Acknowledgements I am greatly indebted to my wife, June, for her unfailing support and patience during the writing of this book, and to my daughters Julie and Hannah for help with several drawings. My colleague Professor David Reay wrote the bulk of chapter 7, provided the index and was a constant source of encouragement and information. Mary Thomson's help and guidance in typing and in preparing the script for camerareadiness was invaluable. I am most grateful to Dr. Ramesh Shah for agreeing to write the Foreword. Dr. Peter Kew read the part of chapter 6 on evaporation and condensation in compact passages, and Dr. Eric Smith read chapters 4 and 6, both providing valuable comments. Mr Tim Skelton of the Caddett organisation generously gave permission to use information from the Caddett guide: Learning from experiences with Compact Heat Exchangers. Others supplying valuable information were Dr. Chris Phillips of BHR, Mr. Keith Symonds of Chart Heat Exchangers, and Drs. B. Thonon, V. Wadekar and F. Aguirre. I am indebted to the Department of Mechanical and Chemical Engineering at Heriot-Watt University for library and other facilities given. Finally, I would like to thank Mr. Keith Lambert of Elsevier Science for his unfailing support and encouragement during the preparation of the book.
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Airport Design and Operation
In general, if one side is estimated, the other follows from equation 6/26. From N, Ap and mean properties, estimate G from equation 4/13 or 4/40, and hence A~for each side.
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This resistance is added to the film and wall resistances in the design process, giving an increased surface area to compensate for the fouling. Tables of recommended fouling factors for shell- and tube exchangers are published by the TEMA and other bodies. The danger of blindly applying the quoted values Rf for compact exchangers has been clearly demonstrated (for example by Bott (1990)). This arises because I the over-design gives rise to lower clean flow velocities, with then allow greater precipitation and fouling rates.
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These have the form of non- uniform fin spacing, recurved fin shape, or an open fin, as shown schematically in Figure 6/22. Thus some channels will be wider, and some narrower, than their design or nominal values. The consequence of these non- uniformities is twofold: a reduction in Ntu, and a reduction in pressure drop, both arising from the fluid taking preferential flow paths through the wider channels. Shah (1985) gives some indicative numerical data. Mondt (1990) showed results of tests designed to correlate these performance effects with a statistical measurement of the non- uniformity of prepared heat cores, timing reasonable agr~mem. The problem is only likely to arise to any serious extent in the manufacture and utilisation of high aspect ratio finned surfaces of the kind described, which are characteristic of gas turbine and related applications.
Figure 7/4 Schematics of pump- heat exchanger interactions, with fouling Clearly there are complex inter-relationships between the various parameters of pump characteristic shape, fouling type and surface type. If the surface is compact, then any given fouling deposit will have a strong influence on pressure drop (Cowell and Cross (1981)). In this case the loss of flow in a practical system will dominate the effect on thermal performance. It has been shown in a simple analysis (Hesselgreaves (1992)) that depending on the interaction of these relationships there could be an initial rise both in fluid velocity and heat flow.
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In a speech at the cere- mony, witnessed by Moscow-based foreign reporters flown in front the Soviet Union overnight, Mr NnJibnllah said the army 4 * success at Khost symbolised the Afg unity. He told the troops It was also a victory for the policy of “national recondliatioir which he launched a year ago in an effort to p ers uad e the insurgents to lay down their arms and help form a coalition administration. He said die situation in the town had returned to normal, now that a guerrilla blockade had been lifted. Soviet and Afjghaw govern- ment estimates have pnt the number of dead among the insurgents - who had prevented food reaching the town by road for three months — at up to 2,000. The guerrillas in P akistan have denied this, saying they inflicted heavy casual- ties on Soviet and Afghan army units which took pdrt in the battle. Seven die in black townships By Anthony Robinson in Cspe Turn ANOTHER upsurge of violent death in the black townships around Pietermaritzburg claimed seven lives on Sundry night and dashed hopes of a truce after the recent joint UDF-Inkatha statement calling on their supporters to stop fighting. Among the victims was a two-year-old child shot in his mother’s arms at a village near Greytown 80km north ofPieter- maritzburg. A man was burnt to death and three others injured when a petrol bomb was thrown at their house by a mob, police said. Other victims died in Pie- termaritzburg townships after a series of knife, gun and arson attacks between members of rival gangs. At least 42 people have died in the fighting so far this year.
In the above sections it is shown (and developed in chapter 4) that for a given thermal and pressure drop specification, the size- principally volume- of an exchanger is a function of both geometrical compactness of the surface(s) and of the performance parameters independently of the surface. Although the compactness and performance parameters appear in separated form in the volume expression, they are indirectly linked in that the operating parameter is a function of Reynolds number, which is proportional to hydraulic diameter. The performance parameters described, for example, by the ratio j / f (for flow or face area), or f / f (for volume), in turn depend on the Reynolds number, as shown in chapter 4.
The following organisations conduct research into compact heat exchangers, and offer software for design, or are concerned with promoting the interests of those connected with all aspects of design, manufacture and operation. A short description of activities is given for each in the following pages.
