128 STEAM BOILERS, ENGINES, AND TURBINES
return air from the mine, and to provide a column of hot air in the
up-cast shaft, the difference between the weight of this column of air
and that of the column of air in the down-cast shaft, which was at
the temperature of the outside atmosphere, providing what was called
the motive column.
Air, it will be remembered, has weight, and when it passes over
the surface of the flues, through fire bars, the interstices of coal, etc.,
it creates friction, and this necessitates the expenditure of a certain
force to move it. Further, the weight of a given volume of air varies
directly with its absolute temperature. At 32° F., 1 lb. of air
occupies approximately 12 cubic feet; at double the absolute tem-
perature, or about 525° F., the same weight of air would occupy 25
cubic feet. The absolute temperature corresponding to 32° F., it will
be remembered, is 493° F., absolute zero being 461° F. below the zero
of Fahrenheit's scale; consequently double the absolute temperature
at 32° F. is 986° absolute Fahrenheit, corresponding to 525° gauge
temperature Fahrenheit.
With the arrangements of a boiler furnace, flues, chimney, etc.,
the property of fluid pressure that has been referred to in a previous
portion of this chapter comes into play. It will be remembered that
in a fluid any pressure communicated to any part of the fluid is trans-
mitted through the fluid in all directions. In the case of the boiler
furnace, the air passes through the ashpit and the fire bars into the
coal, and it is the weight of the column of air above the entrance to
the ashpit which causes the air to move into and through the furnace.
If the hot gases met a similar column of air on the other side of the
boiler, the pressure on the two sides would be equal, and there would
be no force tending to move the air and the gases through the furnace.
But by the provision of a column of hot gases, such as exists in a
boiler chimney, and whose weight is less than that of the column of
air pressing against the ashpit entrance, there is a force tending to
cause the air to pass into the ashpit, and thence through the fire and
flues, etc., this force being measured by the difference between the
weight of the column of hot gases in the chimney, and that of the
equivalent column of air outside the ashpit.
It will be evident from the above, that the larger the area of the
chimney, and the greater the height of the chimney, the greater is
the force tending to move the air into the furnace, and to move the
hot gases through the furnace flues. Further, it is evident that the
higher the temperature of the hot gases in the chimney, the greater is
the difference between their weight and that of the column of air
outside, and therefore the greater is the force moving the air and hot
gases or, as it is expressed, creating a draught. On the other hand,
it will easily be understood that whatever quantity of heat is left in
the hot gases, when they enter the chimney, is wasted, so far as
