Heating Surface

—That part of a steam boiler exposed to the heat generated by the furnace. Area of heating surface is frequently used to expre the horse power. This is figured from the number of square feet of boiler and tube surface, exposed to the action of the fire; the extent of the heating surface of a boiler depends on the length and diameter of the shell and the number and size of the tubes or flues. For the ordinary tubular boiler, fifteen square feet of heating surface has. been held to be equal to one horse power; it is i A E