Arch

A stainless steel arch in an NKP Berkshire firebox (coal burner).

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.

Experiences

Doug Edwards posted on Chaski.org

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.

John posted on Chaski.org:

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.

Brew posted on Chaski.org:

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.