BİLGİSAYAR YARDIMIYLA ISI DEĞİŞTİRİCİSİ TASARIMI ÖZET Bu çalışmada genel anlamda ısı değiştiricilerinin tasarımının bilgisayar yardımıyla gerçekleştirilmesi incelenmiştir. Öncelikle ısı değiştiricilerinin tanımı, sınıflandırılmaları, çalışma şekilleri ve kullanım alanları belirtilmiştir. Visual Basic 5.0 bilgisayar diliyle dört ayrı bölümden oluşan bir program hazırlanmıştır. Amaç, bilgisayarda programlama yoluyla ısı değiştiricilerinin ısıl hesaplarının yapılması ve boyutlandınlmasıdır. Programın ilk bölümü matematiksel modeli incelenmiş olan gövde boru tipi ısı değiştiricisi ile ilgilidir. Akışkanların giriş, çıkış sıcaklık değerleri, kütlesel debileri, ekonomik faktörler programa veri olarak girilir ve maliyet analizi ile birlikte optimum ısı değiştiricisi boyutları bulunup boru dizilişlerinin resmi çizdirilir. Yöntem olarak şöyle bir yol izlenmiştir: Değişik gövde ve boru çapları için tek tek ısıl ve maliyet hesapları yapılmıştır. İçlerinden en ucuz maliyete sahip olan ısı değiştiricisi çözüm olarak kabul edilmiş ve çözümler sanayide kullanılan ısı değiştiricileri ile karşılaştırıldığında bir yaklaşıklık gözlemlenmiştir. İkinci bölümde soğutucu ve nem alıcı serpantin tasarımı incelenmiştir. Serpantinin ısıl hesapları yapılıp, kullanılan akışkanlar (su ve hava) için yapılıp soğutma ile birlikte neni alıcı olarak çalışıp çalışmayacağı belirlenmiştir. Islak, kuru ve toplam yüzey alanları bulunduktan sonra akışkanların çıkış sıcaklıkları ve ısı yükleri hesaplanır. Basınç düşümleri tespit edildikten sonra maliyet analizi yapılır ve son olarak örnek bir resim bilgisayar ortamında çizdirilir. Üçüncü bölüm ısıtıcı serpantin ile ilgilidir. Boyutları veri olarak girilen ısıtıcı serpantinin ısı transferi hesaplan yapılır ve bunun neticesinde bulunan boyutlar ile ilk girilen boyutların uygunluğu kontrol edilir ve bu değerlerin kabul edilecek yakınlıkta olması sağlanır. Son bölümde ise akışkan özelliklerinin bulunduğu program hazırlanmıştır. Bu özellikler ilk üç bölümde gerekli olmuş ve o bölümlerde kullanılmıştır. Bu özellikler ayrı olarak dördüncü bölümde bir kez daha incelenmiştir. Su için sıcaklık değerlerine bağlı olarak yoğunluk, ısı iletim katsayısı, Prandtl sayısı, özgül ısı ve dinamik viskozite gibi değerler bulunmuştur. Hava için ise yine sıcaklığa bağlı olarak yoğunluk, ısı iletim katsayısı, ısı yayılım katsayısı, Prandtl sayısı, dinamik viskozite ve özgül ısı değerlerinin yanında havanın psikrometrik özellikleri belli bir iterasyon sonucunda bulunmuştur. Kullanılan tüm bağıntılar amprik ifadelerdir ve belli bir oranda hata vermektedir. Ancak bu hata oranları ısı değiştiricilerinin tasarım sıcaklığı için oldukça düşüktür ve hesaplamalarda kullanılır. xı COMPUTER AIDED DESIGN OF HEAT EXCHANGERS SUMMARY In this study, computer aided design of heat exchangers is examined generally. At first the heat exchangers' definitions, classifications, working manners and usage areas are determined. A computer program which is formed from four parts is prepared by using Visual Basic 5.0. The object is the obtaining of thermal calculations and sizing of the heat exchangers. The heat exchangers are classified to six parts: Heat transfer process, surface compactness, geometry (construction), flow type, number of fluids, heat transfer mechanism. According to the heat transfer process classification, the direct contact and the indirect contact type of heat exchangers are available. In direct contact type of heat exchangers the heat transfer is obtained from the contact of hot and cold fluids to each other directly which are used in cooling towers and jet type of condensers mostly. In indirect contact type, the thermal energy transfers from a surface or a wall between the fluids that do not touch each other. The ratio of heat transfer surface area to the volume of the system in heat exchangers is the measure of compactness. If this ratio is more than 700 m2/m3, this heat exchanger is called as a compact heat exchanger. In contrary, the heat exchanger is not a compact one if this value is less than 700 m2/m3. The concept of compactness is very important when the replacement area of the heat exchanger is in the first consideration. Classification to the geometry has four types: Tube type heat exchangers, plate type heat exchangers, extended surface type heat exchangers and regenerative type heat exchangers. The tube type of heat exchangers are double pipe, shell and tube type and spiral tube type. The shell and tube type of heat exchanger is the most frequently used one and its thermal design and the sizing is calculated in the first part of the program. The plate type of heat exchangers are gasketed, spiral and lamella. Especially, the gasketed plate type of heat exchangers are being used widely after the shell and tube type heat exchangers. This type is preferred when there are too much disadvantages of the shell and tube type heat exchanger in that application. The heat exchangers that have extended surfaces are plate-fin types and tube-fin types. The tube-fin type heat exchangers are examined in the second part of the program. They are especially used in xnthe air conditioning systems. The regenerative type of heat exchangers are mostly used in steam boilers as air heaters and also they have a very large compactness. Paralel flow, counter flow and cocurrent flow are the types of flow in heat exchangers. Counter flow gives the highest effectiveness. In the classification according to the heat transfer