though these are not required unless they are necessary to secure an
added furnace volume, to get in sufficient grate surface, or where such
an arrangement must be used to allow for a fuel bed of sufficient
thickness. Depth of fuel bed with the dry fuel is as important as with
the moist fuel. If extension furnaces are used with this dry wood, care
must be taken in their design that there is no excessive throttling of
the gases in the furnace, or brickwork trouble will result. In Babcock &
Wilcox boilers this fuel may be burned without extension furnaces,
provided that the boilers are set at a sufficient height to provide
ample combustion space and to allow for proper depth of fuel bed.
Sometimes this is gained by lowering the grates to the floor line and
excavating for an ashpit. Where the fuel is largely sawdust, it may be
introduced over the fire doors through inclined chutes. The old methods
of handling and collecting sawdust by means of air suction and blast
were such that the amount of air admitted through such chutes was
excessive, but with improved methods the amount of air so admitted may
be reduced to a negligible quantity. The blocks and refuse which cannot
be handled through chutes may be fired through fire doors in the front
of the boiler, which should be made sufficiently large to accommodate
the larger sizes of fuel. As with wet fuel, there will be a quantity of
unconsumed wood carried over and the heating surfaces must be kept
clean.

In a few localities cord wood is burned. With this as with other classes
of wood fuel, a large combustion space is an essential feature. The
percentage of moisture in cord wood may make it necessary to use an
extension furnace, but ordinarily this is not required. Ample combustion
space is in most cases secured by dropping the grates to the floor line,
large double-deck fire doors being supplied at the usual fire door level
through which the wood is thrown by hand. Air is admitted under the
grates through an excavated ashpit. The side, front and rear walls of
the furnace should be corbelled out to cover about one-third of the
total grate surface. This prevents cold air from laneing up the sides of
the furnace and also reduces the grate surface. Cord wood and slabs form
an open fire through which the frictional loss of the air is much less
than in the case of sawdust or hogged material. The combustion rate with
cord wood is, therefore, higher and the grate surface may be
co