Isı enerjisinin geri kazanılması

Abstract

Bu çalışmada çeşitli endüstri dallarında ısı enerji sinin geri kazanılması ve önemi üzerinde durulmuştur. Konunun genişliği göz önünde tutularak, bu çalışmada dışarı atılan sıcak gazlardan hava ısıtılması ve kullanma suyunun ısıtılması sureti ile ısı geri kazanılması üzerinde durulmuştur. İlk bölümde ısı geri kazanma sistemleri hak kında genel bir bilgi verilmiştir. 2. ve 3. bölümde bu tip uygulamalarda kullanılan çeşitli ısı değiştiricileri ile malzeme, korozyon, kirlilik gibi ortak sorunlar verilmiştir. Ayrıca 2. bölümde atık ısının geri kazanılmasın da çeşitli yöntemler tanıtılmıştır. 4. ve 5. bölümde örnek bir uygulama verilerek ısıl tasarım anlatılmıştır. Son bölümde ısı geri kazanmanın önemi üzerinde durulmuştur, Ayrıca ısı geri kazanma ünitesinde maliyet açısından bir inceleme yapılarak uygun ısı değiştiricisinin seçimi anlatılmıştır. Genelde endüstride atık ısının geri kazanılmasınının enerji giderleri açısından önemi vurgulanmış tır.

There exists today worldwide concerns about the best ways of using the depletable sources of energy, and of developing techniques to reduce pollution. This interest has encouraged research and development efforts in the fields of alternative energy sources, cost-effective use of the exhaustible sources of energy, and the re-use of the usually wasted forms of energy. A large number of industrial processes, covering most indust rial sectors, use significant amounts of energy in the form of heat, which is rarely utilized efficiently. Thus there is considerable scope for the use of heat exchangers and other forms of heat equipment to enable waste heat to be recovered. The potential savings possible are greatest for the temperature range from 200 to 500°C. The developed countries are the pacesetters in energy consumption, and discharging at the same time vast amounts of waste energy. In the USA.., for example, the total energy consumption is 2.2810 kwh. Of this, the indust rial sector consumes 8,20 10^-2 kwh. The thermal energy discharged energy 3,221012 kwh, meaning that the thermal waste of the industry amounts to %14 of u.s energy con sumption, and %39 of the industrial energy consumption. The consumption and discharged waste of energy by industry in Europe is close to that in the U.S.A. Ways and means to conserve depletable energy sources and to recover some of otherwise wasted energy are currently active areas of research. The energy that is wasted by industry takes the forms but combustible fuel, sensible heat dischorge from drain water, and more notably, the sensible and latent heat discharge from flue gases. Waste energy can be recovered by the installation of combustion equipment to utilize the wasted fuel, and the provision of heat recovery equipment to regain sensible and lateat heat. Much effort has been expended during the past two decades to re-use the wasted heat. Analyses of heat recovery systems sui table for industrial plants have been suggested in this context. An overview of available waste heat equipment, as well as of current applications were presented. Charts, tables and curves were made available to assist the engi neer in selecting the appropiate heat recovery system. xv The decision as to whether or not to apply a waste heat recovery and utilization system is usally economical in nature. The payback-period and life-cycle cost concepts are considered during the decision-making process. It must be emphasized for effective energy management that other more effective steps, like process control, mainte nance improvement and process changes such as adjusting the excess-air rate, and streamlining of operations, must be considered before investing in heat recovery equipment. Engineers made a survey of the waste heat related industries, and concluded that refuse incineration, sewage incineration, cement factories, glass furnaces, foundries and industrial incinerators provided ample opportunities for waste heat recovery. They suggested that the reco wered waste heat be used for water desalination, especia- ally in arid zones. A further study on the same subject suggested the use of waste heat recovered from the incine ration of solid wastes to be used in connection with desa lination of water by the reverse-osmosis process. Other engineers pointed out that gaseous streams represent the largest and most readily exploited source of recoverable heat. They asserted that economizers can be used to reco ver sensible heat only, and that waste heat boilers are most appropriate at temperatures above 3oo°C. They discussed the possibility of utilizing waste heat from the cooling water from electric generators for heating greenhouses. They proposed various applications in the process industry where, steam generated by the use of waste heat from a cogeneration facility could be used Another designers investigated ways to pipe waste thermal energy generated in nuclear power plants to locations up to 40 km away. They concluded that