Yüksek basınç püskürtmeli bir sıvı yakıt brülorünün yakıt sisteminin modellenmesi

Abstract

Bu tezin amacı, sıvı yakıtlı bir yüksek basınç püskürtmeli brülörün varolan yakıt sistemini modellemek ve bu yolla pratik değerlere ne kadar yaklaşılabileceğini göstermektir. Bu nedenle tez içinde yanma olayına mümkün olduğunca girilmemeye çalışılmıştır. Sistemde kullanılan yakıt fueloildir. Bu nedenle ilk iki bölümde fueloil hakkın da geniş bilgiye yer verilmiştir. Daha sonraki bölümlerde sıvı yakıtla çalışan brölür tipleri ve yakıtın yakıcıya aktarılma sistemlerinden bahsedildikten sonra tipik bir yüksek basınç püskürtmeli brülörün yakıt sistemi genel ve incelenecek sistem bazında ayrıntılı olarak anlatılmıştır. Son bölümde ise hesap yöntemi ayrıntılı olarak anlatılmıştır. Hesaplarda yardımcı olacak bilgi, şekil ve tabloların tümüne metin bölümlerinde yer verilmiştir. Kullanılan yöntem, sistem elemanlarının hidrolik davranışlarını tanımlamak, sistemdeki önemli değişkenleri (basınç ve debi gibi), girilecek başlangıç değerlerinden itibaren, bu hidrolik davranışlara bağlı olarak, sistemde istenen dengeye ulaşana kadar kademe kademe düzeltmektir. Bu nedenle bir bilgisayar programına ihtiyaç duyulmuş ve Turbo C programlama dili kullanılmıştır. Program akış diyagramı, programın kendisi ve çıktı örnekleri tezin sonuna eklenmiştir. Programı anlaşılır hale getirebilmek için, mümkün olduğunca hesap yöntemiyle paralel gidilmiştir. Sonuç bölümünde elde edilen sonuçların bir yorumu yapılmıştır. Pratiğe yakın sonuçlar elde edilmesi, bu veya benzer programlarla mevcut sistemlerin incelenmesinin mümkün olabileceğini göstermektedir. Ayrıca sistemde yapılacak ilave veya değişikliklerin sistem karakteristiklerinde yaratacağı değişikliklerin mertebeleri deney yapmadan kolaylıkla bulunabilir. Mevcut sistemde karakteristikler gerçeğe çok yakın bulunmuş ve elektrik motoru kapasitesinin küçültülebileceği tespit edilmiştir. Burada, en önemli husus, sistem elemanlarının ve özellikle kayıpların ve sıcaklık değişimlerinin çok iyi belirlenmesinin gerektiğidir.

The aim of this thesis is to model fuel oil system of a present high pressure oil atomizing burner and to show how it is possible to get characteristics very close to the practical values by this method. Any petroleum product, whether it is crude oil, gasoline, lubricating oil, or fuel oil, is composed of only two elements, carbon and hydrogen. Any combination of car bon and hydrogen is called a hydrocarbon. There are many types of hydrocarbons found in petroleum, but most of them fall into four main classes- paraffinic, aromatic, naphtenic, and olefinic. Sulphur is present in varying amounts in all crude oils and petroleum products. Presence of this element, even in such quantities as small as 1 to 2%, can be quite troublesome to both refiner and consumer. Generally, few oils origi nally contain oxygen and nitrogen, but some of compounds present have a tendency to pick up oxygen, thereby creating different compounds. As the amounts of sulphur, oxygen, and nitrogen in oils are usually small, they are classed as impurities. As there are many types of burners used in many difrTerent operations, so must there be different grades of fuel oil to meet their needs. Some burners can bum all gra des of oils, while others are intended for one particular grade. Burners that can operate on all or more than one grade of fuel oil usually cannot do so economically with every grade. The proper grade of oil for any installation is usually governed by the following points: Type and size of burner, type and method of atomizing; size of combustion chamber, type of equipment; type of operation. In order to keep the various grades on a uniform basis, the National Bureau of Standards and the American Society for Test ing Materials have standardized five grades of oils designated as No. 1, 2, 4, 5 and 6. No. 1 oil is used exclusively for domestic heating. No. 2 oil is called a distillate oil, as it is capable of being distilled or vaporized at normal temperatures and pressures. It is used in low capasity burners without preheating. No. 4 is also called light oil and may need preheating before burning. No. 6 and No. 5 oils are the heavy black residuals. No. 5 is the blending product of No. 4 and No. 6 oil. They are used in high and medi um capasity burners specially in industrial burners and need preheating. The gravity of oil is an important measure of both heavy and light oils, as it is easily obtained, and a number of approximate conclusions and relevant information can be derived from it. But, with modem refining methods, using many different cru des, a gravity spesification does not mean or indicate anything definite or spesific, as two oils with the same gravity can have many different characteristics. The oil indust ry employs the A.P.I. gravity scale. The relation between API gravity and spesific gra vity is the following equation: Deg API gravity=141.5/specific gravity at 60 °F - 131.5 Following informations can be derived from the API gravity of an fuel oil: The lower vu the API gravity, the heavier the oil in viscosity or consistency, the higher in carbon re sidue, and the heaver in weight; the higher the API gravity, the greater the heat of combustion; the higher the API gravity, the lower the unit weight of the oil inkg/hA The viscosity of an oil is the measure of its resistance to the flow. It is someti mes referred to as consistency. Usually if the API gravity of an oil is low, the viscosity is high and conversely. However, this does not always hold true, as a number of fac tors tend to operate against this relationship, including methods of refining, crude ba se, and whether or not oil is a blended one. If