bars, may be arranged with trunnions, so as to rock the individual bars by means of hand levers. A form of cast-iron grate bar for the burning of sawdust is shown in Fig. 3. The bar is semicircular in cross-section and is provided with circular openings FIG. 3 for the introduction of air. Lugs are cast on each side of the bar to serve as distance pieces in providing air spaces between the bars. Dead Plate.-The front ends of the grate bars are usually supported on the dead plate, which is a flat cast-iron plate placed across the furnace just inside the boiler front and on a level with the bottom of the furnace door. The purpose of the dead plate is twofold: It forms a support for the firebrick lining of the boiler front, and a resting place on which bituminous coal may be coked before it is placed on the fire. To support the grate bars, the inner edge of the dead plate is either beveled or a lip is provided, as at a, Fig. 4. Objection to Stationary Grate Bars.-The greatest objection to stationary grate bars is that with them the furnace door must be kept open for a considerable length of time when the fire is being cleaned. Cleaning fires when the boiler has a stationary grate not only severely taxes the fireman, but the inrush of cold air chills the boiler plates, thus producing stresses that in the course of time will crack them. Shaking Grates.-There are on the market many designs of shaking grates for large steam boilers, differing chiefly in detail and arrangement. Usually the grate bars are hung on trunnions at each end and are connected together with bars to which are attached shaking rods that extend forwards through the furnace front. Levers or handles are attached to the shaking rods, and by working them back and forth the grate bars receive a rocking motion that breaks up the bed of coal on the grate and serves to shake the ashes through into the ash-pit. The fires may thus be kept clean without the necessity of opening the fire-doors. FIG. 4 Classes of Mechanical Stokers.-A mechanical stoker is a power-driven rocking grate arranged so as to give a uniform feed of coal and to rid itself continuously of ashes and clinkers. The principal designs of mechanical stokers and automatic furnaces may be divided into two general classes, overfeed and underfeed. Overfeed Stoker.-In overfeed stokers the fixed carbon of the coal is burned on inclined grates. The coal is pushed on to these grates, which are given a sufficiently rapid vibratory motion to feed it down at such a rate that practically all the carbon is burned before reaching the lowerend, where the ashes and clinkers are discharged. In Fig. 5 is shown a sectional view of a stoker of this class. The coal is fed into the hopper a, from which it is pushed by the pusher plate b on to the dead plate c, where it is heated. From cit passes to the grate d. Each bar is supported at its ends by trunnions and is con FIG. 5 nected by an arm to a rocker bar i, which is slowly moved to and fro by an eccentric on the shaft s, so as to rock the grates back and forth; the grates thus gradually move the burning fuel downwards. The ashes and clinkers are discharged from the lower grate bar on to the dumping grate e. A guard f may be raised, as shown by the dotted lines, so as to prevent coke or coal from falling from the grate bars into the ash-pit when the dumping grate is lowered. Air for burning the gases is admitted in small jets through holes in the air tile g, and the mixture of gas and air is burned in the hot chamber between the firebrick arch h and the bed of burning coke below. Underfeed Stoker.-The stoker shown in Fig. 6 illustrates the principle of operation and the construction of the underfeed stoker. Coal is fed into the hopper a, from which it is drawn by the spiral conveyer b and forced into the magazine d. The incoming supply of fresh coal forces the fuel upwards to the surface and over the sides of the magazine on the grates, where it is burned. A blower forces air through a pipe ƒ into the chamber g surrounding the maga zine. From g the air passes upwards through hollow-cast iron tuyère blocks and out through the openings, or tuyères e. The gas formed in the magazine, mixed with the jets of air from the tuyères, rises through the burning fuel above, where it is subjected to a sufficiently high temperature to secure its combustion. Nearly all the air for burning the coal is supplied through the tuyères, only a very small portion of the supply coming through the grate. The ashes and clinkers are gradually forced to the sides of the grate against the side walls of the furnace, from which they are removed from time to time through doors in the furnace front similar to the fire-doors of an ordinary furnace. In other words, owing to the construction of the underfeed stoker, the fire must periodically be cleaned from clinkers and the ashes removed by hand. COVERING FOR BOILERS, STEAM PIPES, ETC. The losses by radiation from uncovered pipes and vessels containing steam are considerable, and in the case of pipes leading to steam engines, are magnified by the action of the condensed water in the cylinder. It therefore is important that such pipes should be well protected. The