By Doug Bach
I sing the praises of the arch! No, I’m not talking about the arch in architecture or bridge building, although they’re very nice too. Instead, I’m talking about the arch built into the firebox of most steam locomotives. The arch was an elegant solution to an early problem, but it has an application to model engineering as well.
The early locomotives burned either coke or anthracite coal.
Bituminous coal is much more common and more easily mined, but
early attempts to use it in locomotives ended in dark failure.
I say “dark” because when these locomotives tried bituminous
the cloud of smoke they put out went from “highly impressive” to
“spectacularly colossal”. It
seems the ability to announce the arrival of the train by gassing the
whole town is an exception to the rule “there is no such thing as bad
locomotive trials such as the Rainhill trials contained a requirement
that locomotives must “consume their own smoke”.
This phrase sounds odd to modern ears, but made perfect sense to
the 19th century citizens who were tired of consuming the
Nineteenth century science soon found the source of the problem.
All coal has “volatile components” which are driven out of
coal (when heated) as a gas. These
volatile components will burn, and can be up to 60% of the heat value of
the coal. The flame seen
above a coal fire is these volatile components burning as they are
carried away from the firebed. Anthracite
coal has a very low level of volatile components (hence, a short flame).
Coke has been artificially baked in the absence of air to drive
off the volatiles, which means it burns hot and with little apparent
flame, but the processing is expensive and throws away a significant
portion of the energy available. Volatiles
were not burning in the early locomotives, and a little investigation
In locomotive boilers the hot gases from the fire are directed
into a series of narrow firetubes running through the barrel of the
boiler. These tubes rapidly
absorb heat out of the gases, which is exactly what is wanted.
However, if the volatiles have not finished burning before the
gases enter the tube, the temperature rapidly drops below ignition
point, the flame goes out, and the partially burnt volatiles go on to
soot up the tubes and cloud the lungs of the spectators.
If the gases can rise from the grate and dive straight into the
tubes smoke is inevitable unless the short-flamed anthracite or coke is
used. To burn bituminous, a
longer path is needed before the flame enters the tubes.
A firebox can only be made so deep before overall height becomes
a problem. So the next try
was to put in a barrier running from just underneath the tubes back and
up about 2/3 of the way to the top rear corner.
The flame was forced to pass back to the rear of the firebox, up
over the rear lip of the barrier, and then forward and down to the
tubes, a much longer flame path. When
the first try showed molten metal has a strange reluctance to stay in
place, a second try was made with the barrier built of two stayed plates
with boiler water in-between for cooling.
This did half the trick, but the water-cooled barrier itself
cooled the gases passing near it below ignition point.
Then someone had the bright idea; build the barrier out of
shallow arches of firebrick supported on the firebox sides. The white-hot firebrick didn’t cool the gases.
Secondary air admitted through the firedoor burned the volatiles
without having to pull all the oxygen through the firebed.
This did the trick; locomotives could now burn bituminous with
only mild curses from women hanging out washing.
When oil burning came in, it was found the fire could be treated
just like the volatiles in coal. However,
no matter how well designed the system, for both coal and oil there was
one limit. When the combination of grate area times flame length
exceeded the volume of the firebox, you got incomplete combustion.
This is why later locomotive designers placed so much importance
on firebox volume, even to the extent of extending the front of the
firebox into the barrel of boiler to form a “combustion chamber”.
I’ll deal with one puzzle here.
Yes, the sidewalls of the firebox cool and hence extinguish the
flames passing near them. This
applies to both standard locomotive fireboxes and watertube fireboxes
(of which Briggs boilers are one example). As long as the cross section of the flame is “thick”
enough that most of the flame doesn’t get close to the walls, this
isn’t a serious problem. Thermic
siphons in the firebox could cause problems if there are too many of
them, but if the gaps between them are big enough there will be enough
unextinguished gas to reignite the extinguished gas “downstream” of
the siphons, and the improved circulation gives a net improvement in
watertube boilers (e.g. marine boilers) have the flame pass along the
tube nests rather than through them, at least until there is enough path
length for complete combustion.
Doug Bach is a member of the British Colombia Society of Model Engineers in Burnaby (Vancouver) Canada. This article first appeared in "The Whistle" their monthly newsletter and is re-printed here with permission.