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] |
|_____________________|_______________________|__________|____________|

It is essential in the burning of this class of fuel that a large
combustion space be supplied, and on account of the usually high
moisture content there should be much heated brickwork to radiate heat
to the fuel bed and thus evaporate the moisture. Extension furnaces of
the proper size are usually essential for good results and when this
fuel is used alone, grates dropped to the floor line with an ashpit
below give additional volume for combustion and space for maintaining a
thick fuel bed. A thick fuel bed is necessary in order to avoid
excessive quantities of air passing through the boiler. Where the fuel
consists of hog