The resultant parameters required are the heat load O and the pressure drop Ap. For interest we examine the common limiting flow conditions of fullydeveloped laminar and fully-developed turbulent. For brevity only the first scenario is analysed in full, the others being equally simple to derive and thus merely quoted.
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LORD JUSTICE MUSTILL said that In April 1982 shir powners assigned to the bank all the earnings of their vessel, Dominiqne. In June they chartered her to the charterers for a voyage from India to Europe. The charterparty was in the Genoon form. Clause 16 provided that freight should be pre-pald within five days of signing and surrender of final bills of lading, "full freight deemed to be earned on signing*. On July 14 the vessel sailed, bills of lading having been signed. On July 19 she arrived at Colombo and was arrested by creditors -The shipowners were insolvent and could not procure her release. On July 22 the charterers brought the charter to an end on the ground of wrong- ftd repudiation. Their right to do so was not challenged. Arbitrators found that the charterparty came to an end on that date. The arbitrators also found that by July 26 all the bills of lading had been surren- dered to the shippers- They did not find whether, the five day period stipulated by danse 16 was stSI run- ning when the ' contract came to an end.
T where 1 - ~ is the easily recognisable Carnot efficiency of a system T exchanging heat O at a temperature T and rejecting it at the ambient To. Thus any given heat flow Q has the potential for delivering power via a heat engine up to the limit defined by equation 3/14. Clearly, the higher the temperature T of the heat exchange, the higher the possible power extractable.
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Thomas, B, (1998), Compact Heat Exchangers Applied to Industrial and Environment Processes. Eurotherm Seminar 62 "Heat Transfer in Evaporation and Condensation".
Figure 6/27 Schematics of surfaces for pressure containment Suppose the pressure differential between streams is 8p, the fin thickness is tf and fin density N (fins/m). Then for both the plate-fin (PFHE) and Primed Circuit (PCHE) surface types the stress E (Pa or kPa) in the fin is given by (6/90) For given pressure differential, therefore, the stress reduces as the product Nt increases. This product is often simply called the '~l-t" value of a surface to denote its pressure containment potential. In practice it is related to the yield stress of the material by an appropriate factor of safety (for example in a design code for the exchanger type) to give a maximum allowable pressure difference.
Three of its leaders, Mr Mare Bazin, Mr SUvlo Claude and Mr Gerard Gourgue, slammed the elections as a “fraud” and a "sham” in radio broadcasts on Monday, There were 11 presidential candidates. If none wins an out- right majority, a run-off win be held January 31. “We thank the people who know who their credible lead- ers are,” said Mr Sylvio Claude, who ran In the November 29 election that was cancelled at least 34 people were killed at polling stations. Afterward, the junta replaced the independent Electoral Council with its own cm scheduled Sunday’s vote issued laws over the ^ Western diplomats that between five and 10 per cent of Haitians voted in Port- ■ au-Prince and 20 to 40 per cent in the provinces. The outstanding question in Haiti yesterday posed by most e du c a ted Haitians as weU as by diplomats, was whether the mili tary junta and the new gov- ernment could win any degree of credibility for the elections. The fact that the junta effec- tively organised the poll was the reason most Hums— boy- cotted it Many say tt takes the nation back 30 years to the start of the Duvalier dynasty. Manigat was a dose friend of Mr Francois "Papa Doc” Duva- lier and was an official in the Foreign Ministry for fear yean under the dictator. According to the electoral law, “zealous” Duvalierists were banned from the tial race but Mr Mawiwtf -gras not considered to be in t& at cat- egory. The boycotting of the poll by toopt Haitians had been widely expected hoe but dear fraud, seen by diplomats and foreign iamaiUats, surprised many. Most pojfeag rtntfoaad& not it control have ballot sBps for aB 11 rrn- didates. At some stations, jour- nalists saw slips for only one or two candidates.
Clearly both power and refrigeration systems need both hot and cold streams: some (heat transformers and absorption refrigeration systems) need 2 or more sources or sinks. In all applications of heat exchangers mechanical power is necessarily expended to pump the working fluids through each exchanger by virtue of pressure losses in its ducting or heat exchange passages. For liquids this power is usually relatively small.
Run HEAd online to select the best heat exchanger type for a given process application, based on cost and technical suitability, and provides detailed information on design, manufacture and operation. Download DEVIZE to guide the engineer through the thermal design of shell and tube heat exchangers using a graphical, interactive and highly visual engineering approach.
These lengths are treated separately as for single-phase flows, for liquid or vapour as appropriate. Taylor (1990) gives detailed procedures. It should be noted that when designing for convective boiling, the pure vapour calculation should be performed for the fmal 5% of the surface length, since the wall is often effectively dry even though the bulk fluid is still such that the mean quality has not attained unity. The length for the preheat section is determined by a straightforward heat balance.