accompanying table gives the loss of heat from steam pipes naked, and covered with wool or hair felt of different thickness, the steam pressure being assumed at 75 lb., and the exterior air at 60°. There is a wide difference in the value of different substances for protection from radiation, their values varying nearly in the reverse ratio to their conducting power for heat, up to their ability to transmit as much heat as the surface of the pipe will radiate, after which they become detrimental, rather than useful, as covering. This point is reached nearly at baked clay or brick. A smooth or polished surface is of itself a good protection, polished tin or Russia iron having a ratio, for radiation, of 53 to 100 for cast iron. Mere color makes but little difference. Hair or wool felt has the disadvantage of becoming soon charred from the heat of steam at high pressure, and sometimes of taking fire therefrom. This has led to a variety of cements for covering pipes-composed generally of clay mixed with different substances, as asbestos, paper fiber, charcoal, etc. A series of careful experiments, made at the Massachusetts Institute of Technology showed the condensation of steam in a pipe covered by one of them, as compared with a naked pipe, and one covered with hair felt, was 100 for the naked pipe, 67 for the cement covering, and 27 for the hair felt. The presence of sulphur in the best coverings and its recognized injurious effects make it imperative that moisture be kept from the coverings, for, if present, it will surely combine with the sulphur, thus making it active. Stated in other words, the pipes and coverings must be kept in good repair. Much of the inefficiency of coverings is due to the lack of attention given them; they are often seen hanging loosely from the pipe that they are supposed to protect. RELATIVE VALUE OF NON-CONDUCTORS The preceding table is deduced from tests of raw materials contained in commercial pipe coverings and does not indicate the relative values of coverings into whose composition they enter. Mineral wool, a fibrous material made from blast-furnace slag, is a good protection, and is incombustible. Cork chips, cemented together with water glass make one of the best coverings known. A cheap jacketing for steam pipes, but a very efficient one, may be applied as follows: First, wrap the pipe in asbestos paper, though this may be dispensed with; then lay from 6 to 12 strips of wood lengthwise, according to size of pipe, binding them in position with wire or cord, and around the framework thus constructed wrap roofing paper, fastening it by paste or twine. For flanged pipe, space may be left for access to the bolts, which space should be filled with felt. If exposed to weather, tarred paper should be used or the exterior should be painted. A French plan is to cover the surface with a rough flour paste, mixed with sawdust until it forms a moderately stiff dough. It should be applied with a trowel, in four or five layers each in. thick. If iron surfaces are well cleaned from grease, the adhesion is perfect; for copper, a hot solution of clay in water should be applied. A coating of tar renders the composition impervious to the weather. BOILER FEEDING AND FEEDWATER INJECTORS As Classification of Injectors.-Injectors may be divided into two general classes, namely, non-lifting and lifting injectors. Non-lifting injectors are intended for use where there is a head of water available. When the water comes to a non-lifting injector under pressure, as from a city main, it can be placed in almost any convenient position close to the boiler. Lifting injectors are of two distinct types, called automatic injectors and positive injectors. positive injectors generally have two sets of tubes, they are frequently called double-tube injectors. Automatic injectors are so called from the fact that they will automatically start again in case the jet of water is broken by jarring or other means. Positive, or double-tube injectors are provided with two sets of tubes, one set of which is used for lifting the water, and the other set for forcing the water thus delivered to it into the boiler. A positive injector has a wider range than an automatic injector and will handle a hotter feed-water supply; it will also lift water to a greater height than the automatic injector. Advantages and Disadvantages of Injectors.-The advantages of the injector as a boiler feeding apparatus are its cheapness, compared with a pump of equal capacity; it occupies but little space; the repair bills are low, owing to the absence of moving parts; no exhaust piping is required, as with a steam pump; it delivers hot water to the boiler. The disadvantages of the injector are that it will not start with a steam pressure less than that for which it is designed, and that it will stand but little abuse, being poorly adapted for handling water containing grit or other matter liable to cut the nozzles. Size of Injector.-Most engineers prefer to select a size of injector having a capacity per hour about one-half greater than the maximum evaporation per hour in order to have some reserve capacity. The maximum evaporation, WATER DELIVERED BY INJECTORS |