Günümüzde ısı değiştirici imalatında kullanılan yöntemler

Abstract

ÜZET Endüstride ısı değiştiricileri uzun yıllardan buyana yoğun olarak kullanılan elemanlardır. Isı değiştirici alanında ilk kullanılan modeller borulu tip olmuştur. Burada borular en iyi akış şeklini verecek bir matris şeklinde dizilir ve içinden akışkan geçer. Boruların dışında ise bir soğutma veya ısıtma ortamı söz konusudur. Bu sistemlerin yoğun kullanım yerleri buhar üretim merkezleridir. Buhar üretim merkezlerinde suyun buharlaştırılması ve yoguşturulması amacıyla iki değişik yerde ısı değiştiricisi kullanılır. İlerleyen yıllarda havacılık ve uzay sanayilerindeki gelişmeler daha verimli, hafif ve küçük boyutlu sistemleri zorunlu hale getirmiştir. Böyle sistemlerde maksimum soğutma yüzeyini elde etmek için ilk akla gelen düzlemsel levhalar kullanmak olmuştur. Daha sonra gelişme hep lamelli ısı değiştiricileri üzerinde yoğunlaşmıştır. Lamelli ısı değiştiricilerinde levhalar presleme veya haddelemeye benzer bir sistemi andıran makinalardan geçirilerek çeşitli akış yolları elde edilir. Sistemde iki farklı profile sahip levhalar vardır ve bunların arasında ayırma plakaları bulunur. Soğutulan akışkan için gereken akış yollarını oluşturan levhanın en alta konmasıyla montaj başlar. Bunun üzerine bir ayırma plakası koyulur ve onun üzerinede soğutucu akışkanın geçeceği kanalları oluşturan şekillendirilmiş levha konur. Bu işlem tekrarlanarak montaj tamamlanır. Anlaşılacağı gibi plakalar arasında yüzlerce temas noktası ve birleştirme vardır. Bu birleştirmeler için en uygun yöntem sert lehimlemedir. Bu amaçla önce tuz banyoları kullanılmışsa da bunun lehimleme sonrası getirdiği temizleme ve kalıntı tuzun korozyon problemlerinden dolayı uygun olmadığı görülmüştür. Daha sonra kontrollü atmosfer ve vakum lehimleme söz konusu olmuştur. Bu çalışmanın içeriğinin büyük bir bölümü sert lehimleme konusuna ayrılmıştır. Sert lehimleme malzeme ile büyük ilişki içerisindedir ve çalışmada sert lehimlemenin tamamlayıcısı olarak malzeme konusuda geniş bir şekilde incelenmiş tir. Bu noktada çekirdek ve kaplama olarak iki farklı malzemenin beraber kullanılması söz konusudur. özel fırın sistemleride ele alınmış fırın ve koruyucu atmosfer yapıları incelenmiştir. Her ne kadar lamelli ısı değiştiricileri çok kullanışlı sistemlerse de bazı dezavantajları vardır. Bu nedenle çalışmanın sonunda bir karşılaştırma yapılmıştır. vı

SUMMARY THE METHODS THAT IS USED AT FABRICATION OF THE HEAT EXCHANGERS Many types of heat exchanger are employed in such varied installations as steam power plants, chemical proces sing plants, building heating, air conditioning, refrigeration systems and mobile power plants for automotive, marine and aerospace vehicles. Most heat exchangers may be classified as being in one of several categories on the basis of the configuration of the fluid flow plants through the heat ex changer. The four most common types of flow path configura tion: 1) Parallel flow (cocurrent) 2) Counterflow 3) Single-pass crossflow 4)' Multipass crossflow These types will reviewed in following sections of this, study. Heat exchangers are of then classified on the basis of the aplication for which they are intended and special terms are employed for major types. These terms include boiler, steam generator, condenser, radiator, e vapar at or, cooling tower, regene rator, recuparator, heater and cooler. The specialized require ments of the various applications have led to the development of many types of construction some of which are unique to particular applications.Typical units are describedin subser quent sections to illustrate the characteristics and features of the principal types. The fabrication of the shell and tube heat exchanger and the lamellar heat exchanger ismore interesting than the other types, therefore these two types of heat exchangers are described and told. The shell and tube heat exchangers are primitive models of heat exchangers. In these models tubes are arranged in amatrix form which gives the suitable flow of cooling fluid. The outher side of these tubes is surraunged by a cooling chamber. This cooling can be water or air or any other fluid. But heat: exchangers were not used for cooling only. These system can be used olso for heating. Steam boilers are typi cal uses of the heat exchangers for heating a fluid. This type of heat exchangers is older than the other types. Therefore several examples was given about fabrication of shell and tu be heat exchangers and lamellar heat exchangers fabrication was detailed. By the great development at the space and aviation in dustries more efficent and smaller systems were required. The first idea about more efficent