mechanism, there are four items: one phase flow convection in both sides, two phase flow convection in one side and one phase flow convection in the other side, two phase flow convection in the both sides and finally convection and radiation together. The number of fluids are mostly two in heat exchangers but in special processes three or more number of fluids may be used. The shell and tube type heat exchangers are used in heating, refrigeration, air conditioning, power plants, chemical processes and so many similiar areas. They are the structures which the tubes are placed in a shell. They provide high heat transfer surface area in respect of their volumes and the weights. They can be cleaned easily, can be designed for high pressure applications, have a great flexibility for the service requirements and they can be designed and manufactured without any difficulties. This type heat exchanger is most widely used one. The most important parameter in the design of heat exchangers is the shell type. The shell type is chosen according to the TEMA (Tubular Exchanger Manufacturera Association) standarts. Tube arrangements and the baffles are also very important parameters in heat exchanger design. Square and triangle tube arrangements are available but the triangle tube arrangement are mostly preferred because of its tube density, low cost and higher heat transfer per surface area. Also the distance between the tubes is very considerable for the strength of the tube sheet. The distance between the baffles is the function of the shell diameter and it plays a great role in heat transfer so the designer should give great importance to this parameter during the design. In the basic design of heat exchangers, at first the problem should be defined clearly. The flow rates, arrangements (like condensation or boiling), inlet and outlet temperatures and pressures and the other extra datas necessary for the design should be given. In design process, the basic construction of the heat exchanger is selected as a trial form. This trial design is evaluated and the thermal performance and the pressure drops are found. If these found values are in satisfied ranks, this design is accepted. Otherwise, the trial form is changed and and the calculations are done till reaching the desired values. It is preferred to do these calculations by the help of the computers. There are two main problems encountered in the design of heat exchangers: Rating (performance analysis) and sizing problems. In rating problem, an available heat exchanger is examined. Heat exchanger type, dimensions, surface geometry, flow rates of the fluids, inlet temperatures and the fouling factors are known. According to these datas, outlet temperatures, total heat transfer amount and the pressure drops for the both sides can be found out. If an acceptable thermal performance that supplies a value under the maximum pressure drop is found, this is the solution of the design. The second problem is the sizing problem. In this problem, inlet and outlet temperatures, flow rates, surface geometries, allowed pressure drops and the material properties are the datas of xinthis kind of design. Consequently the sizes of the heat exchanger is obtained. Since the necessity of the selection of the heat exchanger type before the thermal analysis, second problem is more difficult than the first one. The cooling and the dehumidifying coils are used in many fields like refrigeration, air conditioning systems, drug and food industry and product storing. The heat of the air transfers to a metal surface, then transfers to the cooling fluid. In this study the cooling fluid is selected as water. Also the moisture in the air, condenses on a cold surface and constitutes wet layer. So it is possible to take the moisture from the air besides cooling the air. Usually fins are used on these coils. The fins are located to the air side of the tubes since the heat convection coefficient here is lower than then the value on the other side. In the design of shell and tube type heat exchangers and the cooling and the dehumidifying coils, the properties of the fluids (usually water and air) are used repeatedly. The amprical equations which are related with the temperature of the fluids are used. The result of these equations are a little bit different form the real values read from the fluid tables. But these differences are acceptable in the design range of the heat exchangers. For water, viscosity, density, specific heat, heat conduction coefficient and Prandtl numbers are found with the data of temperature. For air, heat conduction coefficient, Prandtl number, viscosity, density, specific heat and psychrometric values like wet bulb temperature, relative humidity are found out. As a computer program, Visual Basic 5.0 is used in this study. It is possible of writing computers with Visual Basic which run under Windows operation system. Since widely known software programs that are used in PC's are prepared to be run under Windows, the other conventional computer programs like GWBasic, Qbasic, Fortran, C, Pascal are all outdated. Creating new softwares with these programs are very troublesome because of very long command lines. But with visual computer programs (Visual Basic, Delphi, Visual C) it is very suitable to make programs with rich graphical media. This graphical user interface facilitates the usage and learning of the different kind of applications. In Visual Basic