, so
are not much help in estimating air flow. Some estimate of changes in air
flow can be made using an anemometer, but the results can be sensitive
to position of the instrument. The extent of fouling can often only be
determined from a fall-off in overall exchanger performance. This may of
course be due to either air side or tube side fouling (see sub clause 10.1).
There is not a great deal of information in the literature about ACHE
cleaning


Tests have been conducted by manufacturers on high pressure water
cleaning, and made tests on various fintube matrices with air, steam and
low pressure water cleaning.
When a unit is fouled, then the fouling should be examined and analyzed.
If possible, tests of the available cleaning methods should be made. As a
result of these examinations and tests, and depending on the thermal
performance and importance in the process of particular units, the
methods and frequency of cleaning will be decided. The experience of
neighboring plants, which are likely to suffer fouling from similar sources,
should also be investigated.
10.2.2

Methods Of Cleaning
Seven methods can be considered for cleaning the outside of fintubes:
mechanical, air, combined air and water, low pressure water, high
pressure water, steam, and flame cleaning. A preliminary detergent soak
may be used before washing or air blast. The methods are outlined
further in 10.2.2.1 to 10.2.2.7, in general order of preference, so the first in
the list that can do the job may be adopted.

10.2.2.1

Mechanical Cleaning

Especially in the early stages of dust fouling, it is easy to remove the
fouling from the lowest row by brushing. If there is the usual 9 mm gap
between tubes, it is possible to brush much of the dust from the second
row. Brushing is quicker than other methods, and leaves less mess.
10.2.2.2

Air Cleaning

Cleaning by air lance is simpler and less messy than water cleaning. Air
alone can only remove rather friable deposits. Manufacturers tests
showed that the air pressure was, within reason, unimportant, and nozzles
incorporating eductors to increase the air flow at the expense of air
pressure were used. If oil contamination of the bundle is suspected, a
degreasing solvent used with the air can be helpful.
10.2.2.3

Combined Air and Water Cleaning

A combination of air and water cleaning, with detergent pre-soak
introduced by the air lance, has been especially successful at some
client’s locations.

GBHE recommends this as the preferred method, to be done on a regular
basis. The basic equipment employs an air lance, consisting of a NPS
pipe connected to the maintenance air supply. Liquids can be induced into
the air flow through a connection near the tip. The procedure used
is as follows:
(a)

Using the air lance, inducing a mixture of "Lissapol" and "Aromasol
H", the fins are sprayed and left to soak for half an hour.

(b)

The fins are re-sprayed using the same mixture.

(c)

The fins are water washed using water through a inch bore hose.

(d)

Using the air lance, inducing water, the fins are sprayed to remove
most of the debris and dirt.

(e)

Finally, the fins are blown clear using the lance with air only.

10.2.2.4

Low Pressure Water

Water at "mains" pressure, or similar (say 4-10 bar) can be used in a
hosepipe. Results with even moderately hard deposits are often not very
good.
10.2.2.5

High Pressure Water

The techniques of shell and tube exchanger cleaning with high pressure
water are well understood. A contractor will usually do the work, using a
truck mounted diesel powered ram pump. This will be capable of
pressures up to some 1,000 bar. Water delivered from such a
pressure will destroy aluminium fins, so when cleaning ACHEs the
pressure is normally limited to well below this value. Tests at some
manufacturers indicate that aluminium fins can withstand nozzle
pressures of up to 300 bar, with a fan jet reasonably normal to the tube
bank. Steel fins could withstand full rig pressure with a pencil jet. Most
contractors will limit pressure at site to 30 bar, at which pressure no
damage to aluminium fins should occur. Rather surprisingly and
disappointingly, the cleaning of the steel finned ACHEs at one client’s
plant is done with pressure limited to 30 bar, to avoid damage to the fins. It
is assumed that the zinc fixing the fins to the tubes deteriorates with time,
so that the fins are displaced under rather low pressure.