systems was using flat plates at heat exchangers. After this bright idea the development on heat exchangers was condensed on plate types. In these types of heat exchangers the main component is corrgated she ets. There are many many ways to produce these sheets. Stam ping and continious corrugating machines are the two way to form these plates. Flat plates have the advantage that sheet stock is less expensive than tubing per unit of surfave area and flat she ets offer many posibilities for constructing heat exchanger flow passages that are aerodynamically clean so that pressure losses can be minimized. Fluid passages can be formed by cor rugating or stamping the sheets and soldering, brazing or wel ding them together. Unfortunately, units of this construction have the disadvantage that stress concentrations at the seam- welded joints will induce cracks if large pressure differen tials exist. Thus flat-plate construction is not suitable for applications involving large pressures. Where as in many app lications it may be possible to balance the pressures in the two fluid circuids so that the pressure differantial across the flat plates is small under normal conditions, of f design conditions may still present a problem since the failure of a pump or a loss of system pressure in one circuit gives a large pressure differendial for a short interval. Flat plates also present severe thermal stress problem in many units be cause of their stiffness in shear and the internal fluid pas sages cannot be cleaned mechanically. For these and other re- asonstubular heat exchangers are more widely used then flat plate units. But flat plate units is newer and they are deve loped. Basic structure of a plate type exchanger can be des cribed like this: A formed sheet for the flow passages of Vlllthe fluid which is cooled. A seperating plate on this and another formed plate for the fluid that is used to cool the other. And another seperating plate on all of the system. This structure goes on until the wanted shape and capacity is achieved. As this exaple shows, there must be hundereds of joining in this structure. And the experiments show that most suitable way to have sufficent joints is brazing. So that the milestone of the manufacturing of plate type heat exchangers is brazing. For critical aplications like the plate type heat exchangers, it is usual to heat the component in a non oxidising atmosphere wehere the joint area is protected. The atmosphere may be in the form of a cover gas an inert or reducing nature, or the component can be hea ted in a vacuum. This vacuum is formed within a vessel of suitable design which may be cold or hot walled, with the heating elements either inside or outside the containment.The component parts of the joint, not neccesarily the whole as sembly, must be heated to the brazing temperature. But for heat exchangers generally the whole assembly must be heated. This temperature of the parent materials but will in all ca ses be above the solidus of the brazing filler metal. In many cases the brazing temperature will be up to 100 C superheat above the liquidus of the filler metal so that flow into the joint will occur readily. The methods by which the assembly can be heated to the brazing temperature will depend upon many factors. The mobt importants of these are: 1) The size of the assembly 2) The brazing temperature 3) The parent materials 4) The economics 5) The ability of the designer to model the component so that it can be brazed efficently. Low carbon steel components can be brazed using copper as a filler metal in a continious furnace in a suitable at mosphere. This is a low cost, high production process in use since 1930s, particularly in America. In the main automotive centers such as Detroit, furnaces work on a continious rou tine and are dedicated to keeping the nearby plants supplied with brazed components. These include parts of the cooling and air conditioning systems. Adjecent to each brazing plant is a press shop where sub components are pressed from sheet to produce finished sizes that have clearances compatible with the process.Pre cleaning of the components is not requiredin many cases, as machining pressing lubricants volatilise in the furnace. On leaving the furnace, the components have a bright finish, and can be surface protected without a major cleaning operation. Components that have failed to pass visual examination can be immediately re-cycled through the furnace, usually with additional filler metal, meanwhile components satisfac tory on the visual examination can be further tested as requ ired by the spesification. The output of one continious fur nace is many tons each day, so a