The length of any smokebox depends mainly on how much room is needed for the “front end appliances” inside. These consist of the exhaust nozzle from the cylinders, the blower nozzle, and petticoat pipe which almost all engines have, to the various baffles and screens needed to control sparks and cinders.
The smokeboxes on engines with diamond stacks were short because there was nothing in them but the nozzle, and the petticoat, all of the spark arresting taking place in the stack. The disadvantage of the diamond stack is that the netting has to handle both the volume of the smoke, as well as volume of the steam. This creates a bottleneck that reduces draft on the fire. It is also very labor intensive in that the netting is always covered in corrosive wet soot, and had to be replaced every few months.
To get away from this, there was the idea of putting the screen in the smokebox so the smoke would pass through it before it mixed with the exhaust steam. The first generation of engines to do this had the extended smokeboxes. The extension was necessary to house the baffles and netting in the way they were laid out at that time. There was a vertical baffle called a diaphragm, which ran from the top of the smokebox, ½ way to the bottom, just behind the stack. Next was the table plate, which ran horizontally from the bottom of the diaphragm towards the front of the engine about a foot or 18”. The table plate had the nozzle coming up through it. The screen was attached to the front end of the table plate and ran horizontally, or at a slight up angle 2’ or 3’ to the front of the smokebox. The smokeboxes were extended to make room for that almost level screen.
The smoke would come out of the boiler flues, go down behind the diaphragm, under the table plate, up through the screen, back to the petticoat pipe, and up the stack. The cinders would bounce off the screen and pile up in the smokebox until cleaned out. Most of these old engines had a “cinder hopper” under the smokebox extension for cleaning. This is a big cast iron elbow with a steam ejector attached. When the front end was ready to be cleaned, the hostler would take the cover off the cinder hopper, attach a stove pipe to the outlet to get the cinders clear of the side of the engine, and turn on the steam, which would blow all the junk out of the smokebox. These cinder hoppers can still be seen on the East Broad Top’s engines.
As front ends were further perfected, the “master mechanics” or “self cleaning” front end evolved. The diaphragm and table plate were lowered to 2/3 or ¾ of the way to the bottom of the smokebox. With the table plate lowered, the screen could be tilted more towards vertical in front of the stack, and still have enough open area for the smoke to go through. With the screen in its new position, the smokebox could be shortened again, though not as short as with a diamond stack. A beneficial side effect of this arrangement was that with the shorter space between the front of the table plate and the smokebox front, the smoke traveled through this area at a high rate of speed. This high-speed smoke would pick up any cinders lying in the bottom of the smokebox and continually fling them against the screen until they were broken into small enough pieces to go through. This did away with the need to manually blow out the cinders at the ashpit, and made the smokebox “self cleaning”.
I understand that the K’s were equipped with master mechanics front ends when built, but were switched to “Anderson” front ends (a fine name I might add) by the Grande. An Anderson front end has no screens at all, but a series of sheet steel baffles surrounding the base of the petticoat pipe. These baffles induce an intense swirling action to the smoke, causing the cinders to be flung against the baffles to be broken up and extinguished. I don’t know how self-cleaning Anderson front ends are, but they don’t take up much space, and will fit in a master mechanics sized smokebox.
Another variation is the “cyclone” front end. It works on the same principle as the Anderson front end, however is in a very bulky housing, which looks like a gargantuan dust buster, requiring an extended smokebox. The Great Western applied cyclone front ends to all their engines during WWII with dubious cosmetic results.
