256. *Q.*—What is the proper proportion
of fire grate per horse power?

*A.*—Boulton and Watt allow 0.64 of
a square foot area of grate bars per nominal horse
power in their marine boilers, and a good effect arises
from this proportion; but sometimes so large an area
of fire grate cannot be conveniently got, and the
proportion of half a square foot per horse power,
which is the proportion adopted in the original boiler
of the Great Western, seems to answer very well in
engines working with a moderate pressure, and with
some expansion; and this proportion is now very widely
adopted. With this allowance, there will be 22
to 24 square feet of heating surface per square foot
of fire grate; and if the consumption of fuel be taken
at 6 lbs. per nominal horse power per hour, there will
be about 12 lbs. of coal consumed per hour on each
square foot of grate. The furnaces should not
be more than 6 ft. long, as, if much longer than this,
it will be impossible to work them properly for any
considerable length of time, as they will become choked
with clinker at the back ends.

257. *Q.*—What quantity of fuel is
usually consumed per hour on each square foot of fire
grate?

*A.*—The quantity of fuel burned on
each square foot of fire grate per hour, varies very
much in different boilers; in wagon boilers it is from
10 to 13 lbs.; in Cornish boilers from 3-1/2 to 4
lbs.; and in locomotive boilers from 80 to 150 lbs.;
but about 1 cwt. per hour is a good proportion in
locomotives, as has been already explained.

258. *Q.*—In what manner are the proper
sectional area and the proper capacity of the flue
of a boiler determined?

*A.*—The proper collective area for
the escape of the smoke and flame over the furnace
bridges in marine boilers is 19 square inches per nominal
horse power, according to Boulton and Watt’s
practice, and for the sectional area of the flue they
allow 18 square inches per horse power. The sectional
area of the flue in square inches is what is termed
the *calorimeter* of the boiler, and the calorimeter
divided by the length of the flue in feet is what
is termed the *vent*. In marine flue boilers
of good construction the vent varies between the limits
of 20 and 25, according to the size of the boiler
and other circumstances—the largest boilers
having generally the largest vents; and the calorimeter
divided by the vent will give the length of the flue
in feet. The flues of all flue boilers diminish
in their calorimeter as they approach the chimney,
as the smoke contracts in its volume in proportion
as it parts with its heat.

259. *Q.*—Is the method of determining
the dimensions of a boiler flue, by a reference to
its vent and calorimeter, the method generally pursued?