What is the purpose of a Brick Arch in steam loco fireboxes?

[PaxmanValenta]

Many steam locomotives have a brick arch inside the fire box, what is the purpose of this arch? Do all steam locos have an arch?

 

[Adlington]

https://en.wikipedia.org/wiki/Firebox_(steam_engine)#Brick_arch

 

[The Lad]

In most loco's the arch is built across the firebox from studs on each side. In use it gets very hot (incandescent) due to radiation from the fire and this assists with combustion in addition to increasing the path length of the burning gases. The resultant flow also aids mixing of secondary air from the firehole door.

 

[chorleyjeff]

In most loco's the arch is built across the firebox from studs on each side. In use it gets very hot (incandescent) due to radiation from the fire and this assists with combustion in addition to increasing the path length of the burning gases. The resultant flow also aids mixing of secondary air from the firehole door.

And is a source of instant heat for gases passing through boiler tubes.

 

[randyrippley]

A brick fire arch is less likely to fail from metal fatigue than an equivalent metal construct

 

[Adlington]

By definition brick fire arch will never suffer metal fatigue

 

[randyrippley]

By definition brick fire arch will never suffer metal fatigue

that was the point I was making......too subtle I guess

 

[Taunton]

Without a brick arch not only was less heat got from the coal, but considerable smoke was emitted. This had been a concern from the earliest times, and to use an old Victorian expression, locomotives were "required to consume their own smoke". The initial way to do this was to use coke. A brick arch allowed the use of regular coal, much cheaper as well as more efficient.

The additional enforced path, making things more complete, has always seemed to me analogous to a superheater, where the steam is taken on an additional path beyond the straightforward one, again to enhance efficiency by gaining more heat.