BOILER ACCESSORIES 133
methods known generically as mechanical draught, very much higher
rates of consumption of fuel are obtained with a given grate area
than are usual with chimney draught, and as this produces a larger
quantity of hot gases, if they are not to be throttled by the chimney
the sectional area of the latter must be increased in proportion. On
the other hand, where the chimney is only employed to get rid of the
hot gases, these will be at a much lower temperature than is usual
with chimney draught, and therefore their volume will be less from that
cause than it would have been where the hot gases create the draught.
The Factors ruling the Height of a Chimney
It has been explained that the chimney is required to carry a
column of hot gases of sufficient size to furnish the necessary motive
column. The weight of the column of gases in the chimney, however,
will depend inversely upon their absolute temperature. Further, the
velocity of the gases have an important bearing upon the matter, as
will be seen, The velocity at which the gases pass through the
chimney rules the velocity with which they pass through the boiler
tubes, or the space around the tubes, flues, etc., and this again rules
the rate at which air is admitted to the furnace, and at which com-
bustion takes place. Hence the velocity of the gases in the chimney
rules the rate of combustion, this again being ruled by the difference
in the pressure between the ashpit and the furnace. Again, the
velocity of the gases is ruled by the difference of pressure between
the column of the atmosphere, and the column of hot gases in the
chimney. The pressure exerted by the motive column is known as
the head, and is usually denoted by the letter H, and the head of
any motive column is equivalent to the height through which the
gases composing that column would have fallen in acquiring the
velocity at which the gases are moving. The velocity at which
the gases are moving is found from the formula v = √2gh, where v
is the velocity of the gases, h is the height of the motive column,
under which they are moving, and g is the accelerating force of
gravity, taken usually as 32-2 feet per second. The formula will be.
recognized as that which is applied to all falling bodies. It will be
seen from it, however, that the velocity of the gases, the accelerating
force of gravity being constant, varies as the square root of the head
of the motive column. In mining work this is expressed by saying
that the velocity of the air varies inversely as the square root of the
water-gauge, and this may be applied equally to boiler-work. The
height of any column of any fluid required to furnish a given pressure
depends directly upon the pressure, and inversely upon the density
of the fluid.
