I would consider 400c a good starting point. Noel

Be an interesting thing to study. The tubes are cooled by water boiling and the engine consuming steam to do work. If the tubes were 100% efficient, the smoke-box gases would be at boiler water temperature, at 150psi about 185°C. Impossible to be 100% efficient, so the smoke-box will be hotter, depending on how much heat the whole engine loses by radiation and convection.

The tubes cool exhaust gases less efficiently when the blast is on because the increased vacuum causes combustion gases to flow faster, allowing heat less time to pass through the tube wall into the water. Therefore the smoke-box of a labouring engine with blast will be hotter than a cruising engine.

I've an account somewhere of the smoke-box of a full-size engine being seen to glow dim red, which must be at least 410°C. It was pulling a heavy load up an incline after dark.

As model sized locomotives can't be well insulated and have a large surface area compared with their volume, the rest of the engine will keep the smoke-box cool. And as models are rarely thrashed up long steep hills, their smoke-boxes are unlikely to get as hot as a full-size might.

Assuming coal rather than Oxy-Propane, 120°C to 400°C seems a likely range, but I guess very high temperatures would be unusual, perhaps only the result of protracted bad driving.

Dave

Edited By SillyOldDuffer on 12/08/2023 22:15:27

Not many models work at 150 psi. Jim Ewins actually measured temperature of gas inside a smoke box, I think the results are reported in one of Martin Evans's books. Otherwise Martin Johnson's articles in ME might give some guidance. If someone had one of those non contact pyrometer gadgets it would be easy to clamp it to a bracket and measure on a working model loco. Or clamp a thermocouple to the smoke box is probably easier

Edited By duncan webster on 12/08/2023 22:22:49

Dave wrote "I've an account somewhere of the smoke-box of a full-size engine being seen to glow dim red, which must be at least 410°C."

This will almost certainly be caused by char igniting and burning in the smokebox, caused by an ill fitting smokebox door permitting air entering and promoting combustion, rather than the effect of exhaust gases. I suspect that this would be pretty unlikely in a small scale model. The sheer volume of material needed over an extended period of time is never going to be achieved, operator fatigue will set in long before then.

Regards

Martin

RH&DR locos that have run over the Ravenglass and Eskdale tend to return to their flat homeland with charred smokebox doors.

This is caused by prolonged hard work over undulating terrain and an unavoidable build up of char / ash in the smoke box between cleaning sessions.

So the same will be true for any steam loco that works hard for long periods. The blast will pull hot gas and ash through the tubes and deposit it in the smokebox until the ash builds up and burns the paint on the door.

In the same way, any paint on the air cooled exhaust manifold of a hard worked internal combustion engine does not stay intact for long, once the engine is put to work.

Howard

Do not ( unless there is some cast iron guaratee ) use powder coat ! VHT paint is good to much higher temps than any body has mantioned. Noel.