The company's Hong Kong end is also being closed, although some offices In Europe and the Far East will be kept going. Midland, like many UK banks, early joined enthusiastically to Big Bang. In 1984 it bought Greenweil & Co, a top City gilt-edged broker. However, the relationship never thrived and last year suffered a series of blows: March: Sir Kit McMahon, Midland’s new chairman, decided to quit the volatile equity market-making business after losing £6m. In the light of the later market crash it was a wise move but the sudden retreat damaged staff morale. July: Midland botched a pos- sible sale of Greenweil Mon- tagu's remaining agency busi- ness to Morgan Stanley. That led ro further staff losses and exposed Midland's lack of com- mitment to it. October? The market crash* caused a steep fall in commis- sion revenue, plunging the Operation into loss. Management struggled to hold the company together and to rebuild esprit de corps, but Midland waa by then forced to decide whether it wanted to continue in this ill -starred busi- ness at aU. The decision to opt out was eased by the link with the Hongkong Bank, which gave Midland a way to present clo- sure in a more positive light. The Hongkong Bank has owned 14/9 per emit of Midland since December 22. It is the parent of James Capri, one of the City's most successful stocktooking firms.
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Mulberry, which is already manufacturing at full capacity, will be forced to find factory premises elsewhere, probably outside the UK. Given the specialised nature of its output, the company must manufacture m an area where it can draw on a pool of experi- enced leather workers. Mr Saul said that it would be impossible to find such a reser- voir of skill elsewhere in the UK. The company would thus be forced to turn to Italy, where there is a long tradition of leatherworking and a pool of highly skilled workers. In the past 16 years Mulberry has established itself as one of the leading manufacturers of high-quality leather goods. Radio plans to be set out BY RAYMOND SNODDY MR DOUGLAS HURD, the Home Secretary, is expected to out- line his plans for the expansion of UK radio in parliament today. Mr Hurd will set out his legis- lative plans in a written answer making clear the Government's intention to set up a radio authority to oversee the devel- opment of commercial radio. Commercial radio is at pres- ent regulated by the Indepen- dent Broadcasting Authority. The policy statement is expected to follow closely the radio green paper. Choices and Opportunities, published early last year. The green paper argued for the creation of up to three national commercial stations and as many as 500 local and community stations. Synthetic genes for US BY DAVfO F1SHLOCK, SCIENCE EDITOR BECKMAN INSTRUMENTS, a subsidiary of SmithKline Beck- man of the US, is to market the catalogue of synthetic genes available from British Biotech- nology.
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Particulate fouling (silting) Silting is the deposition of solid particles on a surface- the phrase 'silted up' is commonly used to describe a pipe or channel which has a thick covering of particles at one or more locations. Small particles can be harder to remove than larger ones, as the forces holding them together and to the surface can be greater. Much has been written about r and thermophoretic effects in attracting small particles to surfaces, and these forces do play a role in many particulate fouling events. However, these, and other temperature-driven effects are less in compact heat exchangers because of the (generally) lower temperature differences. Particulates by themselves are not too difficult to remove. Problems arise when the particulates are combined with other fouling mechanisms, in particular tar formation. Then removal needs more drastic measures, such as chemical/solvent treatment. Pure particulate fouling can be reduced by using high fluid velocities, except in cases where an adhesive component may be mixed with the particles. The effect of velocity on fouling resistance under particulate fouling conditions is graphically illustrated in Figure 7/3, for a PHE, (Karabelas, 1997). Filtration of particles can be applied and for CHEs a suitable strainer can be installed upstream.
In a parallel flow exchanger the temperature distribution in the wall is fairly fiat regardless of both the stream conductance ratio (rlol~)* or the heat capacity rate ratio C*. Because of this the wall conduction is negligible and no allowance is normally needed. Crossflow The flows in most compact crossflow exchangers are exactly or are very close to unmixed, and as such this case has received most attention. Because of the twodimensional nature of the temperature distribution, the actual temperature gradients are higher than those of a counterflow unit, but since crossflow exchangers are generally designed for much lower effectiveness and hence lower Ntus the loss of effectiveness is usually small. The case of an unmixed- unmixed crossflow exchanger has been investigated numerically by Chiou (1976,1978, 1980).
Then the parameter P~ is ea - C~a(1 - a)A, _- 2(1 - a)A~ = 2 • (1 - 0/8)• 190 - 0/0211 Lppu pC, a/ 2 Lppu p 1 x 1/2x3 x 1000 (Notice that P~ can be expressed in this case in terms of the mass velocity G (= pu)). For an Ntu of 4 the ideal (zero conduction) case would give an effectiveness of 0/80. To achieve this value with the above value of Px would require an Ntu from equation 6/66 of 4/33- a substantial increase in surface area. Because this increase in turn changes P x, the correction has to be iterated.
Compact Heat Exchangers: Selection, Design and Operation
M6. The procedure for single pass crossflow design is then followed, using the selected surfaces for each pass. Each pass is taken to be identical. M7. The actual value of Ntu for each pass is then evaluated, from the geometric and thermal values for the two sides. The total pressure drop, over all passes, is also calculated for each side. M8. From the Ntu, calculate a new % and hence intermediate and final temperatures. M9. Iterate dimensions (layer numbers and widths) until thermal and pressure drop requirements are met.
