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Was looking to build something different for the back yard and had some parts lying around. Smoke/heat enters center low under 3/8 divider plate and rises on each end, moves to center then up the stack. Fire box and exhuast stack are both dampend to regulate heat and dwell time in the cook chamber. Still under construction hoping to light it off over the weekend.
Well I finished it up today, painted it up, set it in the back yard and did a trial run. At first I couldn't get temps above around 175 in the cook chamber (with all dampers wide open and a large bed of coals) then started messin with the damper on the exhaust stack, once I closed it about 2/3 the temps ran right up to 250 and held. So I opened the damper to about 1/2 and she settled in at about 225. Turns out a 6" exhaust stack is way over kill for this unit, glad I put the damper in.
It was dark by the time I got done messin with it so I'll post a couple of photos of the finished unit tomorrow.
It had more to do with the length of the stack, and not the diameter. A stack that long would increase the velocity of the air so much, that I am sure it was sucking air right past the fire before it had a chance to heat. With 6 inch pipe, the length would be around 12 to16 inches or less for that size tank.