it is necessary to choose the one most important single spesification of an oil, viscosity must be selected. For proper and efficient combustion an oil should have a reasonable viscosity at the burner so as to obtain the best atomization. The difficulties encountered with an oil of too high visco sity can be listed as follows: Difficulty in pumping from tank to burner, if too thick and viscous, insufficient oil will reach the burners, causing erratic and spasdomic ope ration; flash-back from the burner; trouble in starting the burner due to insufficient quantity of oil available at the burner, poor atomization; high viscosity oil can be high in carbon residue, and due to poor combustion caused by the foregoing conditions, carbonization of burner tips and carbon formation on walls of fire chamber may result. An oil too light in viscosity can be responsible for the following troubles: Too much oil may be pumped to the burners, causing incomplete combustion, resulting in smo ke, carbonization and dirty combustion chamber. As fuels are heated, vapors are produced which at a certain temperature flash when ignited by an external flame. This temperature is called the flash point of the oil. If the heating is continued, sufficient vapors are finally driven off to produce a continu ous burning not just a flash. This temperature is called fire point. Light oils usually have lower flash points. However the flash point of an oil also changes with blending, contamination and refining. There is no relationship between the flash point and spon taneous ignition point. This ignition point is the temperature at which a mixture of oil vapors and air will ignite without the application of an external flame. This point is very high and there is no need to worry about it. The temperature at which an oil will just flow under standardized conditions is known as the pour point. The high pour point of an oil, especially distillates, is usual ly ascribed to the presence of wax. Some of troubles in cold weather due to pour po int include clogged strainers, oil unpumpable, clogged lines, erratic combustion as insufficient amounts of oil reach the burners, spitting and smoke carbon from poor atomization, due to heaviness of the oil. Application of heat, so as to make the oil more fluid, will prevent and eliminate these troubles. It is impossible to ignite the oil properly in the fluid state, and the function of the burner is to break the oil into a fine spray, this action is called atomization. Atomizati on is accomplished at the burner tip, or just beyond it, by air or steam pressure or by the oil pressure itself. If the oil were not heated and thinned, it would be extremely difficult to obtain this fine spray, with resulting poor ignition and combustion. The fine spray makes possible for more oxygen to react with the extremely small oil partic les to produce rapid and complete combustion. Preheating brings the oil temperature closer to the ignition point of the oil and thins out it for better atomization. If there is too much preheating there is danger of cracking and coking the oil in the preheater, causing blocked burners and loss of heat; the oil can become too fluid, thereby passing vm through the burners too fast, giving improper combustion and carbonization on the combustion chamber walls, as well as slipping at the pump, vaporization, and pulsation. If the preheating temperature is too low, as the viscosity will be too high for good atomization, inefficient combustion will be obtained, resulting in high fuel con sumption, loss of heat, smoke, and carbonization. Sometimes it is not possible to start the burner. The functions of a burner are to deliver oil and air to the combustion space (thus positioning the flame), to mix the oil and air, to provide for continious ignition of the oil-air mixture and to atomize and vaporize the oil. The oil is delivered to the burner by three different methods. If the oil tank level is above the burner, oil runs to the burner by its weight, and the extra oil returned by the pressure relief valve or by-pass valve of the oil pump is delivered back to the tank. In this case, outlet from the tank should be up to 30 meters above the level of the pump and the oil level in the tank should not exceed 4 meters. If the level of the tank is under the burner, the oil is sucked to the burner by the pump. But the difference between the levels of the tank and the burner should not be higher than 2 meters and the pipe length should not exceed 5-6 meters. This system is called suction system. If this difference is over than 4-5 meters, a pump is added to the system as the burner pump cannot suck the oil itself. This system is called pumping system. There is another classification of oil delivering systems to the tank: One pipe or two pipes system. In one pipe system, the oil is brought to the burner by one pipe and not returned back. This is used in light oil burners such as diesel oil. Heavy oil burners are accomplished with two pipes system. During the first movement of the burner, preheated oil is delivered to the nozzle line. As the nozzle is closed during the first movement, the oil is returned to the tank via second pipe, thus heating the oil and lines to the tank and from the tank. Oil is stored in a tank where its temperature is kept about 50°C. In industrials plants, there is another tank called daily tank besides the main one. The oil sucked from the tank is first delivered to the preheater after passed through a strainer, bringing its temperature a proper degree suitable for the burner. Oil burners are conveniently classified according to the method of dispersing the oil: 1. Vaporizing, 2. Oil-pressure atomizing, 