Report Compact Heat Exchangers: Selection, Design and Operation
THERMAL DESIGN FOR HEAT EXCHANGER REACTORS In the growing pursuit of Process Intensification (P l) in the chemical process industries it is becoming desirable to achieve heat exchange and chemical reaction in one unit (Butcher and McGrath (1993), Phillips and et al. (1997), Arakawa et al (1998)). The purpose is twofold, one being simply to save equipment (and with it fluid inventory, important for safety and environmental considerations), the other being that the heat of reaction, either exothermic or endothermie, can be removed or added during the reaction process. In this way the temperature of the reactants can be closely controlled, which has substantial benefits for by-product production. It has been shown (Edge et al. (1997)) that very significant reduction in by-product production can be obtained by custom-designed heat exchanger reactors. The danger of runaway reactions is also almost completely avoided. In the following discussion we will use the terminology that a flow rate of a reactant (usually of relatively low flow compared with the primary flow) is injected into the primary flow, with which it reacts. The secondary flow is a service flow from the point of view of the unit, although it may also be a process fluid. The simplest form of heat exchanger reactor is one in which the reactant is injected at entry to the exchanger (for example into the inlet manifold or header).
A commonly accepted lower threshold value for fl is 300 m2/m3, which for a typical porosity of 0/75 gives a hydraulic diameter of about 10 mm. For tubes this represents the inside tube diameter, and for parallel plates it represents a plate spacing of 5 mm - typical of the plate and frame generation of exchangers. An informative figure given by Shah (1983) shows the 'spread' of values and representative surfaces - mechanical and natural. It should be noted at this point that the porosity affects the actual value of surface density, independently of the a~ctive surface. In Figure 1/2, the value of 0/83 is chosen which is typical of high performance plate- fin surfaces with aluminium or copper fins. As hydraulic diameter is progressively reduced, it is less easy to maintain such a high value, especially for process exchangers. This is for two reasons, both associated with the effective fin thickness. Firstly, for high temperature and high pressure containment, stainless steel or similar materials are necessary for construction, and diffusion bonding is the preferred bonding technique. This in turn requires significantly higher fin thicknesses to contain the pressure. Secondly, the lower material thermal conductivity calls for higher thicknesses to maintain an adequate fin efficiency and surface effectiveness. Thus typical values for porosity for diffusion bonded exchangers are from 0/5 to 0/6, so having a strong effect on surface density and exchanger weight.
As mentioned earlier, the PCHE surface is normally treated sufficiently accurately as all- primary, although it is in reality a variable- area all- secondary surface with very thick fins. Certain proprietary surfaces such as the Marbond TM and G-pak | slotted- plate structures have secondary and tertiary surfaces, which need special analysis.
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Use of the single channel correlations would then be conservative. For flow quality higher than 0/95 the heat transfer is largely that of pure vapour, and the appropriate single phase correlation should be used with the saturated vapour properties (Taylor (1990)). This may not apply to evaporation in plate exchangers in which the active part of the surface is probably maintained at a wet condition by the high flow turbulence.
Calculate the ratio of heat capacity rates C*. From the appropriate e- Ntu chart or formula estimate the overall Ntu. From Ntu estimate side N's as starting values.
In addition there is normally a fillet of brazing metal at the fin- plate interface of greater effective thickness than the fin, thus reducing the local stress. Corresponding arguments can be made for the brazing alloys used for stainless steel P FHEs, provided that the stringent requirements of preparation of the matrix and for temperature control are met (Shah (1990)). It has also been demonstrated that the bond strength of commercial PCHE structures, which are diffusionbonded, is that of the parent metal, because of the crystal grain growth across the interface during bonding.
Plans are in hand at the time of writing to use the method in a number of heat exchanger types, and to counter inorganic fouling. Other reports (MiillerSteinhagen, 1997) suggest that the addition of wood pulp fibres can also have a dramatic effect on reducing scale deposits. At this time, more rigorous analyses arc required of the effect of fibre addition, and the type of fibre, on the fouling mechanisms, both for scaling and biological fouling, but the data to data shows that a promising technique may be evolving. Another recent approach to biological fouling minimisation is to use tube inserts, of the type made by Cal Gavin Ltd, the 'HiTRAN' clement. While data on the trials are not yet published, there seems to be no reason to doubt that benefits will accrue to the use of these tube inserts, although of course in many applications the tubes will be somewhat larger than those associated with compact heat exchangers.
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The industry must thin out, unless there is to be a j reduced level of profttabi Most of the hand knitting companies - were caught unawares by the downturn. It follows an unprecedentedly, buoyant period for the industry in the mid-1980s when the pre- vailing fashion - for bright, baggy knitwear made from "easy-to-knit" chunky yarns - coaxed lots of new knitters into the market. In 1986 the hand knitting market was in its heyday. A record 22m kg of yam, worth £335m, was sold in Britain dur- ing that year, according to research by Textile Market Studies. But by the end of 1985 the West German market began to falter. Other European coun- tries followed suit. By the autumn of 1986 the British market slipped into decline. It fell further in the following spring. Today most of the main hand knitting markets In Europe have dwindled by between 15 and 40 per cent since their peak in 1986. Handknitting yam mflBorr hg 24 las in G. Britain E miHon 350 300 1980 82 SOWCBJsatt The chief cause of the decline is -a change in fashion away from chunky knitwear towards a more stylised look.
