Transcript of Air Cooled Heat Exchanger Design
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
GBH Enterprises, Ltd.
Air Cooled Heat Exchanger Design
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
CONTENTS SECTION
3 DEFINITIONS 3
4 SUITABILITY FOR AIR COOLING 4
4.1 Options Available For Cooling 4 4.2 Choice of Cooling System
9
5 SPECIFICATION OF AN AIR COOLED HEAT EXCHANGER 16
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
6 CONTROL 35
8 ASSESSMENT OF OFFERS 37
8.1 General 37 8.2 Manual Checking Of Designs 37 8.3 Computer
Assessment 39 8.4 Bid Comparison 40
9 FOULING AND CORROSION 40
9.1 Fouling 40 9.2 Corrosion 41
10 OPERATION AND MAINTENANCE 42
10.1 Performance Testing 42 10.2 Air-Side Cleaning 45 10.3
Mechanical Maintenance 48 10.4 Tubeside Access 48
11 REFERENCES 50
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
4 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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
20 NON-LINEAR TEMPERATURE ENTHALPY CURVES 55
21 CORRECTION FACTOR FOR SMALL EXCHANGERS 55
DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 57
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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:
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
(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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
(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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.)
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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 make- up 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"
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
4.1.7 Cooling Water From An Air Cooled Heat Exchanger
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
direct- contact 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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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 Condit ions
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).
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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 25-
30°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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
5.2 General
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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 provis ion 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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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 dut ies only): The chosen
temperature is exceeded for only 30 hours per year. (0.3%
frequency).
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
(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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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 Characterist ics
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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).
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
(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.).
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
(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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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:
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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:-
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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.
Characterization Refining & Gas Processing & Petrochemical
Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the
Development & Commercialization of New Technology in the
Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
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