Oil Tank Arch - Smokebox or not?

[MarquisVII]

I'm building an evap for a 2x4 divided pan using an old fuel oil tank. It appears that most builds have a "smokebox" at the back of the unit that the stack sits atop (the the mailbox-shaped part that you don't cut off the top of the tank when making the pan table). Is there a thermodynamic benefit to this? Seems it would be easier to cut the top flush off and have a flat area behind the pan that the stack is mounted on - then there's a bit of flat surface that can be used as a warming table.

Thoughts on this are appreciated!

I'm sure you all know what I'm talking about, but just in case the first pic is with smokebox, second without:
https://booksatmiddlemayfarm.files.wordpress.com/2021/03/fuel-tank-maple-syrup-evaporator.jpg

https://i.imgur.com/hEZiabu.jpeg

[Pdiamond]

I have seen them done both ways, I don't know that there is an advantage/disadvantage either way. The one I inherited was flat and I used it to warm sap in metal coffee cans. that was my very first evaporator. 1 big square pan.

[buckeye gold]

I am in agreement with Pdiamond, I don't know if it matters. However, lining it with ceramic blanket behind the bricks will make a big difference. If you hadn't planned on ceramic blanket, I recommend it.

[Super Sapper]

It is called the stack base. It's purpose is to allow the heat and smoke to evenly travel under the pan. If you just have the hole in the middle the heat will short-circuit to the center and the back corners will not get any heat. You can put a brick or some other blockage right in front of where the stack goes out to force the heat to the outside if you do not have a base but is not as efficient.

[NhShaun]

Super sapper said it. The mailbox style in the photo will help spread out the flames/heat to the bottom of the pan evenly not just the direct line to the smoke stack. Having that extra height over the other photo with the flat top will likely help create a stronger draft.

[MarquisVII]

Thanks Super Sap! This is exactly what I was looking for and it makes complete sense.