A. i.—_Enlarged Grate Area._—In the older methods of working, there
was a general tendency to employ a furnace of standard size, and
improvements in the economy of the process were in the direction of
reducing the fuel bill as much as possible for the given size of
furnace. This was effected by keeping the grate area fairly small.
In modern practice, economical working still involves having the ratio
of size of hearth to size of fire-box as large as possible, but instead
of reducing the dimensions of the fire-grate to suit the hearth, a
large grate is built to commence with, and the hearth is constructed of
such a size as will utilise all the heat available. From this principle
of burning a large quantity of fuel and melting with it as much charge
as possible, the efficient and economical working of large furnaces has
been developed.
A grate area of about 28 square feet is now regarded as the minimum for
economical work at modern smelters, and fire-boxes up to 128 square feet
in area are usual in practice.
In small fire-boxes, only small quantities of fuel can be burned at
once, and in consequence, fresh firing is continually required, which
interferes greatly with the work of the furnace and decreases the
rapidity of heating. Each addition of cold fuel has a cooling effect on
the fire and furnace gases, the temperature in the hearth being found
to drop for a period of five or ten minutes by as much as 100° C., the
flame becoming smoky, red, and cold. A similar time is required for
the original temperature to be attained once more. Cold air is also
admitted every time the fire-box doors are opened for charging.