nst
setting deterioration but to protect the operators from leakage of gas
which is disagreeable and may be dangerous. Aside from the intensity of
the draft, a poor mixture of the gas and air or a "laneing" action may
lead to secondary combustion with the possibility of dangerous
explosions within the setting, may cause a pulsating action within the
setting, may increase the exit temperatures to a point where there is
danger of burning out damper boxes, and, in general, is hard on the
setting. It is highly essential, therefore, that the furnace be properly
constructed to meet the draft which will be available.

Stacks for Wood-fired Boilers--For boilers using wood as fuel, there is
but little data upon which to base stack sizes. The loss of draft
through the bed of fuel will vary over limits even wider than in the
case of coal, for in this class of fuel the moisture may run from
practically 0.0 per cent to over 60 per cent, and the methods of
handling and firing are radically different for the different classes of
wood (see chapter on Wood-burning Furnaces). As economy is ordinarily of
little importance, high stack temperatures may be expected, and often
unavoidably large quantities of excess air are supplied due to the
method of firing. In general, it may be stated that for this class of
fuel the diameter of stacks should be at least as great as for coal-fired
boilers, while the height may be slightly decreased. It is far the best
plan in designing a stack for boilers using wood fuel to consider each
individual set of conditions that exist, rather than try to follow any
general rule.

One factor not to be overlooked in stacks for wood burning is their
location. The fine particles of this fuel are often carried unconsumed
through the boiler, and where the stack is not on top of the boiler,
these particles may accumulate in the base of the stack below the point
at which the flue enters. Where there is any air leakage through the
base of such a stack, this fuel may become ignited and the stack burned.
Where there is a possibility of such action taking place, it is well to
line the stack with fire brick for a portion of its height.

Draft Gauges--The ordinary form of draft gauge, Fig. 35, which consists
of a U-tube, containing water, lacks sensitiveness in measuring such
slight pressure differences as usually exist, and for that reason gauges
which multiply the draft indications are more convenient and are much
used.