ged wood and sawdust alone, it is best to feed it
automatically into the furnace through chutes on the top of the
extension. The best results are secured when the fuel is allowed to pile
up in the furnace to a height of 3 or 4 feet in the form of a cone under
each chute. The fuel burns best when not disturbed in the furnace. Each
fuel chute, when a proper distance from the grates and with the piles
maintained at their proper height, will supply about 30 or 35 square
feet of grate surface. While large quantities of air are required for
burning this fuel, excess air is as harmful as with coal, and care must
be taken that such an excess is not admitted through fire doors or fuel
chutes. A strong natural draft usually is preferable to a blast with
this fuel. The action of blast is to make the regulation of the furnace
conditions more difficult and to blow over unconsumed fuel on the
heating surfaces and into the stack. This unconsumed fuel settling in
portions of the setting out of the direct path of the gases will have a
tendency to ignite provided any air reaches it, with results harmful to
the setting and breeching connection. This action is particularly
objectionable if these particles are carried over into the base of a
stack, where they will settle below the point at which the flue enters
and if ignited may cause the stack to become overheated and buckle.

Whether natural draft or blast is used, much of the fuel is carried onto
the heating surfaces and these should be cleaned regularly to maintain a
good efficiency. Collecting chambers in various portions of the setting
should be provided for this unconsumed fuel, and these should be kept
clean.

With proper draft conditions, 150 pounds of this fuel containing about
30 to 40 per cent of moisture can be burned per square foot of grate
surface per hour, and in a properly designed furnace one square foot of
grate surface can develop from 5 to 6 boiler horse power. Where the wood
contains 50 per cent of moisture or over, it is not usually safe to
figure on obtaining more than 3 to 4 horse power per square foot of
grate surface.

Dry sawdust, chips and blocks are also used as fuel in many wood-working
industries. Here, as with the wet wood, ample combustion space should be
supplied, but as this fuel is ordinarily kiln dried, large brickwork
surfaces in the furnace are not necessary for the evaporation of
moisture in the fuel. This fuel may be burned in extension furnaces