In designing furnaces for burning wood, the question resolves itself into: 1st, the essential elements to give maximum capacity and efficiency with this class of fuel; and 2nd, the construction which will entail the least labor in handling and feeding the fuel and removing the refuse after combustion. Wood, as used commercially for steam generating purposes, is usually a waste product from some industrial process. At the present time refuse from lumber and sawmills forms by far the greater part of this class of fuel. In such refuse the moisture may run as high as 60 per cent and the composition of the fuel may vary over wide ranges during different portions of the mill operation. The fuel consists of sawdust, "hogged" wood and slabs, and the percentage of each of these constituents may vary greatly. Hogged wood is mill refuse and logs that have been passed through a "hogging machine" or macerator. This machine, through the action of revolving knives, cuts or shreds the wood into a state in which it may readily be handled as fuel. Table 42 gives the moisture content and heat value of typical sawmill refuse from various woods. TABLE 42 MOISTURE AND CALORIFIC VALUE OF SAWMILL REFUSE _____________________________________________________________________ | | | | | | | | Per Cent | B. t. u. | | Kind of Wood | Nature of Refuse | Moisture | per Pound | | | | | Dry Fuel | |_____________________|_______________________|__________|____________| | | | | | | Mexican White Pine | Sawdust and Hog Chips | 51.90 | 9020 | | Yosemite Sugar Pine | Sawdust and Hog Chips | 62.85 | 9010 | | Redwood 75%, | Sawdust, Box Mill | | | | Douglas Fir 25% | Refuse and Hog | 42.20 | 8977[41] | | Redwood | Sawdust and Hog Chips | 52.98 | 9040[41] | | Redwood | Sawdust and Hog Chips | 49.11 | 9204[41] | | Fir, Hemlock, | | | | | Spruce and Cedar | Sawdust | 42.06 | 8949[41] | |_____________________|_______________________|__________|____________|