plant consisting of several furnaceshas the capacity to service a car plant of conside rable size. This type of furnace could adequately and econo mically process mild steels, but with the need for joining complex materials to meet more demanding service conditions, the processors of alloy steels became interested in high tem perature brazing as a joining method. Little progress was made in deceloping a process capab le of joininig materials containing chromium, titanyum and aluminium used for brazing aplications during the 1940s, but the dekands of aircraft industry for improved mechanical per formance and the difficulty in producing high-quality joints by welding gave a considerable incentive to the brazing in dustry at this time. Than in the United States the technology and its potential for joining complex aeroengine components manufactured from stainless steel and aluminium was inves tigated. The main problems are: 1) The heating furnace, the requirement was for avessel able to reach a high brazing temperature and contain suitable atmosphere 2) The atmosphere, had to be clean enough to reduce the degree of oxidation to an acceptable level so that impurities at the joining region were very low. 3) A filler metal compatible with the parent materials that would perform adequately in the operational environmentThe first filler metal used was Colmonoy 6, a nickel, silicon, boron alloy which was used for applying a hard-wear ing surface onto less hard materials. The principles of high temperature brazing process and its potentialapplications were demonstrated by this way, and so it was introduced into the aerospace industry, after many proving trials to demons trate the ability of the brazed component to withstand ther mal and mechanical shocks. It is still difficult to convince the design engineer that a sandwich structure of ductile pa rent materials and a brittle joining material can perform satisfactorily in a properly designed component. From the early work the technology was improved. Bra zing filler metals with spesific properties were formulated, having good corrosion resistance at operating temperatures, adequate mechanical properties, improved machanbility, the ability to fill wide and narrow joint gaps satisfactorily, and other properties spesific to the service requirements. So that step brazing could be possible, several series of filler metals with decreasing liquidus temperatures were for mulated. The first joint was made at the highest temperature and then otherjoints on the same assembly at lower temperatu res. This enabled very complex assemblies to be manufactured and non destructively examined after each brazing operation. The final joints were made at the lowest temperature. After these developments instead of using high purity hydrogen, development was consentrated on vacuum technology, The vessel was evacuated by means of rotary and oil diffusion pomps to apressure of less than one millionth of an atmosphe re. This atmosphere contains fewer impurities than the purest hydrogen and the risk of contamination from the furnace walls and pipework is much less than when using a purged system. The problems associated with vacuum are the transfer of heat, be cause there is no conduction or convention inside the chamber and extremely slowrates of cooling, particularly below 7500. Other constrains in the aplication of vacuum technology are the size limitations of the heated chamber, because of the cost of building large vacuum furnaces and the problems of heat transfer when a multi component load is heated and co oled in vacuum. The high temperature controlled atmosphere process is now widely used in the plants manufacturing heat exchanger components because of its ability to produce clean, distor- sion free components by a standardized method of manifacture, xxThe aerospace and automotive industries are the prime users, but the obvious adventages of the process it is olso used to produce parts of atomic energy plants. The performance of a brazed joint will not depend en tirely upon the properties of brazing process. The characte ristics of theassembly will depend upon the skill of desig ner and freedom he has to impose a design solution upon a particular problem. There are, however, cedtain basic requirements that must be considered when selecting a brazing filler metal for a spesific aplication. These are: l)Temperature of operation during the life of the com ponent. 2)Time at temperature and any thermal cycling. 3)Type of mechanical loading. A)Parent materials. 5)Environment. 6)Joint configuration. 