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.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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

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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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


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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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 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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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. 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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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 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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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. 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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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. 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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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 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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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

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. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance 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

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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