132 STEAM BOILERS, ENGINES, AND TURBINES
important, Its sectional area must be sufficient to allow the
whole of the gases delivered by the boiler, or battery of boilers, to
escape freely through the chimney, without throttling. In addition,
if the chimney is also to furnish the draught necessary for the boilers,
it must be of a sufficient height to furnish the required motive
column described on p. 128. The total energy present in the gases
passing through the chimney will depend on both of these factors.
The larger the area of the chimney, and therefore the larger the
volume of the gases passing through it, the greater the energy
present in a column of a given height; and also the greater the
height of the chimney, the greater the energy present for a given
area. The H.P. of the boiler, as it is sometimes expressed, or the
battery of boilers, determines the sectional area of the chimney, and
this is quite independent of the height of the chimney. The modern
method of describing the capacity of the boiler, as able to evaporate
a certain quantity of water per hour to steam at a certain pressure,
is very much more accurate, and more scientific. For it will be
evident that the H.P. furnished by the steam generated in any
boiler will vary with the engine in which the steam is used. Thus,
taking the consumption of non-condensing engines at from 30 to
40 lbs. of steam per H.P. per hour, simple engines condensing at
from 24 to 30 lbs., compound engines at from 18 to 25, and triple
expansion engines at from 15 to 20, the H.P. of a boiler, or battery
of boilers, could be stated in very different figures, and would be
furnishing a very different number of H.P., according to the type of
engine to which it was furnishing steam.
From what has been stated on previous pages, it will be under-
stood that, in order that a certain quantity of water shall be raised to
a certain temperature, and converted into steam, a certain number of
heat units must be delivered to it, and this requires, in each kind of
boiler, the consumption of a certain quantity of fuel, again varying
with the composition of the fuel, this again requiring a certain
quantity of air, and furnishing a certain volume of heated gases.
This again implies the presence of a certain grate area in the furnace,
or furnaces, in which the fuel is consumed. Hence it will be evident
that the area of the chimney will depend upon the area of the grate
or grates on which the fuel is burned, to furnish the hot gases that
are to pass through the chimney. Professor Thurston gives as a
standard for chimneys of 200 feet high and upwards, a sectional area.
of 2 square inches for each pound of fuel consumed on the grates
supplying the chimney; and he gives as a further standard, the
sectional area of the chimney as from to of the grate area.
It should be mentioned en passant, that this last standard will be
subject to modification where the chimney is only employed for
carrying the gases off harmlessly. As will be explained, with all the