The pitfalls of doing this are obvious. Maintenance of the heat exchanger is useless if carried out in isolation. A heat exchanger is but one component of a process stream, and associated with the heat exchanger will be controls, valves, filters and sensors, such as pressure transducers and thermoeouples. Checking of the calibration of equipment which is used to monitor the performance of the heat exchanger is essential - this goes without saying- but it is also vitally important, as mentioned in the section on 'operation', to ensure that such equipment is used during heat exchanger running. If monitoring equipment is in place, but is neglected, no-one other than the site person responsible can be blamed for the consequences of failure. The cleaning of filters is a regular maintenance operation, not to be forgotten. If the pressure drop across the heat exchanger is being monitored (as it should be), the location of the pressure tappings may be such that the filters are included in the pressure drop being measured; if they are outside the region between tappings, a second pressure drop monitoring system covering the upstream filters, could be usefully installed. Preventative maintenance is designed, of course, to reduce the chances of breakdown in any of the components making up a process stream. However, contingency plans should always be present in case failure of the whole, or part, of a heat exchanger does occur while on line. The remedial action may involve directing the process stream(s) through an adjacent second heat exchanger, (installed for such an eventuality), or blocking off the offending layers in the compact heat exchanger.
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On the refrigerant side, fouling is unlikely. However, oil can be carried around the circuit from a lubricated compressor, and the presence of this could affect compact heat exchanger behaviour. On the secondary side, the fluid is determined by the cooling requirement, and any form of fouling might occur. Thus brazed plate heat exchangers would be selected with care if properties of the secondary fluid were unknown.
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A further aspect strongly affecting counter- versus crossflow choice is that of pressure drop. These points are further explored in the discussion on mass velocity equation below.
One possible response to the challenge posed by (hen trade Imbalances waa protec- tionism, the demand for which was growing. Alternative mea- sures were needed as the nega- tive impact of general protec- tionism on world growth was damaging short-term benefits accruing to individual nations. But if a more measured response than protectionism was to be achieved the NICs themselves would somehow have to be brought to accept that they have been the main beneficiaries of the moves to nwml'jiln an n pw> flrihul trading system and that, by trading in manufactures they and the industrialised countries were interdependent. As competition from poorer -Asian countries Increased the NICs would have to move Co higher value-added production, mirroring Japanese experience. The needed for high technology and higher expenditures cm research and development would be a big opportunity fir British and European compa- nies. Filipinos vote after 93 die • in campaign By RfeiMvtf OoiMay M MmBb MOKE THAN 20m Pfllplaoe voted yesterday in mostly peaceful local ejection* at the end of seven weeks of often violent nationwide campaigning which left SS candidates and party MP- Aa polling c losed. Presi- dent Corazon Aquino wel- comed the peaceful election day and praised the people's "faith in the demo- cratic, process". The polls complete the replacement of appointed with elected fas the transition fhm dictatorship to democ- racy that Mrs Aquino initi- ated soon after President Ferdinand Marcos was «mst«d nearly two years: The relatively peaceful poll* will also strengthen confldenee in the Govern- ment and holster the eonn - try** economic recovery, * * said. Tan admits breach of trust ByWongMoagtoitotia THE MALAYSIAN businessman and Ton Koon Swan Holdings, the Chinese invest- ment group of which he was fonneny msnagmgdireetor. The offence carries a maxi - ««*«* jail term of th ree ye w. Scnfcn^r wiU be a nnoun ced on February*.
The OSF data correlated well with the convective component only. The perforated fin data correlated with an approach which took the greater of the Cooper correlation (o~b, with S = 1) and the convective term Fo~.
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Some typical values of conductivity for various deposits are shown in Table 7/3, adapted from data given by Marner and Suitor (1987). The operating temperatures and measured thicknesses are also shown.
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The former o~en have low-finned tubes and the latter an enhanced surface on the bottom few rows of tubes. Smaller duties, as mentioned in chapter 1, are nearly exclusively now the domain of the brazed-plate exchanger. In the closely- related field of air conditioning, especially for the space and cost- conscious transport and domestic sectors, development is progressing of variants of flat and galleried tube- fin exchangers for both evaporation and condensation, to replace the round tube and fin exchangers historically used. Automotive and prime mover sector In this sector the heat transferred is normally from atmospheric air to water, oil, refrigerant or compressed air, and louvred fin surfaces are used for the airside. In the case of automobiles, with production runs in the millions, first cost is paramount. Thus material quantity and speed of production are dominant factors in the selection, and louvred fin, flat tube surfaces are almost invariably used. This is because they are the highest performers from the point of view of compactness and enhancement (see chapter 4), and also because the fins are produced by a very rapid rolling process. A crossflow arrangement is standard for these exchangers. The fiat tubes (when carrying cooling water) are normally used without augmentation, since the air side usually dominates the overall heat transfer resistance. Oil coolers, on the other hand, as mentioned in chapter 1, require oil side enhancement to balance resistances.
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Applications of Compact Heat Exchangers and Fouling Possibilities The types of compact heat exchangers used in various sectors of industry do vary, depending upon the sector experience and degree of confidence. To a considerable degree the confidence is based upon a knowledge of fouling. In Table 7/1, the compact heat exchangers most commonly used in a variety of sectors, and two genetic unit operations (refrigeration and prime movers) are identified, and comments made on the types of fouling likely to be encountered.