WARNING! A contractor can do a great deal of damage very quickly to
tube bundles; only reliable and experienced contractors should be used
for cleaning ACHEs.
10.2.2.6

Steam Cleaning

Tests by one manufacturer showed no advantage for steam cleaning over
air or water cleaning. Unless the fouling has properties that indicate that
steam is necessary to remove it, there seems little advantage in this
method.
10.2.2.7

Flame Cleaning

It could be that some organic fouling can best be removed by flame
cleaning. No cases where this has been done are known, and special
precautions would obviously be necessary to avoid the risk of fire or
damage to the exchanger. This is very much a last resort solution.
10.2.3

Results
Surveys of plant operators conducted by GBH Enterprises found that
cleaning was a problem that they could cope with. However, if fouling
problems were difficult on a plant, then it was a considerable nuisance,
and some plants were limited in throughput in summer by ACHE fouling.
The advantages of induced draught ACHEs were evident both for access
for inspection and for cleaning. Experience from one European plant at
shows that jetting should be done from the air inlet side, which is easier
with induced draught units.
Some sites claim to be successful with on-line cleaning. Generally, it is to
be anticipated that a crew will clean a section (with two bundles and two
fans) in a shift. To this has to be added the time taken to isolate the unit,
bag the motors for protection, lash the fans and scaffold for access. If
regular cleaning is expected to be necessary, the provision of permanent
access platforms could be cost effective.
Many sites reported that polyurethane coating was stripped off with the
fouling, when bundles were cleaned. Since polyurethane coating need
only be used on those sites where rather harsh cleaning methods will
probably be necessary, there seems little to be said in favor of such
coatings. However, GBHE reported that good quality coated bundles were
easier to clean than uncoated ones.


The frequency of cleaning varies from plant to plant. Obviously it will
depend on the degree of over-design in the unit; tight designs needing
more frequent cleaning to maintain performance. It also depends very
much on how clean the air is near to the exchanger. Fertilizer plants are
particularly prone to cause high rates of fouling.
Some chemical plant operators generally clean their air cooled
exchangers every 12-18 months during a scheduled shutdown. The
exchangers on the sulfuric acid plant’s are often cleaned every 2-3 years.
On particularly dirty sites, more frequent cleaning may be required.
Certain exchangers on Ammonia Plant’s require cleaning as frequently as
every three months, but following improved atmospheric conditions, 12
months is now the norm. If it becomes necessary to clean more frequently
than every 6 months the use of ACHEs should be questioned. At the other
extreme, ACHEs on one European site showed little evidence of fouling
after three years operation without cleaning.
10.2.4

Preferred Contractors
Experiences with different contractors are variable, and to some extent are
subjective.

10.3 Mechanical Maintenance
10.3.1 Fans
Plastic fan blades are subject to failure by cracking, and should be
inspected every six months. The riveting and general condition of built-up
aluminium blades should be inspected every year; and all fans should be
generally inspected for cracks, corrosion and damage at plant shut-down,
or as frequently as conveniently possible.
Auto-variable pitch fans have proved unreliable in some instances, with
failures of the actuators or bearings. This has lead some plants to replace
them with fixed pitch fans, control being achieved by turning off selected
fans in winter.


10.3.2 Fan Drives
The power transmission to the fans seems to give more problems than
any other aspect of ACHE maintenance. Vee-belts are the most disliked,
with Poly-Vee banded belts a close second. "T" toothed belts (timing belts)
are generally liked, and give the least trouble. However, some problems
have arisen with them when they have been over tightened, resulting
in premature bearing failure. For some reason, Poly-Vee belts are the only
belts allowed. Gearboxes, though expensive, are relatively trouble free if
the makers' instructions are followed; however, if trouble does occur, it is
often a major problem.
If sealed-for-life bearings are not used, then the use of a centralized
lubrication system should be considered. Some clients demands remote
lubrication points. This is particularly important for bearings in the plenum
of induced draught units.