3. Low-pressure air-atomizing, 4. High-pressure steam- or air-atomizing, 5. Centrifugal atomizing (rotary) burners. Liquid fuels must be vaporized before they can be burned. Some small capacity burners accomplish this vaporization in a single step by direct heating of the liquid. Such burners are called vaporizing burners. Typical examples of these are blowtorch, gasoline stoves. Fluid under pressure when released through a small orifice tends to break into a fine spray. This is the principle of oil-pressure atomizing burner. The oil must acquire a swirling motion inside the nozzle before it is released from the orifice in order to de velop a satisfactory spray. This is ensured by diagonal grooves cut in a disk, through which the oil pass. In the low-pressure oil atomizing burners, these grooves are coarser compared with the high-pressure oil atomizing ones. Therefore a heavier oil can be used in low-pressure oil atomizing burners. IX In the low-pressure air-atomizing burners, air under low pressure is used as the atomizing medium. Oil pressure need to be sufficient only to deliver the fuel through the ports in an inner tube. Around this tube, there is an inner chamber. The air is int roduced into this inner chamber through slots tangentical to a small circle, which impart a rotary motion to it. The air picks up the oil, partly atomizing it. Atomization is completed when this stream from the inner chamber engages a second air stream from an annular space, which is not whirling. In high-pressure steam or air-atomizing burners, steam or compressed air tears droplets from the oil stream and propels them into the combustion space. The high velocity of the oil particles relative to the air produces the scrubbing action required for quick vaporization. These burners can atomize very heavy oils, sludges, pitch, and some tars. A subclassification is afforded by considering the point of mixing of oil and steam. Outside-mixing burners contact the two fluids only at the point of their release into the furnace atmosphere, whereas inside-mixing burners contact the oil and steam inside the burner and obtain secondary dispersion when the mixture passes through an outside port or nozzle. In the rotary burners, the oil is delivered to a rotating cup, and centrifugal force then throws the oil from the lip of the cup in the form of a conical sheet of liquid which quickly breaks into a spray. Low pressure air is admitted through an annular space around the rotating cup. If the air velocity is high, it tends to blow the spray into a narrow cone, but if the speed of rotation of the cup is high, this tends to overcome the effect of the air stream, producing a wide angle spray. Both horizontal and vertical rotary cup burners are used for boilers and domestic furnaces. High pressure oil-atomizing burner equipments: The most important parts of this type burners are as follows: An electric motor is used to drive both the oil pump and the fan, that are connected each other with a coupling. The oil sucked from the tank or a preheater is delivered to the heater on the burner by the oil pump. Fan is located in the burner body, and with the first movement it will take away the combustion gasses remained in the combustion chamber. The function of the fan is to provide the air for combust ion and regulate the amount of the air to the oil with the help of a shutter. Fuel oil must be supplied to the burners at low but steady rates. Positive-displacement-type pumps are the most satisfactory for this service. Fuel pumps are equipped with an au tomatic pressure-operated by-pass or relief valve, so that a constant discharge pressure is maintained. Thus, it is possible to meet different capacities using one oil pump. Another reason to use by-pass valve is to protect the burner from excessive pressures. The function of the heater on the burner is to bring the oil temperature to a proper degree for atomization. The oil from the heater is delivered to the nozzle where it is given its whirling motion and sprayed to the combustion chamber. Burner tile (com bustion block) is a refractory block with a conical or cylindrical hole (flame tunnel) through its center. The tile serves to maintain ignition and to reduce flash-back and blow-off. If a flame is temporarily extinguished, the hot surface of the tile will reignite the fuel-air mixture. Burners are equipped with more than one strainer, one located at the inlet of the pump, one at the outlet of the heater, one in the strainer itself etc. The ignition of the oil is accomplished by the difference in voltage up to 10.000 V produced by a transformer between two flame electrodes, and the continuity of the flame is detected by a photocell. The turbulent flow of air is achieved by a tabulator. The form of flame formed in the tile is adjusted by changing the spraying angle of the nozzle and the location of the turbulator The figure above shows the most known equipments of an high pressure oil atomizing burner. In the last chapter of this thesis, fuel oil system of a present high-pressure oil atomizing burner will be studied with all details and limits of the suction line, oil pressure and temperature at the outlet of the pump, rotating speed and the torque of the electric motor, total flow rate, flow rates given by first and second nozzle, powers required for the pump and the fan and the total power of the system will be calculated. The method is to define the hydraulic behaviours of the system components, to bring the relationships between the components to a equilibrium step by step, starting with some arbitrary values. For this reason a computer program will be used with the help of Turbo C programming language.