In all cases a stream absorbs power as noted above. These concepts point to the need for a rational way of analysing systems, especially those involving both heat and power exchange, to enable operation at minimum total energy consumption. Exergy analysis, a natural extension to classical thermodynamic Second Law (entropy) analysis, is the unifying tool for this purpose, and the basic concepts are developed in this chapter. The tool is increasing being used in extended forms to include life- cycle analyses, that is, to include the exergy cost of producing the (capital) equipment and its ultimate disposal, but such development is beyond the scope of this book. The reader wishing to take the subject further is recommended to study papers including aspects of Thermoeconomics such as those by Witte (1988), Ranasinghe and Reistad (1990), and books by Aheme, Szargut and Bejan et al (1996) In this chapter the basic principles of exergy analysis are first outlined. Their application to heat exchangers is then developed, firstly for zero pressure drop and then for finite pressure drop for which an entropy generation minimum criterion is derived, and its implications for design choices is discussed. Finally, a brief discussion is given of the application of the principles to heat exchanger networks.
Heat exchangers for gas or liquid waste treatments Within the new environmental requirements gas or liquid waste treatments become fundamental and several aspects must be taken into account. The treatment of gaseous effluents (VOC's) is either realised by combustion at high temperature, absorption or condensation at low temperatures. Generally, the treatment is realised in a two stages process which associates two different techniques. From the view point of heat transfer, one of the problems is the knowledge of the effluent composition, which is generally not known precisely or which can change with time. This lack of information does not enable an effective design, and very often the equipment does not match the environmental requirements. To obtain a better sizing for VOC equipment, heat and mass transfer aspects must be studied under representative flow conditions. For instance, it has been observed that condensation of VOC's could be either in mist or film depending on the gas composition. Recently, Greth has commissioned a new test rig devoted to VOC's treatment. This test rig can handle any type of VOC's with a flow rate of 500 m3/h of effluent to be treated and temperatures ranging from ambient to cryogenic. This test rig is used either to test single components such as condensers or full size equipment.
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The association was registered in Japan and Europe in 1992. The first edition of the ALPEMA standards was published in 1994. Membership is open to quality manufacturers of BAHXs of over 200 kg used primarily in cryogenic applications.
Thus the temperature at inlet of the primary stream (before reaction commences) and its final temperature are equal, and we call this the nominal temperature. This is ideally the temperature defined by optimum reaction kinetics. The consequent temperature distribution along the exchanger length depends on the speed of reaction together with the mixing characteristics of the exchanger primary flow surface. Two possibilities are illustrated diagrammatically in Figures 6/25 and 6/26. For this purpose we assume that the reaction is exothermic.
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The processing of food and drink is a vast application area for some types of compact heat exchangers. Cleanliness is of course critical, and the ability to use heat exchangers which can be cleaned, and which are made using stainless steel, tends to limit the choice. The plate & frame unit is the most commonly used, because it is easy to dismantle, is flexible, and is well known in the sector. Crystalline, biological tbuling and silting are common types of fouling associated with the sector.
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Biological fouling Biological fouling is caused by the deposition and growth of organisms on surfaces. Bacteria arc the organisms most likely to cause problems in CHEs, and their presence can also assist corrosion by, for example, reducing sulphate to hydrogen sulphide, which is corrosive to common stainless steels and many other materials. The best control method, especially for closed systems, is treatment with biocides. Non-oxidising biocidcs arc normally alternated to prevent the development of resistance, and may kill the bacteria but not remove the biofilm. Some biocidcs have detergent properties which can disrupt the film. Chlorine and ozone arc oxidising biocidcs, which kill the bacteria and oxidisc the film. High concentrations may be necessary for them to be fully effective, but the smaUer volume and fluid inventory in a CHE, compared to shell and tube heat exchangers, helps to minimisc the quantities needed. The chemical diffuses to the biofilm, and in narrow channels this should be relatively rapid. Applicable not just to biological fouling is an innovative method for fouling inhibition patented by workers in the UK. The company Applied Coolant Technologies has recently presented data on what it calls the 'micro-mechanical approach to fouling'. The concept uses micro-fibres, of an insoluble polymer, which arc added to a circulating cooling water stream.
Figure 6/25 Temperature profiles for instantaneous reaction and mixing In Figure 6/25 the reaction is instantaneous, with instantaneous mixing with the primary flow, and with a consequent instantaneous temperature rise. The performance is then identically that of a conventional (sensible heat) exchanger with the peak temperature T" determined by the heat of reaction and thermal equilibrium with the primary stream. The subsequent temperature profile of the reacting (primary) flow stream depends on the flow arrangement. If this is cocurrent (Figure 6/25(a)) then the temperature falls rapidly with distance since the initial temperature difference is high: in a counterflow arrangement (Figure 6/25(b)) the fall is more gradual, as shown: it is linear if the heat capacity rates are equal. From the point of view of by-product generation the co-current arrangement is clearly preferable, although the required surface area will be higher, to attain the nominal starting temperature of the primary flow. If the reaction is not instantaneous, but takes place at a rate such that it is completed exactly at the end of the exchanger- the other limiting condition is reached.
