Besides tradition, why offset smokers firebox are below the chamber?

[PEU]

This may be a stupid question, but why the firebox is below the chamber?
Yes, I know heat rises, and yes I know you need to fight radiant heat.

But what happens if you make a tube like this And adjust the opening for correct flow?:

(for reference that a 24 inch diameter tube 1/4 thickness , and stack can be any diameter/thickness needed
Why 24? because I can find any lenght at ridiculously cheap prices at a local scrapyard here in Argentina)

Will the heat carried by the air be too laminar and pass thru the top of the chamber not interacting with the meat in the grill? Can this be improved by welding turbulence barriers to the top of the chamber?

Thanks in advance!

Pablo

 

[gmc2003]

My guess would be stagnant smoke. You would need some type of draft if it wasn't lowered.