10.4

Tubeside Access
Access to the inside of the header boxes of ACHEs may be required for
tube cleaning, inspection, tube fixing repair, or tube plugging. Should tube
replacement be required, it is normal to remove the complete tube bundle.
All these operations are greatly eased if the header is of the cover plate
type (Figure 17(b)) rather than the plug type (Figure 17(a)). Should cover
plate headers be preferred, then they will generally be found to be more
expensive than plug headers, and there is a design pressure limit, usually
about 30 bar, above which the cover plate becomes so heavy that it is
considered impractical. If the temperature difference between top and
bottom of the cover plate is great - more that 140°C, then joint leakage is
to be feared.
"D" type headers (Figure 17(c)) are rarely used in process ACHEs,
although they are common enough in other industries. They are cheaper
than cover plate headers, and can stand higher pressures. They have the
advantages of cover plate headers, but suffer from the added
disadvantage that pipework has to be dismantled to gain access to the
tubesheet. Many of the advantages of easier cleaning are achieved, at
small or no cost, if plug headers are used for the inlet headers, and "D"
type headers at the return end. With even passes, there is no piping on
the return header; with odd passes, the break in the piping joint is a simple
lift-off.


Manifold headers (Figure 17(e)) are the most expensive type, usually used
for high pressures (1500!bar and above) or for services with stringent
leakage requirements. Tubeside access is virtually impossible, so
tubeside cleaning will be by chemicals and repairs will be a workshop
job.
Platform access should be adequate; the usual header platforms suffice
for any probable in situ work on the tubeside.


APPENDIX A PRELIMINARY ESTIMATION OF ACHE SIZE AND COST
The method outlined in this Appendix is based on the method given by
Russell and Tiley (Ref. [13]). It gives an estimate of exchanger cost
(uninstalled) and plot area without having to size the unit. It has been
converted into SI units by Hills, and the cost data updated.
A.1

BASIC METHOD

A.1.1 Linear Heat Release Curves
(a)

Calculate the "Thermal Ratio" R and the "Reduced Heat Load" S
(kW/K):

(b)

Estimate "r", the sum of the process film resistance and the process
fouling resistance, from Table 3.

(c)

Estimate the Cost Function "C" and Area Function "K" from Figures 18
and 19, interpolating if necessary.

(d)

Find the current value of the Cost Index "i".
The index base used in this calculation was set to 220 in 2010, which was
also the base used for Figure 7. The current value of the index can be
obtained from Cost Estimating. (Remember to make sure that the index
quoted by Cost Estimating uses the same base year.)
The required value of the index may be obtained by dividing the value for
the required year by that for 2010 and multiplying by 220. The value for
the index, based on 2010.


(e)

The plot area, fan power and cost are then obtained from the following
equations:
Estimated plot area

= K × S (m2)

Estimated power absorbed

= 0.723 × K × S

Estimated cost

= i × C × S (USD ex works)

(kW)


TABLE 3

SUGGESTED FILM RESISTANCE FOR USE IN PRELIMINARY
EXCHANGER SIZING


FIGURE 18 CURVES FOR COST FUNCTION "C"


FIGURE 19 CURVES FOR AREA FUNCTION "K"

A.1.2 Non-linear Heat Release Curves.
The method above assumes that the heat release curve is linear with
temperature. The heat release curve of a cooler-condenser is often not
linear with temperature. In this case, an estimate of effective values of Ti
and To can be made as shown in Figure 20. Here a line is drawn with a
slope equal to the mean slope of the Temperature - Enthalpy curve such
that the areas between the curve and the line above and below the curve
are approximately equal. The temperatures of this line corresponding to
the start and finish of the required heat release curve are then taken as
the values of Ti and To for use in the method as given above.


A.1.3 Correction Factors
The method is based on the assumption of a "standard" type of heat exchanger
with the following major characteristics:
(a)

Mild steel tubes 25.4 mm o.d. × 12 b.w.g. × 9.14 m (30 ft) long.

(b)

Exchanger width at least 4.8 m. (16 ft).

(c)

Aluminium "G" fins, 433 fins/m (11 fins/inch)

(d)

Forced draught fans with manually adjustable blade angle.

(e)

Fan drive by TEFC motors and V-belts.

(f)

Design pressure below 10 bar g.

