Burn Tunnel and Riser cast as one

I have a chance of casting a 6 inch burn tunnel and riser as one complete casting. I wanted to use cardboard tube, suspended in a wooden box or crate former. I believe that I can get a good quality cast product out of this one use pattern. Once set I will fire it to burn out the cardboard. My concerns are, will a 6 inch round burn tunnel work, and how long should it be ? I will use a vibrating table to make sure all the air is out of the cast. The end product should be a large square L shape with a nice clean 6 inch bore running through it. Could this work?
Thanks

What material are you casting it out of?

Simon, dunno about clay based materials.

But with castable refractory and ciment fondu, you should leave expansion joints.

Make the interior of the Burn tunnel and heat riser rough. the rough of the bricks and the brick edges contribute to good mixing. and if you are using cast-able refractory make sure its high heat alumna.

If it were up to me I'd cast the burn tunnel and the heat riser separately. The expansion and contraction cycles are going to make some cracks in the joint of the "L"

You could "key" the two pieces so they sit into each other and have some room for uneven expansion rates due to size and distance from heat (not form material differences, we know that would be all made from the same thing).

The effort to make a squarish burn tunnel is not that hard or at least a square joint in your "L" for more turbulence.

I want to see some pictures when you get started, thanks -Dan.