Grate

Grate. It is of importance in the combustion of coal for heating purposes that the furnace should be properly constructed so as to fulfil the special objects for which the furnace is required. In the ordinary fireplaces required for the warming of houses, it is customary to have grates of about 24 inches in width and 6 inches in height placed close to the floor so that the requisite draught to the fire may be low-lying, and so that there may be less danger of accidents by fire. The draught is very slight, just sufficient to induce a continual upward current of burnt gases through the chimney and to effect a slow and steady combustion of the fuel. When the fuel is incandescent it is desirable that it shall be burnt away as slowly as possible, for there is then less waste by convection of hot gases up the chimney and more radiation of heat into the room. Kitchen ranges require the heat to be localised to a greater extent, and the grate is therefore much closer. Also, it is usual to conduct the hot gases past the ovens so that part of their ' heat may be utilised. In engineering the design of grates for steam boilers is of much importance. It is necessary that the area of the grate shall be carefully estimated. This is obtained from determinations of the amount of water that has to be converted into steam per hour. It also depends upon the shape of the boiler and its flues, and in fact the area of fire-grate is usually approximated by comparison with already-existing examples that are working well.' The number of pounds of coal burnt per square foot of fire-grate per hour varies from about 4 in large Cornish boilers to 140 in locomotives Such grates as these consist of parallel fire-bars about 3 feet in length supported on cross-bars, the whole forming an area about 6 feet in length and from 15 to 50 inches in width.