rwork will in a short time be
decreased, due to the silt which is present in practically all fuel oil.
The loss in draft through the boiler proper at a given rating will be
less than in the case of coal-fired boilers, this being due to a
decrease in the volume of the gases. Further, the action of the oil
burner itself is to a certain extent that of a forced draft. To offset
this decrease in draft requirement, the temperature of the gases
entering the stack will be somewhat lower where oil is used than where
coal is used, and the draft that a stack of a given height would give,
therefore, decreases. The factors as given above, affecting as they do
the intensity of the draft, affect directly the height of the stack to
be used.

As already stated, the volume of gases from oil-fired boilers being less
than in the case of coal, makes it evident that the area of stacks for
oil fuel will be less than for coal. It is assumed that these areas will
vary directly as the volume of the gases to be handled, and this volume
for oil may be taken as approximately 60 per cent of that for coal.

In designing stacks for oil fuel there are two features which must not
be overlooked. In coal-firing practice there is rarely danger of too
much draft. In the burning of oil, however, this may play an important
part in the reduction of plant economy, the influence of excessive draft
being more apparent where the load on the plant may be reduced at
intervals. The reason for this is that, aside from a slight decrease in
temperature at reduced loads, the tendency, due to careless firing, is
toward a constant gas flow through the boiler regardless of the rate of
operation, with the corresponding increase of excess air at light loads.
With excessive stack height, economical operation at varying loads is
almost impossible with hand control. With automatic control, however,
where stacks are necessarily high to take care of known peaks, under
lighter loads this economical operation becomes less difficult. For this
reason the question of designing a stack for a plant where the load is
known to be nearly a constant is easier than for a plant where the load
will vary over a wide range. While great care must be taken to avoid
excessive draft, still more care must be taken to assure a draft suction
within all parts of the setting under any and all conditions of
operation. It is very easily possible to more than offset the economy
gained through low draft, by