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1. Тексты для письменного перевода Текст 1. Combustion

Burning or combustion is a special form of oxidation: oxygen combines rap- idly with certain types of fuel, such as coal, oil, gas or wood, and substantial amounts of heat are liberated.

Under some conditions, combustion may be self-starting. For example, coal piled outdoors combines slowly with oxygen in the air giving off heat. If the heat does not dissipate fast enough, temperature rises, and the reaction speeds up until it eventually becomes rapid enough to be called burning.

Such spontaneous combustion is relatively uncommon, except in outdoor coal piles. (It also occurs occasionally in storage bins and bunkers.) Combustion usually begins when heat from an outside source is applied to a fuel. The burning process is initiated by striking the match to generate enough friction heat to set it aflame. The flame's heat is used to light kindling, and the kindling's heat to start the logs. This method of using an easily lighted object to provide heat for ignition of a harder-to-light fuel is common in engineering.

You are well aware that some things burn more readily than others. In gen- eral, the degree of flammability depends on how easy it is to burn the particular substance into a gas, because nothing truly burns until it is a gas. This, in turn,de- pends on the nature and quantity of the substance, compared with the amount of heat available to start combustion. It is easier to start wood-burning than coal, and easier to ignite a twig than a log.

Note that, while combustion is essentially a chemical reaction, most of the practical problems of fuel-burning are mechanical.

Текст 2. Burning equipment

There are two general methods of firing fuel commonly employed: 1) on sta- tionary grates, or 2) on stokers. Also coal may be pulverized to the consistancy of

70 per cent through a 200-mesh screen and burned in suspension. The types of sol- id fuel encountered in various parts of the world and the general conditions under which they must be burned are so variable that it is impossible to-design one type of grate or stoker that is exactly suited to all fuels. The problem becomes one ra- ther of suiting the equipment to the type of fuel to be handled.

To a certain extent, the design of the furnace must be considered coinci- dentally with the selection of fuel-burning equipment, so that satisfactory ignition and heat release may be ensured. The choice of equipment for a given set of condi- tions is limited, and, although any stoker will burn any fuel only one design as a rule will give satisfactory results. Coals may be broadly classified as follows:

Group 1. This group includes the anthracites and semi-anthracites which should be burned without agitation of the fuel bed.

A fuel of this class is satisfactorily burned on travelling grate or chain-grate stokers, on which the coal is fed in a comparatively thin, uniform layer. As com- bustion progresses, the ash covers the surface of the stoker and acts as a protective blanket, the fuel being supplied with combustion air as it travels toward the ashpit.

Croup 2. This group includes the bituminous coals of the caking type which require agitation of the fuel bed to break up the mass of coke as it forms as well as to resist the tendency of this fuel to fuse into a mat, or cake, that resists the passage of air and retards the process of combustion. Underfeed stokers of the multiple- retort type are designed to burn coals of this class, for the plungers have a charac- teristic forward and upward motion. By breaking up the surface of the fuel bed, more air passages are created, with a tendency to increase combustion rate. A few coals of this class have a low ash-fusion temperature with a resulting tendency to fuse and jam the operating parts of the stoker. These coals, particularly if high in sulphur, should be avoided as stoker fuels.

Group 3. This group includes midwestern coals and most of the westernbi- tuminous coals. These do not tend to soften but form masses of coke, they require

no agitation of the fuel bed and are burned to best advantage on chain-grate stok- ers.

Group 4. This group consists of most of subbituminous coals and lignites which do not fuse when heated and do not require agitation. They have a tendency to disintegrate or slack on the grate as well as drift and sift through if disturbed. They have a tendency to avalanche on inclined grates and are most satisfactorily burned on chain- or traveling-grate stokers.