In a steam engine, the firebox is the area
where the fuel is burned, producing heat to
boil the water in the boiler.
Most are somewhat box-shaped, hence the name.
The hot gases generated in the firebox are
pulled through a rack of tubes running through
the boiler.
== Steam locomotive fire tube firebox ==
In the standard steam locomotive firetube
type boiler, the firebox is surrounded by
water space on five sides.
The bottom of the firebox is open to atmospheric
pressure, but covered by fire grates (solid
fuel) or a firing pan (liquid fuel).
If the engine burns solid fuel, like wood
or coal, there is a grate covering most of
the bottom of the firebox to hold the fire.
An ashpan, mounted underneath the firebox
and below the grates, catches and collects
hot embers, ashes, and other solid combustion
waste as it falls through the grates.
In a coal-burning locomotive, the grates may
be shaken to clean dead ash from the bottom
of the fire.
They are shaken either manually or (in larger
locomotives) by a powered grate shaker.
Wood-burning locomotives have fixed grates
that can't be shaken.
Wood ash is generally powder which will fall
through the grates with no more agitation
required than the vibrations of the locomotive
rolling down the track.
The fire grates must be replaced periodically
due to the extreme heat they must endure.
Combustion air enters through the bottom of
the firebox and airflow is usually controlled
by damper doors above the ash collection pocket
of the ash pan.
A locomotive that burns liquid fuel - usually
"Bunker C" fuel oil or similar heavy oil - does
not have grates.
Instead, they have a heavy metal gauge firing
pan bolted tight against the bottom of the
firebox.
The firing pan is covered with firebrick and
the firebox has a firebrick lining, usually
up to the level of the firebox door, all the
way around the firebox.
The oil burner is a nozzle containing a slot
for the oil to flow out onto a steam jet which
atomizes the oil into a fine mist which ignites
in the firebox.
The oil burner nozzle is usually mounted in
the front of the firebox, protected by a hood
of firebrick, and aimed at the firebrick wall
below the firebox door.
Dampers control air flow to the oil fire.
=== Brick arch ===
There is a large brick arch (made from fire
brick) attached to the front wall of the firebox
immediately beneath the firetubes.
This extends backwards over the front third
to half of the firebed.
It is supported on arch tubes, thermic syphons,
or circulators.
The brick arch directs heat, flames, and smoke
back over the fire towards the rear of the
firebox.
Visible smoke contains unburned combustible
carbon particles and combustible gasses.
The purpose of this redirection is to cause
more complete combustion of these particles
and gasses which make the locomotive more
efficient and causes less visible smoke to
be emitted from the stack.
Without the arch, flames and visible smoke
would be sucked straight into the firetubes
without having been fully burned, causing
visible smoke to be emitted at the stack.
The brick arch and its supports (arch tubes,
thermic syphons, and circulators) require
periodic replacement due to the extreme heat
they endure.
=== Firetubes ===
Firetubes are attached to one wall of the
firebox (the front wall for a longitudinal
boiler, the top for a vertical boiler) and
carry the hot gaseous products of combustion
through the boiler water, heating it, before
they escape to the atmosphere.
Firetubes serve the additional purpose of
staying the flat tube (flue) sheets (front
and rear) so that only the top of the front
flue sheet and the bottom of the rear flue
sheet must be separately braced.
=== Sheets and stays ===
The metal walls of the firebox are normally
called sheets, which are separated and supported
by stays.
The stays support and brace the "sheets" (plates)
against pressure.
Ideally, they should be located at right angles
to the sheets, but since the outer sheet (wrapper
sheet) is often rounded and the top of the
firebox (crown sheet) is relatively flat by
comparison, such a relationship to both sheets
is impossible.
The actual location of the stays is a compromise.
Since stay breakage is hidden, the stays have
longitudinal holes, called tell-tales, drilled
in them which will blow water and steam, revealing
if they are broken.
A boiler with more than 5 broken stays, or
two next to each other, must be taken out
of service and the stays replaced.
