Steam:
Its Generation and Use

furnace is for the proper combustion of the fuel, and its duty is performed to perfection when the greatest amount, but not necessarily intensity, of heat is obtained from the given weight of com- bustible. The boiler proper is for the transfer of the heat thus generated into useful effect by evaporating water into steam, and its function is fulfilled completely when the greatest possible quantity of heat is thus utilized. To a lack of depend upon the amount of air admitted to the furnace, and the increase of temperature at which it escapes. The more air admitted the greater the loss ; hence the fallacy of all those schemes which admit air above the fire. The rate of combustion should not exceed 0.3 pound of coal per hour per square foot of heating surface, except where quantity of steam is of greater importance than economy of fuel. Where appreciation of this fact, and of a knowledge of the principles involved, is chargeable much waste of money and disappointment, both to in- ventors and steam users. As a boiler is for making steam, it can only utilize for that purpose heat of a greater intensity or higher temperature than the steam itself, there- fore the gases of combustion cannot be reduced below that temperature, and the heat thereby represented is lost. The amount of this loss will a blast is used the grate surface should be pro- portionately reduced to secure best economy. “The maximum conductivity or flow of heat is secured by so designing the boiler as to secure rapid, steady, and complete circulation of the water within it . . . and securing opposite di- rections of flow for the gases on the one side and the water on the other.”—Prof. R. H. Thurston. The accumulation of scale on the interior, and of soot on the exterior, will seriously affect the 44