The
firebox of a steam locomotive is designed to burn fuel efficiently and
produce adequate heat to boil water and so create steam. The firebox
must be of a size to burn enough fuel for the heaviest duty without
forcing unburnt fuel off the firebed.
Firebox widths vary
from those that overlap the engine frame and wheels to the type that is
waisted to fit between the frames. The top of the firebox may round and
therefore follow the circular profile of the boiler barrel, or roughly
flat known as the Belpaire type. This latter firebox is more costly to
produce but it gives more steam space at the top where it needed most.
The firebox consists
of an inner and outer shell; the outer firebox is made of steel whereas
the inner firebox can be made from steel or copper. The space between
the inner and outer fireboxes, usually 3 to 4 inches at the sides but
1½ to 2 feet at the top, is controlled by over 1000 metal stays that
located the inner firebox at steam pressures of around 250 pounds per
square inch. The level of water that surrounds the inner firebox is
controlled by the injectors that force water from the tender into the
boiler, however as a safety feature, if the water level should drop
below the inner firebox crown plate, fusible plugs made from a low
melting-point alloy, melt and extinguish the fire.
The grate at the base
of the inner firebox consists of cast-iron firebars with air spaces
between. The amount of air admitted through the grate to the underside
of the fire is adjusted by damper doors in the ashpan. Additional air
is admitted through the firehole and guided towards the fire by the
deflector plate. This ensures complete combustion of the gases within
the firebox and eliminating the periods when unburnt fuel is drawn off
the firebed and produce unwanted smoke and block the boiler tubes.
Cleaning the grate
entailed paddling or lifting the ash directly from the firehole with a
long shovel then removing several firebars with heavy tongs and pushing
the remaining clinker and ash into the ashpan. To assist in this
operation, some locomotives were fitted with a drop section grate, or
in later days with a rocking grate that would allow the ash to be
shaken from the fire while the locomotive was in use. |
