Question on draft

[alleyrat58]

I was wondering if there is a formula or magic kinship to the size of the firebox hole and the stack? I currently have a 4X11 inch hole from the fire box and two 3 inch stacks. I regress it is a center feed in a 250 gallon oil drum with a 3 inch on both sides. I can hold temp up two 200 degrees real well but it gets real dicifult at 275. Any ideas will help. Thanks. Alleyrat

[Frank_Cox]

there is actually. it is based on the air volume required for draft and complete combustion and the cubic inches required to come to balance in all the area's of the smoker. the easiest way to get in the ball park is to use the good ol pit calculator. if you want to get exact... I can help you with that tho. here is a link to the pit calc....
http://www.feldoncentral.com/bbqcalculator.html

[alleyrat58]

Thanks Frank I will let you know how it comesout.

[Frank_Cox]

Sounds good!

[alleyrat58]

Hey Frank, thanks for that pit calculator. So to expand a little bit, I've had this pit for the last three years and been making mods to it here and there. As i said the tank is 250 gallon, oblong, fuel oil tank and I stood it upright. I want to make it a center-feed reverse flow my firebox is already in the center. I am going to expand the stack to a 6" and hang it over the center of the smoker. I am looking at putting a convection plate in the bottom and leaving @ 2" on the left and right side for the air to flow up from under the convection plate. I was thinking about putting a 3" drop from the center to the outside edge of the convection plate - sort of an A-frame if you will - so the grease will run off into a drain - I also thought that might help spread the heat out to the side.

What's your opinion - do you think that will be too much of an angle? Once the new convection plate is in place I plan to build a gravity-feeder for the firebox and insulate the whole thing. I try and get some pics tomorrow to post of what I have now.

Thanks!!

[Frank_Cox]

If it was me I would only pitch the plate an inch or so to the middle. you want some restriction with the baffle plate gap to keep the draft from going too fast under the plate so the plate has a chance to absorb as much heat as possible to allow for more even cooking temps. I would put a smaller baffle over the hole between the firebox and the cook chamber to shield the baffle plate from direct contact with the flames and direct heat from the firebox.

[alleyrat58]

Here are the pics of the build we've done over the last three years:

We squared off the front and installed 1/4" doors.


Here is a pic of one side on the inside.


With both doors open


This is the smoke hood we installed 3 years ago, but as you can imagine we still have a very hard time keeping temps even. This little hood is only made of 16 ga steel. My plan is to remove this prior to installing the reverse flow baffle.



We recently cut the rectangular hole from the firebox to the cook chamber to try and get more heat into the box. All it did was make our hotspot hotter and temp control even wilder. Hoping that putting in that baffle will help even that out. Since we've opened up that hole I noticed that the firebox is drafting backwards (out the holes for the fans and around the lid).




We were having problem with the BBQ Guru fans so I built my own fans and power supply and use the BBQ Guru as the brain. It helps, but it isn't great. We had to put some 6" nipple on there to keep the fans farther from the firebox. They kept burning out.


Right now we have 2 3" stacks, which I will be replacing with 1 6" stack in the middle, above the firebox. 6" was the biggest pipe I could find so what I'm going to do is leave enough room to add another 6" stack if necessary.




It's been an adventure thus far. I am hoping to insulate the whole thing before comp season starts back up. Thanks for the advice on the baffle. I'll scale that back to a 1" drop and take your advice on putting a baffle over the throat to keep the flames off the convection plate.

Thanks, Frank! I'm sure I'll be bothering you more as this next set of mods progresses.

[Frank_Cox]

no probs buddy!

[Tom_Heath]

Thanks for sharing the pics, looks tomelike u have the starts to an awesome rig. With a few mods here and there you will have a pristine running food factory.

Sent from my GT-P7510 using Tapatalk

[Gizmo]

Thanks for the pics alleyrat! I think you got the makins of a great smoker there after the "tweeks" and I got a couple ideers just looking it over