BOILER ACCESSORIES 135
way. It will be seen that where the outside atmosphere is subject
to changes of temperature of any magnitude, as all atmospheres are,
even in temperate climates, and still more so on large continental
areas, such as North America, etc., it is necessary to provide a
chimney of such dimensions, that the motive column is always
present, even in the very hottest weather, and this means that the
cost of the chimney is very much greater in consequence, and that
there is a very much larger waste of energy in the hot gases passing
up the chimney than would otherwise rule.
The height of the chimney is also ruled indirectly by the necessity
of providing for a certain definite velocity in the gases. As explained
above, a certain velocity in the hot gases is necessary in order that
the air from which they are formed may enter the furnace in proper
proportion, and this velocity is only obtained by a certain definite
motive column or height of chimney. Hence the chimney is obliged
to be higher in some cases than would be otherwise necessary, and
than would apparently be necessary from an examination of the
height and area of the chimney, in order to provide for this velocity.
The final solution of the problem is somewhat troublesome, and as
usual it has been left to the practical engineer to work out. Mr.
William Kent, the author of the "Standard Mechanical Engineers'
Pocket Book" in America, has produced the table on p. 136.
The table is based upon a consumption of 5 lbs. of fuel per H.P.
per hour. Modern engines work with considerably less than that, as
explained above, but the differences in the temperature of the outside.
atmosphere, and also changes that may take place in the resistance
offered to the draught, both within the boiler and in the chimney,
have led Mr. Kent to provide very liberally in calculating for the
horse-power.
It will be noted that, in the table, the diameters of different
chimneys are given, from 18 inches up to 12 feet, these being for
circular chimneys, and the equivalent sizes for square chimneys, and,
in addition, what is termed the effective area in square feet.
By effective area is meant, the actual space within the chimney,
operating to produce the required velocity of the gases. As already
explained, the gases, in passing through the chimney, create friction
upon the sides of the chimney, and, to meet this friction, the actual
area through which the gases pass is taken as so much less than the
total area of the chimney itself.
The formula upon which this is calculated is as follows:-
E=A06 × √√Ā
The height of chimneys are given in feet, from 50 feet up to 300 feet, and it will be noted that the chimneys of smaller diameter are given smaller heights, the heights increasing with the sectional areas.
