The chief points to be considered in the design of furnace doors are
to prevent the radiation of heat through them, and to provide for
the admission of air above the burning fuel in order to aid in the
consumption of smoke and unburnt gases.
In all cases where the doors are exposed to very rough usage—such,
for instance, as in locomotive and marine boilers—the means for
admitting air must be of the simplest, and consist generally of small
perforations as shown in Fig. 76 which represents a front view, and
section of the furnace door of a locomotive boiler. The heat from the
burning fuel is prevented from radiating through the perforation in
the outer door, by attaching to it a second or baffle plate, _a_, at a
distance of about 1-1/2 inches, the holes in which do not coincide in
direction with the door proper. By the constant entry of cold air from
the outside the greater part of any heat which may be communicated to
the door by radiation or conduction is returned to the furnace.
[Illustration: Fig. 76.]
Doors similar to the above provide for the constant addition of limited
quantities of fresh air above the fuel, but in actual practice,
however, air is only needed above the fire for a few minutes after
fresh fuel has been thrown on the grates and then is required in
considerable quantities. In the case of land boilers, the furnace
doors of which undergo comparatively mild treatment, it is possible to
introduce the necessary complications to effect this object.
[Illustration: Fig. 77.]
Fig. 77 shows an arrangement largely in use in New England, in which,
by means of a diaphragm, the air is passed back and forth across the
heated inner or baffle plate with the very best results.
The air is first drawn by the natural draught into the hollow space
between the iron door and its lining, through a row of holes _A_, in
the lower part of the door, controlled however, by a slide not shown
in the cut, then caused to flow back and forth across the width of
the door by simply arranged diaphragms, and finally injected into the
furnace through a series of minute apertures drilled in the upper part
of the door liner, as indicated in cut at _B_.
It will be seen that while the air may enter the door at a low
temperature, it constantly becomes heated during its circulation until
the instant it enters the furnace, it is ready to flash into flame with
intense heat upon its incorporation with the expanding gases of the
furnace.
An arrangement in common use in Cornish and Lancashire boilers consists
of a number of radial slits in the outer door which can be closed or
opened at will in the same manner as an ordinary window ventilator.
Other and more complicated arrangements have been frequently devised,
which work admirably so long as they remain in order, but the frequent
banging to which furnace doors are subjected, even in factory boilers,
soon deranges delicate mechanism.
Furnace doors should be made as small as possible considering the
proper distribution of fuel over the grate area, as otherwise the great
rush of cold air, when the door is opened rapidly, cools down the flues
and does considerable injury to tube plates, etc.; for this reason it
is desirable, when grates are over forty inches in width to have two
doors to each furnace, which can be fired alternately.
The great loss arising from a rush of cold air on opening the
furnace doors for replenishing the fires with fuel has led to costly
experiments to produce “a mechanical stoker,” or self boiler feeding
arrangement for supplying the coal as needed.