HEATING SURFACE FOR CONSTRUCTION OF BOILERS The portion of a boiler that is exposed to the flames and hot gases is called the heating surface. 'This is made up of the portions of the shell below the brickwork, the exposed ends, and the internal surface of the tubes. If the boiler is of the water tube type, the exterior surface of the tubes is taken in place of the interior surface.
If our boiler is an ordinary multitubular boiler we can assume the heating surface to be the total inside area of the tubes plusone-half the area of the shell. Then: Heating surface of tubes (8.74 x 71 x 16)/12 = 827.38 square feet.
Heating surface of shell (14.137 x 16)/2= 113.10 square feet.
940.48 square feet.
The ratio of heating surface to grate surface will be 940.48/24 = 39.2 or about 39.
As this ratio is high enough we will not alter our figures. If the ratio had been too low we could have added more tubes and found a new boiler diameter. The heating surface should not be less than 1 square yard or 9 square feet per horse-power. So 940.48 ÷ 75 = 12.54 or our boiler has 12.54 square feet of heating surface per horse-power. This is of course abundantly sufficient.
The capacity of heating surface to transmit heat to water depends upon conductivity, position of surface and temperature of furnace. In designing it is safe to follow proportions of heating surface to grate area in the various types, which experience has shown to give the best results. The following are the proportions for a few types.