Combustion

TM-5-3895-374-10 Paving Machine Bituminous Material: Crawler Mounted Diesel Engine Driven Manual
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TM 5-3895-374-10 heat. The aggregate is exposed directly to the radiant and convective heat from the flame of the burner. The basic function of the burner is to proportion oxygen (air) and fuel and to atomize and mix the fuel and oxygen to obtain complete combustion. The flame shape plays an important role in the drying process within the drum mixer. The flame shape is adjusted to provide a short bushy flame which provides a flame over a large section of the drum cross-section and gives a more uniform temperature across the cross-section. There are a number of factors that determine the flame shape. As the combustion air is forced through the burner by a blower, it is delivered at a regulated pressure. Generally, the flame shape and intensity can be regulated by controlling the blower pressure, the amount of air forced through the burner, and the "swirl". The flame size is partially determined by the blower pressure, by the percentage of stoichiometric air provided to combustion by the blower, and by the ratio of the angular momentum of the burner air to its axial momentum. The type of fuel burned, the degree of atomization of the liquid fuel, the particle size of the fuel, the extent of premixing of the fuel and air, the combustion zone temperature, the amount of swirl, etc. will all effect the flame size and shape to some degree. Only a few of these factors are under the control of the operator. Other portions of this technical manual should be consulted for procedure for adjustments. Combustion is basically a chemical reaction between a fuel and oxygen (02) initiated by an ignition source. Complete combustion generates carbon dioxide (Co2) and water in the form of steam (H2OS). For complete combustion of a fuel, there is a specific amount of oxygen required. In essence, there must be two oxygen (O) atoms available for each carbon (C) atom in the fuel (C+20=CO2), and on oxygen (O) atom for every pair of hydrogen (H) atoms(2H+O=H20) After the amount of oxygen needed for combustion is determined, the amount of combustion air that will provide that oxygen to the combustion process can be calculated. Since air is approximately 21% oxygen the amount of required combustion air is calculated by dividing the required amount of oxygen by .21. The resulting number is commonly called the "stoichiometric air required for combustion". Each fuel has its own unique stoichiometric air requirement, and is generally specified as a air-to-fuel ratio -- the minimum amount of air needed to burn one unit of fuel. For example, the stoichiometric air required to burn one gallon of No 2 fuel oil is 1377 cubic feet (CF), so the stoichiometric air-to-fuel ratio is 1377 cubic feet per gallon. In the case of incomplete combustion, there will be a third combustion product -- carbon monoxide. Generally, if carbon monoxide is present it occurs in extremely small concentrations in the exhaust gases. Complete combustion of a fuel with its stoichiometric air-to-fuel ratio requires extensive mixing of the fuel and air, so that every available oxygen atom will meet up with every available carbon and hydrogen atom. Unfortunately equipment that operates this efficiently does not exist. It is necessary to provide the combustion process with more page 1 - 22

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