saturated high pres sure steam may be produced from ovens, furnaces, turbines, incinerators and combustion equipment by the use of heat pipes. Another application of waste energy was considered for the production of fresh water from the sea. They proposed to use waste heat gained from aluminium smelting furnaces to drive vapor compression equipment for desa lination. Designers studied the kinds of equipment available for gas-to-gas heat recovery. They discussed various types of heat exchangers that may find use as waste heat recovery equipment. They concentrated on fule-gas heat recovery equipment, and emphasized the factors to consi der when specifying a system. They recommended the use xvi of gas-to-gas heat exchan gers, including the heat wheel, for high temperature gases. Designers pointed out the importance of material selection as regards the effects of corrosion. They underlined the advantages of the heat pipe heat exchangers over other types. Gas- to-liquid and liquid-to-liquid type of heat exchangers were treated by the designers, with special emphasis on heat recovery from polluted and corrosive effluents. Designers described the use of a gas-to-gas heat exchanger for preheating combustion air to about 12o°C above the ambient. This was achieved by the use of the stack gases, and resulted in a %6 reduction in fuel con sumption. The use of gas-to-gas heat exchangers in the form of heat recovery wheels, heat pipes and recuperators was investigated in connection with energy retrieval from the exhaust air of buildings. Designers reviewed recupe rative and regenerative techniques and equipment for heat recovery at high temperatures. They proposed the use of liquid heat exchangers for heat recovery from high-presure gas compressors. They also suggested that the thermal efficiencv of the gas turbine can be imporoved by inlet gas precoolingand by intercooling of compressor stages. The extent to which heat can be recovered from a given stream of hot gases is limited by the state of cleanliness of the gas stream. Contaminated streams can cause problems associated with fouling, erosion, corro sion pitting and thermal fatigue. These factors limit the lifetime of heat recovery equipment and reduce effici ency. The amount of latent heat recovered from exhaust gases depends on the permissible lower temperature limit to which these gases may be cooled. Polluted gas streams containing oxides of sulphur and nitrogen are rarely cooled below 15o C to avoid the formation of sulphuric and nitric acids. Dilute solutions of these acids attack the heat recovery equipment. Waste heat is usually but not always characterized by low temperature. There are many methods through which this energy can be recovered and utilized. Unburned fuel can be expended in special combustion equipment. The recovered energy can be utilized to reduce the cost of waste disposal. Depending on the temperature level of the wasted heat and the proposed application, different heat exchanger devices can be employed to facilitate the use of the recovered heat. Energy storage is needed when there is a time span between energy recovered and use. xvii The application of heat recovery should be physicall close to the source of waste heat for maximum benefits from recovered energy. It may be concluded from the above review, that con siderable potential exists for recovering some of the wasted energy in industrial processes, and of using it to improve plant performance. Research and development efforts seem to be focused especially on heat exchangers that utilize heat pipes, Rankine cycle and heat pumps. In the study of thermal system a segment of that knowledge is particularly important, namely, predicting the performance of an existing heat exchanger. Not only is the selection of a heat exchanger important, but it is also crucial to be able to calculate how a certain heat exchanger will perform when operating at off-design condi tions. A useful tool to be stressed in this chapter is the effectiveness of. heat exchangers. A comman engineering task is to select, design or specifty a heat exchanger to perform a certain heatrtrans- fer duty. The engineer then decides on the type of heat exchanger and its details. There of the several dozen types of heat exchangers available are used in industrial processes. Shell-and-tube heat exchanger, commonly, used to transfer heat between two liguids. One of the fluids flows inside the tubes and is called the tube side fluid while the other flows over the outside of the tubes and is called the shell-side fluid. The heat exchanger has two tube passes, which means that the tube-side fluid flows through half tubes in one direction and back through the other half. The head of the heat exchanger on the left end is equipped with a divider that separates the incoming from the outgoing tube-side fluid. Baffles are placed in the shell so that the shell-side fluid flows across the tubes a number of times before leaving the heat exchanger, instead