(_a_) Much less cooling of the furnace by frequent
additions of fuel.
(_b_) Higher temperatures, owing to the increased
calorific intensity of large quantities of fuel
burned at once.
(_c_) Less blanketing of the fire by fuel additions.
(_d_) Less chance of the whole of the grate area
being clinkered up at once, and in consequence,
less likelihood of interference with the rapid
combustion of the fuel.
The most rapid and economical smelting at the present day requires that
at least 0·7 lb. of coal be burned per minute per square foot of hearth
area.
A. ii.—_Draft._—The charge in a reverberatory furnace hearth is melted
chiefly by the heat from the hot gases passing over it, and in giving
up their heat to the charge, the gases become cooled down. The
heating of the charge is made continuous by the continual addition
of fresh fuel in the fire-box, and by the drawing of the flames over the
hearth by means of flues situated at the other end of the furnace and
leading to the stack. The flues and stack must be large enough to
cause sufficient draft through the furnace for the heated gases to be
drawn over the charge with sufficient rapidity, and much unsuccessful
work has been due to the fact that these requirements have not been
fulfilled. There should be a suction equivalent to at least 1 inch to
1·5 inches water pressure up the stack, this being readily measured by
water-manometers—a feature of modern working.
Reverberatories may be worked either by forced or natural draft, the
latter being usually preferred, though it necessitates a large stack
and spacious flues.
Forced draft by fan or blower under the fire-grate has been in use at
several smelters, the ashpit then being closed. It was at one time
adopted at Anaconda, but was given up later. The use of forced draft
has the advantage that leakages of cold air into the furnace are to a
large extent prevented, hot gases tending to be forced out rather than
cold air drawn in, but the objections to its use include the facts that—
(_a_) Special power and machinery are required.
(_b_) The intense action near the fire-bars produces,
from the ash of the coal, a massive clinker in a
semi-fused condition, difficult to deal with.
(_c_) It is stated by smelters to have a cooling action
near the fire-bridge.
A. iii.—_Firing and Grating._—This question is closely connected
with the dimensions of the grate, since the use of a small fire-box
necessitates methods of firing and grating which are not conducive to
the most rapid and efficient combustion of the fuel. In addition to the
cooling action of frequent fresh fuel charges in the small fireplace,
attendant disadvantages include the closing up of the spaces in the
grate by which air enters for burning the fuel, and the consequent
necessity for frequent grating with small beds of fuel, which entails
numerous objections.
The addition of fresh coal to the fire causes the production of large
quantities of volatile hydrocarbons which require an increased air
supply for proper combustion, and this air admission is just prevented
by the blanketing action of the fresh fuel added. This is indicated
by the red smoky flame, and means waste and cooling. The difficulty
is overcome by the arranging of a series of air-holes at the fire-box
end of the furnace, near the fire-bridge, and by the opening of these
directly after firing, the volatiles are immediately burnt up. This is
an important feature in successful working, and with a large fire-grate
and this air-admission, the effect of adding even 1½ tons of fuel on to
the fire at once causes little difference in the furnace temperature.
The flame is observed through a window let into the off-take flue,
which allows of the changes in appearance being noted by the fireman on
the fire-box platform.
The fire is kept moderately shallow, to allow of rapid burning of the
fuel, though deep enough to keep up the enormous body of heat necessary
in the furnace.