Document omZ5OQQaNLmdM2QyB5dEngn2R

326 CHAPTER 13 1952 Guide l door, and by leaks in the setting, whereas the gases leave only through the outlet. By throttling the gases with the damper in the outlet all the air entering by each of file three intakes is reduced in the same proportion, thus maintaining about the same percent of excess-air. If inlet air is con trolled by the ashpit draft door, the air admitted through the ashpit is reduced, while it is increased through the other two intake openings, resulting in an increase of excess air. A considerable increase in the efficiency of handffired furnaces and boilers can be realized by regulating the air supply by means of the damper in the outlet instead of the, ashpit damper. Use of the ashpit damper is required, of course, for low rates of combustion. The cold air check damper. is to be used only when chimney draft is excessive. It is normally closed unless closing of the outlet damper and ashpit damper is unable to control the rate of combustion. Furnace Volume for Coal and Coke The principal requirements for a hand-fired, furnace are that it shall have enough grate area and correctly proportioned combustion space. The amount of grate area required is dependent upon the desired combus tion rate. The furnace volume is influenced by the kind of coal used. Bituminous coals, on account of their long-flaming characteristic, require more space in which to bum the gases of combustion completely than do the: coals low in volatile matter. For burning high volatile coals, provision should be made for mixing the combustible gases thoroughly, so that combustion is complete before the gases come in contact with the relatively cool heating surfaces. An abrupt change in the direction of flow tends to mix the gases of combustion more thoroughly. Anthracite requires com paratively, little combustion space. CLASSIFICATION OF FUEL OILS - Fuel oils are mixtures of hydrocarbons derived from crude petroleum by refining processes designed to produce suitable proportions of naphtha, gasoline, kerosene, fuel oil, and lubricating oil. The processes leave a residue of coke, asphalt, or paraffin depending on the source of the crude oik In the past, refining processes have been directed toward producing the maxi mum amount of gasoline, because this product was in greatest demand. Fuels and Combustion 327 The relative proportions of gasoline and fuel oil produced, per unit volume of crude oil could be varied considerably to suit seasonal changes in demand or gradual trends from year to year. At present gasoline represents about 43 percent and fuel oil (including kerosene) about 22 percent of the yield from crude oil. . Crude oil is distilled in towers at atmospheric pressure to produce gaso line, naphtha, kerosene distillates, and colored distillates, and leave straightrun residues. The colored distillates are distilled further to produce light distillate fuel oils, some lubricating oil, wax, etc., whereas the straight-run residues are distilled under vacuum to produce heavier distillates. The residual fuels remaining can then be passed through cracking plants to produce more gasoline, cracked kerosene, cracked distillates, and cracked residual fuel oils. The exact processes used depend on the proportions of the various end products desired, and to some extent upon the composi tion and characteristics of the crude oil. Fuel oils may be described as straight-run fuels, thermally-cracked fuels, catalytically-cracked fuels, or blended fuels depending on the refining proc ess used to produce them. Straight-run fuels are those produced by distil lation under atmospheric pressure or a vacuum without decomposition of the hydrocarbons by craclang. Thermally-cracked fuels are those produced by a cracking process involving elevated temperatures (850-1100 F) to decompose some of the heavier hydrocarbons. Catalytically-cracked fuels are those produced with the aid of an alumina-silica catalyst in the crack ing process at lower temperatures than those used for thermal cracking. Blended fuel oils are mixtures of any of the above three types. Analysis of Fuel Oils Crude oil in its natural state contains primarily paraffin hydrocarbons (chemical formula CnHiu+i, naphthene hydrocarbons (formula CaH&), and aromatic hydrocarbons (formula C'n/7Ja_6) where n is a whole number. Fuel oils produced by pure distillation, that is the straight-run fuel oils contain essentially these same hydrocarbons. Those produced by crack ing processes may. contain generally all the hydrocarbon series from CtMta+i to CaHia^it, and especially do they contain appreciable percentages of the olefin hydrocarbons which are relatively less stable than file paraffin, napthene, and aromatic hydrocarbons. The paraffin hydrocarbons are hydrogen-saturated, are among the most stable, and have the highest hydrogen-carbon ratio of any of the hydrocarbon series. The straight-run fuel oils have the highest paraffin content, the highest hydrogen-carbon ratio and are the most stable of the fuel oils. The thermally-cracked fuel oils have the lowest paraffin content while the catalytically-cracked fuel oils are intermediate' in paraffin content and stability. The hydrogencarbon ratio of straight-run fuel oils ranges from 0.155 to 0.170, and in catalytically-cracked fuel oils ranges from 0.133 to 0.156, while it is some what lower for thermally-cracked fuels. The blending of straight-run oils with cracked oils is common practice to improve the paraffin content, stability, and ignition characteristics of fuel oils. A high paraffin content and a high hydrogen-carbon ratio are generally desirable characteristics for domestic fuel oils and consequently, the straight-run distillates are better suited to this use than the fuel oils produced by the various cracking processes. On the other hand, thermally-cracked fuel oils often have a lower pour point and a lower viscosity than comparable straight-run fuel oils. The color and stability of cracked fuel oils can be much improved by treatment with sulfuric aicd, by neutralization, and by redistillation.