Bemtsson, T, Franek, P-A, Hilbert, L. and Horgby, K, (1995), Learning from Experiences with Heat Exchangers in Aggressive Environments, CADDET Analysis Series No. 16, CADDET, Sittard, Netherlands. ESDU (1992), Data item 92013: Selection and Costing of Heat Exchangers. Engineering Sciences Data Unit, London. ESDU (1997), Data item 97006: Costing of Plate-Fin Heat Exchangers. Engineering Sciences Data Unit, London.
OPTIMUM DESIGN OF A COMPACT HEAT EXCHANGER FOR ENVIRONMENTAL CONTROL OF AN AIRCRAFT AT CRUISING CONDITIONS
For general compact surfaces, perhaps a more rational approach is to consider the fouling resistance in terms of its two components of (mean) thickness and conductivity. The thickness affects both the flow resistance (pressure drop) and thermal resistance, but is in theory independent of type of deposit, although the surface roughness affects the friction. The conductivity affects only the thermal resistance but depends on the type of deposit.
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However, those who maintain condensing economisers (normally not classed as compact heat exchangers) will advise caution when shutting down the plant. If an acid condensate- as occurs in such economisers on the gas side) - is left on the surfaces of the heat exchanger during a shut down period, although the temperature may be atmospheric ambient, some nice pockets of corrosion will be found on inspection some days or weeks later! Chemical reaction fouling tends to be associated with increasing temperatures in the stream receiving the heat.
A further consequence of this is that ~JVr~ and W could both be negative, it only being necessary that l~to,t is positive. Note also that lost work is proportional to the reference absolute temperature, so in the case of an above atmospheric temperature system producing power, the lost work is a minimum (giving maximum available work), when the final (lowest) heat interaction is with the atmosphere. In addition it is clear that the atmosphere is (ideally) the only reservoir which can exchange heat within a process without undergoing a change of temperature itself.
This has resulted in generalized correlations to predict the pressure drop and heat transfer performance in plate heat exchangers. PHE software HTRI's PHE software designs, rates, and simulates the performance of plate heat exchangers. Plate groups are calculated individually using local physical properties and process conditions. PHE handles single-phase, no-phase-change liquids, in laminar and turbulent flow. Geometries handled include those common to most commercially available plate heat exchangers.
A further option is to increase the hub ratio (axial pump) (Stepanoff (1948)). Combinations of these measures could be applied. The effect is to reduce the sensitivity to change of incidence, or flow, on the head generated.
Tube (the same fouling factor for each) Figure 7/5 The consequence of not scaling a fouling factor (schematic) The absurdity of this example is clear. It would seem, therefore, that one logical approach would be to scale the fouling factor, and by implication, the allowable fouling thickness, by the hydraulic diameter of the surface considered. This would automatically give the same proportional increase in pressure drop, with its corresponding relative reduction in flow and change of heat load. The suggestion is, therefore, that the TEMA or similar fouling factors, which can be presumed to be based on a notional internal tube diameter of 25ram, are scaled down by the ratio 25/dh, where dh is the hydraulic diameter, in ram, of the exchanger surface under consideration. This approach is broadly compatible w~th the quoted values of Rf for the PHE given in Table 7/2, with a typical value of dh of 5mm A further stage might be to include the effects of velocity and geometry. Thomas (1998) has shown that the asymptotic fouling factors for PHEs is a strong function of velocity, reducing markedly as velocity increases; there was also a strong reduction observed as corrugation angle increased from 30 to 60 ~ These considerations would apply especially to particulate fouling. For mass-transfer driven fouling, it is likely that the driving temperature difference (AT), which often controls the mass transfer, would also be an important factor in determining the asymptotic fouling resistance. Compact exchangers often have lower ATs than shell and tube exchangers, so we would expect lower fouling factors to apply for these cases.
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Chemical reactions involving the heat transfer surface, or the carrying of corrosion products from other parts of the system, to be left on the heat transfer surface, lead to corrosion fouling. The formation of deposits can by itself lead to corrosion underneath them, for example as the result of formation of electrolytic oxygen concentration cells. To avoid this type of fouling, construction materials which are resistant to corrosion should be selected. This is routinely done, of course, by the heat exchanger vendors, but there always remains the possibility that the user may change process streams passing through the heat exchanger. Where polymers may be appropriate to counter corrosion, a recent US Gas Research Institute report (Ball et al, 1996) gives much background on types of polymer, operating limits etc. Alternatively, inhibitors can be used. Cathodic protection can lead to cathodic scales being formed if hard water/brines are the flow streams. Several types of compact heat exchanger have no dissimilar metals within them, thus making the likelihood of corrosion attack more predictable. Nevertheless, if there is a possibility of a stainless steel unit being stored for a period in a salt air environment, the surfaces should be protected to avoid stress corrosion cracking, (Allan et al, 1995). It should be noted that corrosion products could be significant in fouling whilst not endangering the exchanger from the point of view of leaks or pressure containment.
