RATIO OF GRATE SURFACE TO FOR CONSTRUCTION OF BOILERS The ratio of grate surface to borse-power varies with the type, as is shown below.
Makers of boilers sometimes estimate the II. P. by the heating surface. That is the horsepower is a fraction of the heating surface. The ratio of heating surface to 11. P. for several types is as follows: Plain cylindrical, 6 —10 Multitubular, 14 —18 Vertical, 15 —20 Water tube, 10 —12 Marine return tube, 3.25— 4Lancashire, 2.75— 4.25 Locomotive, 1 — 2 It is evident that some portions of the heating surface of a boiler have greater efficiencies than others. For instance, more heat will pass through the crown sheet as it is nearer the fire than through the last few feet of the tubes. Taking the efficiency of the crown sheet as 1, an estimate of the percentage of the other parts of a boiler is as follows: Crown of furnace in flue, .95 Plates of cylindrical boiler over furnace, .90 Fire box tube plate of locomotive boiler, .80 Water tube surface facing fire, .70 Vertical side of fire box, .50 If a cylindrical multitubular boiler is divided into equal sections, the section nearest the fire will evaporate more water than the one at the other end, as the gases have a higher temperature at the first section. Suppose we divide the boiler into six sections of equal length, and call the total evaporation 100 per cent. Then the per cent of evaporation per section will be approximately as follows : Section 1 2 3 4 5 6 Evaporation 47 23 14 8 5 3 If the length of a boiler is increased another section, the evaporation will be increased a little but at the same time the radiating surface is increased. In case the addition of a section for evaporation causes a loss by radiation nearly equal to the gain in evaporation, it is not economical to add the section on account of the extra cost of the boiler. If forced draft or an increasd of air of dilution is used, the boiler should be made longer to avoid waste. The air of dilution is the amount of air above that which is necessary to burn the coal.