7)Capillary gap size. 8)Economic considerations. Stating this in simple terms, it is neccessary to know what the brazed assembly has to do, for how long, in what at mosphere, and the cost of failure. It isobvious that many of the factors listed above in teract, and so selection of filler metal to be used will be based upon information available, tempered with judgement. For some aplications no filler metal will meet all the requ irements listed, and so a compromise is necessary. The tempe rature at which the brazed assembly is requrired to operate and time at this temperature are two aspects. These will af fect not only the resistance to corrosion, but olso mechani cal properties will be different at say 600 C than at ambient temperature. Corrosion will also be accelerated at these higher temeperatures. Mechanical properties and corrosion are the main points of brazing.The parent metal composition can affect the selection of filler metal. There may be metallurgical incompatibility between the two meterials, so that excessive alloying will occur and filler metal flow will be restricted. For example, nickel-phosphorus filler metal will embrittle nickelas a parent material, and nickel based filler metals will erode zirconium alloys severely. The filler metal must be selec ted with the economics of the process in mind. Also fitness for purpose and cost of failure must be considered carefully. With the increasing, demands aluminium is becoming a material of major importance to the automotive heat exchanger manufacturer. The low density, high thermal conductivity and moderate cost of aluminium offer the automotive designer the potential for component weight and cost reduction over the more traditional copper and brass materials. The large volume application of aluminium radiators which begun in early 1970 s in Europe and is currently under in way in the U.S. clearly illustrates the significance of the shift from the more tra ditional heat exchanger materiels. The large volume demands created by the automotive ap- lications have spurred efforts to advance the manufacturing technology for aluminium heat exchangers. The effort has led to the development of a fluxless process for brazing in va cuum, which has become the favored one in the world for ma nufacturing automotive heat exchangers. It isnonpolluting and possesses the high braze performance capabilities requ ired in todey's manufacturing climate. A key to the succesful development of the fluxless process has been the available of qulity braze sheet with uniform flow and wetting characteristics has been developed by the aluminium suppliers and is commercially available in large volume. Development of the braze sheet product has been continuous, and today large range of materials are avilable that provide good first run product yield and wide tolerance in vacuum processing requirements. In fact, the number of clad/core alloy combinations that are currently available has increased to the point where the braze sheet can be tailored to match the broad range of vacuum furnace sysstem capabili- es and component design requirements. Braze performance depends directly on the extend to which the oxide layer covering the braze clad is dispersed Xlllwhen the clad becomes molten. Good performance, which charac terizes the soundness (leaks, joint strength and fillet shape) and integrity (minimal base metal core dissolution and corro sion resistance) of the brazed joint is possible only when the oxide barrier is dispersed sufficently for wetting and flow of filler metal occur. Extensive studies of the wetting chacteristics of aluminium braze materials indicate that oxide barrier is the only detergent to wetting between liquid and solid and good filleting in a properly designed joint. The discovery that a braze in vacuum can be promoted by magnesium led to the development of the fluxless process cur rently practicated in the automotive industry. In this process, the filler metal itself promotes the dispersion process. Our understanding of the promotion mechanism has been improved through a number of experiments. Studies of magnesium promo tion indicate that magnesium from the filler metal alloys mo difies the oxide barier during brazing. A small concentration of Mg is observed to develop in the aluminium oxide barier film at temperatures above 400 C, where Mg vaporization occurs. This small buildup is interpereted as the onset of an (AlMg) 0 spinel formation in the pores and free surface of the oxide that are contacted by the vaporizing Mg. When the filler me tal reaches its melting point it is observed to penetrate and cover the modified oxide. This process, which results in the dispersion of the oxide is interpreted to be the result of a wetting of the oxide by molten filler metal. XIV