ESDU are continuing the process of developing new selection and costing information, and updating the existing data. This work is available as part of ESDU's Process and Environmental Technology series.
C3 to C5. The calculated value of overall Ntu is below the required value of ~ 1/667, so the flow length should be increased by perhaps 15%. In addition the water side flow area will need to increase by about 8% to accommodate the pressure drop requirement. Further detailed convergence would normally be undertaken to take into account full surface effectiveness calculations.
There are positive indications that the addition of appropriate particles into the fluid stream - seeding or germination - can be effective in minimising deposition on heat transfer surfaces, (Grillot, 1997). Additives can be used to minimise waxing, but environmental pressures are increasingly mitigating against excessive use of chemical treatments. Mechanical cleaning methods such as high pressure lances are unlikely to be usable in compact heat exchangers because of the small passage sizes and their sometimes tortuous flow paths. These features make it difficult to clean any passages which are completely blocked. Note that care should be taken to ensure that cleaning chemicals are compatible with materials of construction, including brazes, gaskets etc.
Thus the more compact the surface is, the lower is the required material thickness to contain a given pressure. This implies that the porosity requirement for stressing purposes is independent of scale. In practice, however, realistic allowance must be made for the possibility of loss of material through corrosion and erosion. This puts lower limits on the separating plate thickness, as does the handling aspect for large block dimensions. For aluminium alloys the allowable tensile stress is given by Taylor (1990) as approximately 2/3 of the yield stress, giving the allowable stress as 24 MPa. A corresponding value for the yield stress of 316L austenitic stainless steel is 234 MPa, giving an operating stress limit of 156MPa. The pressure containment capability of plate- type exchangers (welded plate, brazed plate) depends on their detailed plate patterns and bonding mechanisms, and hence is specific to each manufacturer. Some operational information is given in Chapter 2.
Recuperation based on heat recovery from exhaust gases is common using compact heat exchangers. Here corrosion might occur of the gases are cooled to too low a temperature (below the acid dewpoint). Particulate fouling may exist, depending upon the fuel. In gas turbine intercoolers, the gases tend to be clean, however. The main problem in gas turbines is not associated with fouling, but with thermal stresses, (see Chapter 6). Where liquids are the heat source/sink, fouling can be more of a problem. Scaling, particulates and, on the secondary side, where cooling water or other fluids may be used with less known specifications, even biological fouling could occur. Thus access for cleaning is important.
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The arithmetic mean is used for the Cm,~ side. The density for friction calculations (especially for a gas) should be evaluated after the mean pressure has been calculated, using the gas law, as recommended by Smith (1997). The above mean property evaluation temperature will serve well for calculation of the heat transfer and friction parameters in cases for which the driving temperature differences are small, in other words for Th~lk ~ Tw~. Fortunately this is commonly the case, low temperature differences being characteristic of compact exchangers. For larger driving temperature differences, the effect depends on whether the stream is a gas (or vapour) or a liquid one. Commonly-used corrections are of the following form (Kays and Crawford (1993)).
Chapter 5 Heat Exchangers
In the last few years, there has been increasing interest in using 'natural' refrigerants, such as ammonia, hydrocarbons, CO2 and air. Ammoniais toxic, hydrocarbonsare flammableand CO2may be operatingat or around its critical point, with relativelyhigh pressures. All of these featuresmake compactheat exchangers,in conjunction with other 'compact' units to reduce fluid inventories,desirable. However, while the refrigerantwill be clean, the fluid on the secondarycircuitmay not be.
Compact heat exchangers Fooling of heat exchangers Fouling is highly detrimental to thermohydraulic performance and must be well understood if the heat exchange capability of practical equipment needs to be accurately predicted. In water cooling applications, particulate and precipitation fouling are frequently responsible for the decrease of the heat transfer performance. Enhanced heat transfer surfaces provide higher heat transfer coefficients than conventional plain tubes and will be more sensitive to fouling. Furthermore, the fouling margin implies extra surface area, which generally costs more than for plain stainless steel or copper tubes. In consequence, some specific recommendations need to be given for both fouling resistance values and operating conditions. Mitigation of fouling of enhanced heat exchangers must be taken into account at the design stage, and should include several factors such as the operating conditions, transient and start-up procedures, maintenance capabilities.
Thus at the centre of a block, the banking factor B = 1 (unity), whereas at the outer layers B = 4, giving twice the effective fin height 1 in equation 6/43. Taylor (1990) recommends that a mean value of 2/25 be used for the block as a whole.
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The purpose of HTFS is to increase the profitability of our customers by providing state of the art knowledge of all Heat Transfer and Fluid systems. We primarily transfer this knowledge, which has been accumulated since 1968, through sottware, research papers and training. As well as leasing or purchasing our software you can join others in guiding software development, oversee extensive research programmes and access the results via membership of the Research network. This special partnership between HTFS and its customers ensures that our sot~are anticipates your business needs.
The textile industry produces large quantities of warm effluent, contaminated with dyes and fibres. It is therefore important to use compacts which can be opened up for cleaning. The sector also uses specially-designed heat exchangers with rotating surfaces for such effluents, mainly for recovering heat in liquid-liquid duties.