The fusible plugs, usually located in the
highest part of the crown sheet, have a soft
metal alloy core which melts out if the water
level in the boiler gets too low.
Steam and water blowing into the firebox both
alerts the locomotive crew to the low water
condition and helps put out the fire.
Not all locomotives are equipped with fusible
plugs.
Also, fusible plugs should be replaced at
regular intervals, about every three months
for a locomotive in regular service, because
the soft metal alloy core will slowly melt
out over time even if the boiler water is
carried at proper levels.
The "mudholes," or washout plugs, allow access
to the interior of the boiler for washing
and scraping away boiler mud and scale.
The sheets on the left and right are called
"side sheets" while the sheet in the front
of the firebox is the flue sheet.
The "front flue sheet" is in the front of
the boiler and at the rear of the smokebox.
The "rear sheet" is at the back of the firebox
and has the door opening in it.
The crown sheet is the top of the firebox.
The crown sheet must be covered by water at
all times.
If the water level drops below the crown sheet,
it will become overheated and start to melt
and deform, usually sagging between the crown
stays.
If the condition continues, the crown sheet
will eventually be forced off the crown stays
by the pressure in the boiler, resulting in
a boiler explosion.
This condition, usually caused by human error
or inattention, is the single greatest cause
of a locomotive boiler explosion.
=== Belpaire firebox ===
Normally the top of the boiler (wrapper sheet)
over the firebox is radial to match the contour
of the boiler; however, due to the problem
of placing stays at right angles to both the
wrapper sheet and the crown sheet (see above)
the Belpaire firebox was developed.
In the Belpaire design, the wrapper sheet
is roughly parallel with the firebox sheets
to allow better placement of the stays.
This arrangement gives the firebox end of
the boiler a more square shape and is usually
made as large as possible within the loading
gauge, to offer the greatest heating surface
where the fire is hottest.
The most notable user of the Belpaire firebox
in the United States was the Pennsylvania
Railroad.
Other railroads, such as the Great Northern
and Illinois Central, had locomotives with
Belpaire fireboxes.
Illinois Central 4-6-0 #382, Casey Jones'
engine, had a Belpaire firebox.
=== Wootten firebox ===
The Wootten firebox was very tall and wide
to allow combustion of anthracite coal waste.
Its size necessitated unusual placement of
the crew, examples being camelback locomotives.
=== Combustion chamber ===
Some fireboxes were equipped with a so-called
combustion chamber which placed additional
space between the fire and the rear flue sheet.
This allowed more complete combustion and
increased firebox surface area for greater
heat transfer.
=== Fireman's duties ===
The fireman's role on a steam locomotive is
to ensure the driver has an adequate supply
of steam at his disposal at all times.
This is achieved by maintaining a supply of
fuel to the fire, and by maintaining the boiler
water level so that it covers the firebox
crown sheet at all times – otherwise, the
latter will overheat and weaken, and a boiler
explosion may result.
At the terminal after the day's work is done,
the fireman will fill the boiler with water
and either bank or dump the fire (i.e. extinguish
the fire), according to company policy, and
chock the driver wheels to prevent the locomotive
from moving while it is unattended.
=== Gallery ===
== Road locomotive firebox ==
Road locomotives, such as traction engines,
usually had fireboxes similar to those on
railway locomotives but there were exceptions,
e.g. the Sentinel steam waggon which had a
vertical water tube boiler.
== Stationary boiler firebox ==
There were, and are, many different designs
of firebox for stationary boilers.
In flue-type boilers (e.g. the Lancashire
boiler) the flues themselves form the firebox.
In water-tube boilers, the firebox is usually
a firebrick-lined compartment below the water
tubes.
== Marine boiler firebox ==
In marine boilers there are also various types
of firebox.
The main distinction is, again, between fire-tube
types (e.g. the Scotch boiler, with internal
firebox) and water-tube types (e.g. the Yarrow
boiler, with external firebox).
== See also ==
Firetube boiler
Flue
Fusible plug
