Smoke is often taken as an indication of poor combustion, and so it may be. However, smoke is often due to the presence of fine particles of soot resulting from breaking down volatile mat ter by heating with insufficient air rather than to larger particles of unburnt fixed carbon. Hence a smoking furnace may be working at fairly high efficiency, while one with a clear stack may have a large excess air loss. Smoke may be prevented by using a low volatile fuel such as anthracite coal and coke, or by mixing air with the volatile matter more thoroughly and thereby insuring its complete combustion in the furnace.
Combustion losses are reduced by preheating the air. The maximum permissible preheat temperature is determined by the durability of grates or stoker parts. In pulverized coal firing, the preheat temperature is limited only by the durability of the pre heater elements. There is an economic relation, however, between the cost of air preheater surface, economizer surface and boiler tube surface, which results in a minimum cost for the complete steam generating unit corresponding to a moderate air preheat temperature between 400° and 5oo° F.
If all the heat liberated by combustion and carried into the furnace in the preheated air were imparted to the products of combustion, very high furnace temperatures would be reached. Much lower temperatures actually obtain in boiler furnaces due to radiation. Radiation from the incandescent fuel and flames varies approximately as the fourth power of their absolute tem perature. With water-cooled walls, over one-half the heat of
combustion is absorbed in the furnace by radiation from the incandescent fuel and flames. This radiant heat will fuse any particles of ash which cannot be kept cool by adjacent water cooled surfaces. Hence refractory baffles must be protected by two or more rows of boiler tubes on the furnace side or particles of ash adhering to them will fuse and form large slag masses. With pulverized coal firing, ash will melt on the furnace bottom unless cooled below its fusing temperature and protected from radiation by "screen tubes." While the melted ash may be run out of the furnace as molten slag, there is an additional furnace loss in the latent heat of fusion of this slag.
Boiler test codes adopted by engineering societies in various countries, define boiler efficiency as the ratio of the heat absorbed by the steam produced per unit weight of fuel burned, to the gross, or upper, heating value of the fuel as determined in a calorimeter where the moisture present in the fuel and formed from the hydrogen therein is condensed to water. In England, efficiencies are often guaranteed as based on the lower heating value, which is equal to the upper heating value minus i,o5o X (M+9H), where M = lb. of moisture and H= lb. of hydro gen per lb. of fuel burned and i,o5o= latent heat of steam at atmospheric temperatures. (W. L. DE B.)