"The Arch"

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 publicity”.  Early 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 smoke themselves.

            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 showed why.

            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 efficiency.  Straight 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.

            Now what does all of this mean for model engineering?  First, it becomes obvious why so many coal burning miniature locomotives seem to thrive on anthracite and coke.  The flame length of these coals is scale compared to the full size locos burning bituminous!  (Now if we could just get scale oil, water and fingers...)  Arches in miniature have not been much tried.  Is it coincidence that coal and oil burning miniature locos need daily brushing out of their tubes?  I sing the praises of the arch!  If an arch prevents even some of the flame from diving straight into the firetubes and extinguishing itself, there has been a saving in both fuel required and brushout effort.  Modern stainless steel can stand up to the temperatures in our miniature fireboxes, as can Kaowool board, so we don’t have to build an actual arch.  Be careful in design, though!  The cross section of the flame must not be restricted at any point.  I suggest an “arch” going only halfway from front bottom to rear top corner to prevent too much of the flame from getting near the cool side and crown sheets.  Maybe if some of you coal and oil burners out there try it, you may also be singing the praises of the arch!


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.