An arch is placed in the front third to half of the firebox. Prototype locomotives typically use brick for the arch, but live steam models most often use stainless steel.
The 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, and also more soot to accumulate in the firetubes. The arch and its supports require periodic replacement due to the extreme heat they endure.
- I used to run a LE 4-4-0 with an oil atomizer burner without an arch. We went to a pot burner, and ended up adding an arch that went about 3/4 of the way back to the back of the firebox. The arch, by lengthening the gas path, helped to insure the fuel finished burning before entering the flues. The flues would soot up much quicker without it.
- The arch was 1/4" mild steel, and lasted a number of years, even though rather warped. We had the arch initially too close to the back of the firebox, and the water glass would bubble like one of those old fashioned Christmas tree lights.
- I have built 3 oil fired engines and all had arches. The first two had cast refractory arches. My current engine ( a Shay ) has a SS plate 1/8" thick that works more like a deflector than an arch. It does the main purpose and that is to prevent the hot gases from short circuiting to the lower tubes.
- I have run my Allen American with and without an arch, here is my basic findings. I will add I run my engine hard and just about always on the edge of its abilities
- Firing on oil, Pre install, heavy soot with firing the boiler hard while pushing its limits. Flue cleaning needed every 4 to 6 hours under fire. Fuel usage was around one gallon an hour.
- Post install, much lighter sooting problem and can get as much as 24 hours between flue cleaning. Fuel usage was down to one gallon any two hours.
- I run a 2.5 inch scale K36 with an arch. I started off with 1/4 inch SS but found that it only lasted 2 years. I now get 18 months or more out of mild steel if I weld 2 plates together. I have never run without one but I suspect the fire would clog and burn out my bottom tubes - 9 years on the ends of the tubes do not have any burning compared that what I have seen on loco's without an arch. I burn at most 50 kg of coal over a 6 hour period hauling a train of 50 - 70 passengers most of the time.
- I ran with arches until the last run of last year. It was easier to fire without them. I was into the safeties almost all day, and using a blacksmith coal that was not good coal.
- I've had a piece of stainless arch in my 10 wheeler for 13 years. It may be discolored, but it's still doing the job. Last time I had the boiler off (about 6 years ago)...there was little deformation. Make the arch sheet as wide as you can fit in the firebox and still clear any weld "buds" from the stay bolts. For mounting...my arch is attached to the frame. Dimensions...my arch is about 1/2 to 2/3's as long as the firebox. I left an opening above the arch to the crown sheet that was approx. 10% of the grate area.
- My propane-burning 1/8th scale steamer has an angled stainless steel arch in the firebox plate causing a lengthening of the combustion path. The gas/flame path moves aft and upwards around the rear of the arch plate and then forward under the crown sheet towards the tube sheet. A 3/8" gap on both sides allows some combustion flow directly on the side sheets to maximize radiant heating. If the arch plate heats enough to glow orange, it provides addition radiant heat transfer to the firebox, where 60% - 75% of heat transfer occurs in full-sized locomotive boilers.