All About Engines

84 All About Engines we shall now proceed to examine those which enable steam to be raised with economy. For every pound of coal burnt in the furnace 14,000 units of heat are produced, and the engineer is concerned to convey these to the engine with the least loss on the way. All of them he cannot retain. There must be some heat in the chimney gases, or the chimney would create no draught. The boiler must be warmer than surrounding objects, however carefully it is covered with badly conducting material; and so long as it is warmer, heat will flow in a continuous stream from its sides. But by the devices and methods we are about to describe a good deal of loss is prevented. Twenty-five years ago a horse-power required from 6 to 7 square feet of heating surface in the boiler, while to-day it can be produced from 2 or 3 square feet. At that time 3 or 4 lb. of coal were needed for every horse-power produced by an engine ; to- day it requires only from 1 lb. to lb. Modern boilers, then, are smaller and less than half as costly to run as the old ones were. A manufacturer who was spending £2,000 a year on coal a quarter of a century ago is now obtaining the same power for less than £1,000. This result is achieved partly by improved design of boilers and partly by mechanical stoking, forced draughts, feed-water heating and regu- lation, and superheating, some of which also reduce the cost of labour. Stoking Probably most people would regard keeping the fire going merrily as one of the easiest things in the