of short-circuiting to the outlet. The finned-coil heat exchanger is the type often chosen to transfer heat between a gas and a liguid. Since the resistance to heat transfer on the gasside is usually high because of the low heat-transfer coefficient of a gas, fins are installed on the gas side to increase the heat-tarnsfer area. The third type of heat exchanger is a compact heat exchanger it usually consists of a stack of metal plates that are often corrugated and arranged so that the two fluids flow through alternate spaces between the plates. XVlll We now return to the distinction between selecting and optimizing a heat exchanger. To select a shell-and- tube heat exchanger for example, the flow rates entering temperatures, and leaving temperatures of both fluds would be known. The task of the designer is to select the combination of shell diameter, tube length number of tubes, number of tube passes, and the baffle spacing that will accomplish the specified heat-tansfer duty. The design must also ensure that certain pressure-drop limi tations of the fluids flowing through the heat exchanger are not exceeded. In optimization, on the other hand, the heat exchan ger already exists, either in actual hardware or as a specific design. Furthermore the performance characteris tics of the heat exchanger are available such as the area and overall heat transfer coefficients. Optimization of a heat exchanger consists of predicting outlet conditions such as temperatures, for various inlet temperatures and flow rates. The emphasis of the next several sections will be on predicting outlet conditions of a given heat exchanger when the inlet conditions are known. Optimization is the process of finding the conditions that give maximum or minimum values of a function. Opti mization has always been an expected role of engineers, although sometimes on small projects the cost of enginee ring time may not justify an optimization effort. Often a design is diffucult to optimize because of its complex ity. In such cases, it may be possible to optimize subsy terns and then choose the optimum combination of them. There is no assurance, however that is procedure will lead to the true optimum. A workable system the process often consists of arbitrarily assuming certain parameters and selecting individual components around these assumptions. In cont rast, when optimization is integral part of the design, the parameters are free to float until the combinaiton of parameters is reached which optimizes the design. Basic to any optimization process is the.decision regarding which criterion is to be optimized. In an aircraft or space vehicle, minimum weight may be the criterion. In an automobile, the size of a system may be the criterion. Minimum cost is probably the most common criterion. On the other hand, the minimum owning and operating cost may not always be followed strictly. A manufacturer of domestic refrigerators, for example, does not try to design his system to provide minimum total cost to the consumer during the life of the equipment. xix The achievement of minimum first cost, which enhances sales, is more important than operating cost, although the operating cost connot be completely out of bounds. Industrial organization often turn aside from the most economical solution by introducing human, social, and aesthetic concerns. What is happening is that their criterion function includes not only monetary factors but also some other factors that may admittedly be only vaguely defined. Optimization activities are often practiced under the name of operations research. Many developments in operations research emerged from attempts to opitimze mathematical models of economic systems. It is only recently that mechanical and chemical engineers have used certain of the disciplines to optimize fulid-and energy flow systems. Component simulation and system simulation are often preliminary steps to optimizing thermal system, since it may be necessary to simulate the performance over a wide range of operating conditions. A system that, may be optimum for design loads may not be optimum over the entire range of its expected operation. Sometimes a design engineer will say= I have optimi zed the design by examining four alternate concepts to do the job which probably means that the engineer has compa red workable systems of four different concepts. The statement does emphasize the two levels of optimization, comparison of alternate concepts and optimization within a concept. A complete optimization procedure then consists of proposing all reasonable alternate concepts, optimizing the design of each concept, and then choosing the best of the optimized designs. The method of lagrange multipliers performs an opti mization where equality constraints exist but the method cannot directly accommodate inequality constraints. A necessary requirement for using calculus methods is the ability to extract derivatives of the objective function and constraints.