Boder-covcrings.—Non-conducting- coverings are made of wool-felt, paper-pulp, asbestos, magnesia, plaster of Paris, etc. It may be said of all these that the greater the amount of air inclosed in their pores or left as a jacket between them and the boiler which they encase, the better their heat-insulating properties.
boilers are given a blast of hot air, others a current of warm air, others, again, a jet of cold air under considerable pressure. In every case the temperature and pressure of the draught may be made to have a considerable effect upon the capacity and economy of the boiler.
,Sfram-dwites.—Additional steam-space is often given by vertical cylin drical " domes " at a right angle to horizontal cylindrical shells; and this usage is almost universal on American locomotive-boilers. It has the marked demerit that the (Ionic is expensive and weighty, and, as generally arranged, is a source of weakness to the shell, a large part of which is gen erally cut away, to permit the boiler to be entered through the hole thus left when the dome is removed. The very small steam capacity which a dome really adds makes its value as a reservoir problematical. Sonic of the best steaming- English locomotives are domeless, and a few American railroads are trying the experiment of leaving off these adjuncts.
The carrying over with the steam of mechanically entangled water may be very largely prevented by compelling the steam to pass through a dry pipe, a long horizontal pipe in the steam-room which has in the centre connection with the steam-pipe and into which the steam can pass only through fine slits or drilled holes, which act as strainers.
Where a boiler generating steam at high pressure is intended to sup ply the steam to heating-systems, etc., where low pressure is needed, there is interposed a pressure-reducing valve, which prevents the pressure beyond it from exceeding a desired point, no matter what pressure may be behind.
The is usually made of east-iron sections, called " bars," so constructed that they have a large amount of air-space between them, so that they cannot easily be warped, and that at the same time they can readily be removed. Some of the best are so arranged that they may be
given motion between and among one another, to facilitate cleaning the fire. Rocking-grates have sections, called "fingers," which may be worked up into the fire by a shaking-lever. (See p. 2S6.) Figure i (pi 78) shows a stepped grate, which it is claimed gives more air surface and places the fuel in a better position than does a nearly level grate.
411,whanfral 2 shows a continuous mechanical stoker. The grate is in sections, arranged like the apron of a horse tread-mill, and the fuel, being fed in at B, is gradually run in under the com bustion-chamber, the ashes and the unconsumed fuel dropping off at the back. The entire grate system may be run out upon the tracks if In Figure 3 is shown a mechanical stoker in which the fuel is fed from below by a screw, the intention being to supply it as in the wick of a lamp or candle, so that as it becomes heated the volatile gases will be evolved and ignited. The ashes are run off at the sides of the combustion pan and fall in an annular passage. In Figure 5 (pi. 77) the ash-pan, which is water tight, is filled with water.
In the rather remarkable system shown in Figure 7 (A1. 7S) the fuel is fired upon an inclined grate and the gases follow the course of the arrows along under the upper shell, between that and the flame-bed or partition; then back under this partition and between it to the middle drum; then back between the middle drum and the lower drum, and then under the lower drum forward to the flue. Thus the coldest gases strike the bottom shell. The connections between the shells are by only one nozzle, at the end, so that the water and the steam have to follow the same zigzag course as that taken by the gases.
In sonic cases the bottom of the fire-box is made double and contains water, the grate being at a little distance above it; but, while this may in some measure increase safety from fire, it renders the boiler more unsafe, by reason of the facilities it offers for the deposit of mud and scale upon the lower sheet and the consequent destruction of the plates.