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Gravity Feed Smoker build Questions

bsimms89

Newbie
3
11
Joined Jun 1, 2015
Hi all,

I have been using a Oklahoma Joe Offset drum smoker with a Guru for the last few years and have been wanting to build a gravity feed stumps clone smoker but I had few questions from people who have one or have built one before. I have read through most the build threads on here but still have a few questions.

The inside dimensions I have planned for the smoking chamber is 3' wide x 2' deep x 3' tall (36"x24"x36"). I am going to make the frame out of 1.5" square tube steel and the walls with 14 gauge sheet metal an insulate it with 1.5" mineral wool. For my firebox and feed chute I was going to use an HSS 8x8 steel section about 4' tall. I am trying to figure out what size i need the burn area to be and how large the transfer tube needs to be for this size smoking chamber. I can make the transfer tube probably up to 6" wide since the outside of the section is 8" but idk how tall it should be. Also since the burn area will be 8"x8" how tall should I allow the burnable section to be (the height from the grate holding the charcoal to the top of the transfer tube). Also my final question is in regard to the exhaust size for a gravity feed what would a typical size for a exhaust for a smoker like this, and oulw you have it exit straight out the top, or out a side near the top?

I appreciate any advice anyone could give me and look for ward to working on this project when I get the time.
 

bigjohnsbbqnv

Newbie
18
2
Joined Nov 12, 2018
Hi all,

I have been using a Oklahoma Joe Offset drum smoker with a Guru for the last few years and have been wanting to build a gravity feed stumps clone smoker but I had few questions from people who have one or have built one before. I have read through most the build threads on here but still have a few questions.

The inside dimensions I have planned for the smoking chamber is 3' wide x 2' deep x 3' tall (36"x24"x36"). I am going to make the frame out of 1.5" square tube steel and the walls with 14 gauge sheet metal an insulate it with 1.5" mineral wool. For my firebox and feed chute I was going to use an HSS 8x8 steel section about 4' tall. I am trying to figure out what size i need the burn area to be and how large the transfer tube needs to be for this size smoking chamber. I can make the transfer tube probably up to 6" wide since the outside of the section is 8" but idk how tall it should be. Also since the burn area will be 8"x8" how tall should I allow the burnable section to be (the height from the grate holding the charcoal to the top of the transfer tube). Also my final question is in regard to the exhaust size for a gravity feed what would a typical size for a exhaust for a smoker like this, and oulw you have it exit straight out the top, or out a side near the top?

I appreciate any advice anyone could give me and look for ward to working on this project when I get the time.

I just finished my gravity feed build this past summer. I have cooked on it about 20 times and what I learned is for my design I overestimated the burn height for the charcoal.

My build is close to the same size as what you are proposing, but I used a 6”x6” tube for the feed Shute. My heat transfer tube is 4.5” tall x 6” wide. Since my grate sits about1.5” below the button on the chute the total burn area is about 6” tall x 6” wide. I found this to be too much fire. When I am cooking hot and fast the food tastes ok, but when I try to cook low and slow (225ish) the food has a bitter, dirty fire flavor.

I am in the process of shortening my burn area to 3” tall. I ran a test and determined that I only needed about half as much charcoal to cook at both low and slow temperatures and hot and fast and still get good flavor results.

For the stack size, this is based on the stack effect. The stack effect takes into account inside and outside temperatures as well as inlet and outlet cross sectional area and hieght. Because gravity feeds are technically vertical smokers you can use the cabinet hieght in your stack height calculations. But, based on what I have seen, most of the stacks are between 3 and 5inches in diameter for round stacks and about 5”-10” tall. For square stacks, similar sizing would be between 3”x4” and 3”x6”, but as long as the cross sectional area is between 10 sq in and 20sq in it should be ok. I would probably use something closer to 20 sq in for your cabinet size, you can always add a damper to lessen the amount of air through the stack if it is a little too big. My stack is mounted on the side, but I do think the cooker would draft a little better if it was located on the top.

Sorry! this got a little lengthy. But I hope the information is helpful.
 

bigjohnsbbqnv

Newbie
18
2
Joined Nov 12, 2018
I just finished my gravity feed build this past summer. I have cooked on it about 20 times and what I learned is for my design I overestimated the burn height for the charcoal.

My build is close to the same size as what you are proposing, but I used a 6”x6” tube for the feed Shute. My heat transfer tube is 4.5” tall x 6” wide. Since my grate sits about1.5” below the button on the chute the total burn area is about 6” tall x 6” wide. I found this to be too much fire. When I am cooking hot and fast the food tastes ok, but when I try to cook low and slow (225ish) the food has a bitter, dirty fire flavor.

I am in the process of shortening my burn area to 3” tall. I ran a test and determined that I only needed about half as much charcoal to cook at both low and slow temperatures and hot and fast and still get good flavor results.

For the stack size, this is based on the stack effect. The stack effect takes into account inside and outside temperatures as well as inlet and outlet cross sectional area and hieght. Because gravity feeds are technically vertical smokers you can use the cabinet hieght in your stack height calculations. But, based on what I have seen, most of the stacks are between 3 and 5inches in diameter for round stacks and about 5”-10” tall. For square stacks, similar sizing would be between 3”x4” and 3”x6”, but as long as the cross sectional area is between 10 sq in and 20sq in it should be ok. I would probably use something closer to 20 sq in for your cabinet size, you can always add a damper to lessen the amount of air through the stack if it is a little too big. My stack is mounted on the side, but I do think the cooker would draft a little better if it was located on the top.

Sorry! this got a little lengthy. But I hope the information is helpful.
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