Variations from this standard will have an effect on plot area and cost. Figure 7
gives three correction factors by which the basic cost for an exchanger with a
given extended surface area should be multiplied to allow for variations in fin
pitch, design pressure and materials of construction. However, the correction
factor for fin pitch should be viewed with caution, as exchangers with the same
extended surface area but different fin pitches may not give the same
performance, due to changes in coefficients. Equally, the effect of fin pitch on plot
area is hard to determine.
The original method in Russell et al assumed large exchangers. For these units,
the cost is directly proportional to the plot area, as the units are built up from
standard modules. This linear relationship tends to break down for small units, as
is shown in Figure 7. Allowance can be made for this by multiplying the cost
obtained by the above method by the correction factor given in Figure 21.


FIGURE 20 NON-LINEAR TEMPERATURE ENTHALPY CURVES


FIGURE 21 CORRECTION FACTOR FOR SMALL EXCHANGERS

A.2

LIMITATIONS AND ACCURACY
Russell et al state that the method is not applicable to vacuum steam
condensers.
Any quick estimating method is obviously dependent on the accuracy of
the heat transfer coefficients used. The slopes of the curves in Figures 18
and 19 indicate the penalty paid for inaccuracies in estimating "r".
In developing the method, Russell et al used "reasonable" designs for a
range of exchangers, based on their experience. This implied using
different numbers of tube rows for differing duties. In general, higher
values of "r" and higher values of "R" lead to more rows in the "optimum"
design. Their cost data were based on the knowledge that the cost per
unit surface area tends to fall with increasing number of tube rows. The
estimating methods of Refs. [10] and [11] take no account of this. In
revising the curves for the cost function "C" (Figure 18), a case with R =
0.5 and r = 0.00088 m2.K/W was taken as the base. Using additional data


supplied by Russell this was determined to require a four row unit. The
cost of a "standard" exchanger 6.1 m (20 ft) wide with four rows was
estimated from Figure 7 and also by the original method of Russell et al.
This gave an inflation factor between the date of the original work and the
base date for Figure 7. All other points on Russell's curves were scaled by
this factor. This approach may introduce some unquantifiable errors into
the method, but the only alternative would be the very time consuming one
of redeveloping the method from a series of optimized and costed
designs.
Russell suggests that, provided the value of "r" is correctly estimated and
the other assumptions given above hold, the method should give costs
accurate to 10%. In view of the changes made in updating the method,
which did not involve going back to original data, this may be rather
optimistic, but an accuracy of 30% is probably reasonable.
In practice, the optimization of capital cost against fan power and
variations in the economics of bundle fabrication between differing
manufacturers may lead to exchangers with considerably different
dimensions, fan power and capital cost from those arrived at by these
methods. Moreover, the quoted cost for an exchanger depends not only
on the dimensions of the unit, but also on market related factors such as
the competitive position and work load of the manufacturer. There is no
substitute in the long run for a quotation.


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING
GUIDE
This Process Engineering Guide makes reference to the following
documents:
INTERNATIONAL STANDARDS
API 661

Air cooled heat exchangers for general refinery services
(referred to in 5.7.1)

ENGINEERING PROCEDURES
GBH Enterprises

The Use of Process Data Sheets for an Air Cooled
Heat Exchanger (referred to in 5.3 and 5.6)

ENGINEERING GUIDES
GBH Enterprises

Shell and Tube Heat Exchangers Using Cooling
Water (referred to in 4.1.5)

GBH Enterprises

Guide to Estimating Book 2; (referred to in 4.2.1)

PROCESS ENGINEERING GUIDES
GBHE-PEG-HEA-500

Physical Properties for Heat Exchanger
Design (referred to in 5.6)

GBH Enterprises

Cooling Water Systems (referred to in 4.1.5)

GBHE-PEG-HEA-504

Thermal Design Margins for Heat Exchangers
(referred to in 5.3)

GBHE-PEG-HEA-508

Selection and Design of Condensers
(referred to in 4.1.2)


ENGINEERING SPECIFICATIONS
GBH Enterprises

Limiting Noise Levels of Manufactured Items of
Equipment (referred to in 4.2.5 and 5.7.5)

GBH Enterprises

Specification for Air Cooled Heat Exchangers
(referred to in 4.2.5, 5.7.1, 5.7.3, 5.7.6, 5.9, 9.2 and
10.3).


Air Cooled Heat Exchanger Design

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