program, the known abilities of the usual Basic are combined with the visual design tools. Menus, fonts, dialog boxes, text boxes, scroll bars and the other graphical elements can be designed fast and easily. It has an event driven structure. Namely, instead of programming softwares in sequences with constant steps, the programmer clicks a window, moves the mouse or selects a command. Creating databases, interaction with the other popular software like CAD programs and data exchange applications are possible. Another advantage of Visual Basic is that the programmed software can be compiled and it can be converted to an executable file. For the heat exchanger program, a main menu is prepared at the beginning. There are four numbered command buttons on a form for the four program parts. Clicking one of these buttons make the connected program part run. Also five labels exist. The color of the forms, the font types and the font sizes on the boxes and buttons are set for the main menu and for the other forms which are used for the other parts of the program. xivThe first part of the program is related with the shell and tube type heat exchangers that at first its mathematical model is found out. Fluids' inlet and outlet values, mass flow rates, fouling factors, total operating time, economical factors like materials unit cost, electrical cost and the rate of increase for fuel are entered as datas and after the cost analysis, the optimum exchanger dimensions and the drawing of the tube arrangements are found out. As a method, the thermal and cost analysis is done for the different diameters of the shells and the cheapest one is accepted as the optimum solution. Besides these results, the number of tubes is also determined. This program part is divided to three sections. The first section is for the water in the shell side and the water in the tube side. The second section is for the steam in the shell type and water in the tube side. Finally the third section is for the water in the shell side and the steam in the tube side. The two phase flow analysis is done for the second and for the third sections. When these solutions are compared with the exchangers used in the industry, a great approximation is seen. In the second part of the program, the design of air cooling and dehumidifying coils is studied. The principal logic is same with the former program part. Air inlet pressure, temperature, specific humidity, mass flow rate, air velocity, water inlet temperature, mass flow rate, water flow velocity are the datas which are entered to the computer. According to these entered values, the thermal calculations of the cooling and the dehumidifying coils run. After the thermal calculations, it is established whether the air cooling coil is working as dehumidifier or not. After wet, dry and total heat transfer areas are determined, the outlet values of the fluids and total amount of heat are found out. Following the pressure drop obtention, cost analysis is done and a sample drawing is created in the computer medium. The third part is about the heating coils. The former program parts' logic continues in this part. The dimensions of the coil, heat load, fluids' inlet and outlet temperatures and the properties and dimensions of the fins are entered as the datas. After the heat transfer calculations, the dimensions found ultimately are compared with the ones entered before. It is tried to get these values have acceptable nearness to each other. The last part of the program is prepared for obtaining the properties of the fluids (water and air) which are used in the first, second and third parts of the heat exchanger program. For shell and tube type heat exchanger design, the water properties; for the cooling and dehumidifying coils the water and air properties with the psychrometric properties and for the heating coils, the water and air properties are necessary. For water, as the function of the temperature, density, heat conduction coefficient, Prandtl number, specific heat, dynamical viscosity values are found. For air, besides these values, psychrometric values are found by using iteration method. These properties are found with the emprical equations and these equations have a certain error rate. But these error rates are acceptable at the range of the design of heat exchangers and after the comparison of tese values with the real ones in the diagrams and tables it can be considered that they can be used reliably. xvConsequently, the design, optimisation and the sizing of the shell and tube type heat exchangers, cooling and dehumidifying coils and the heating coils are determined with the computer. For creating this software, MS Visual Basic 5.0 is used due to its flexibilities, facilities in working and the graphical user interface properties. In the applications, there are very different types of the heat exchangers for different fields. Because of this, the heat exchangers should be designed one by one for the pointed out values. To obtain the optimum design, variable economical factors should be always taken into consideration. This program provides the reducing of design time, the minimizing the design errors and the facilitating of the sizing. Briefly, the computers and the softwares plays the most significant role in the engineering fields recently. It is certain that the importance of the computers will be further in the engineering, industrial and technological fields in the future. xvi 111