Top down style offset design help

Horizontal smokers with a side firebox and tuning plates.
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casey3115
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Top down style offset design help

Post by casey3115 » July 21st, 2018, 1:51 pm

Hi all,
I introduced myself and this project over in the introduction section but decided to move the design discussion here.
I want to build an offset smoker. I was thinking reverse flow for a while but recently read some threads about Jambo back yard cookers and the top down style of drafting and how well they work. Outlaw smokers as well seem to be this same top down style.

I really like the look of the new LSG 20" models (as evidenced by the photos of my current design) and it is also this top down style.
Does anyone have any experience good or bad building a top down style offset?

Here are some pictures of where I currently am on this design. I used the Feldon calculator before running across this site and based on the volume of the CC my firebox as modeled is about 105% of recommended and the throat area is also about 105% of recommended on that calculator.

Some things I need some input on would be the shape of the throat baffle that I have drawn here to move the air towards the top of the CC. The height of the throat relative to the bottom grate. The chimney exit height relative to the bottom grate. Also the Feldon calculator had a 4x4 sq tube chimney height of 18.5". Is that total length? Height above cooking grate? Height above the top of the CC? I am not clear on sizing cimneys.

Also the shape of the CC and the height of the bottom grate could be adjusted if anyone thinks it needs to be. Those are positioned like that from a few weeks back when this was modeled as a reverse flow smoker so I needed that clearance (probably needed more) for the baffle and airflow under it.
Thanks for all the help



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Re: Top down style offset design help

Post by casey3115 » July 21st, 2018, 1:55 pm

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Big T
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Re: Top down style offset design help

Post by Big T » July 21st, 2018, 6:30 pm

I really don't know very much about this style cooker but here is a thread that might help you out. The builder mentions some of his dimensions for the build throughout the thread and Tom helped him correct his issues. I'm sure Tom or Frank will come along at some point and they should be able to help you out, both of those guys stay busy so it might be a while. Maybe this will help https://smokerbuilder.com/forums/viewto ... ilit=jambo


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Re: Top down style offset design help

Post by Dirtytires » July 22nd, 2018, 11:48 am

I’ve not seen one of those. Seems like a lot of trapped, stale air in the cook chamber would lead to bitter/dirty meat.

Just my thought.




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Re: Top down style offset design help

Post by Dirtytires » July 22nd, 2018, 8:30 pm

In my humble opinion/experience, the airflow is never going to be happy with the inlet that high and the exaust that low. Hot air rises but is then trapped at the top of the chamber with no where to go so it just sits there. As it cools, it drops all the bitter smoke by-products on the meat before finally getting cool enough to reach the bottom. At that point, it’s not hot enough to rise out the exaust stack.

I’m just throwing out my opinion here and they appear to be made commercially. I experimented with a bottom exaust (as my pit has dampers for both) and am never happy with the results. I’m a true believer that hot air rises and physics just can’t be tricked.



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Re: Top down style offset design help

Post by casey3115 » July 22nd, 2018, 8:41 pm

Dirtytires wrote:In my humble opinion/experience, the airflow is never going to be happy with the inlet that high and the exaust that low. Hot air rises but is then trapped at the top of the chamber with no where to go so it just sits there. As it cools, it drops all the bitter smoke by-products on the meat before finally getting cool enough to reach the bottom. At that point, it’s not hot enough to rise out the exaust stack.

I’m just throwing out my opinion here and they appear to be made commercially. I experimented with a bottom exaust (as my pit has dampers for both) and am never happy with the results. I’m a true believer that hot air rises and physics just can’t be tricked.
Well I am not interested in trying to trick physics. That's a fools errands for sure.
I think the idea is that you end up with a lot of movement in the chamber. The draft will cause the air and smoke to start circulating in a counter clockwise direction. Anything that doesn't escape in the chimney will be pulled back towards the fire box and pulled up and back around with the new incoming hot air and smoke.
It's still just an idea at this point. I'll keep researching.



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Re: Top down style offset design help

Post by Big T » July 22nd, 2018, 8:50 pm

It will work fine as long as it's done correctly. There are a lot of jambo's out there and some of the big name guys use them so I am sure of it. If you were building a traditional offset or a RF I could help you with info but I've never built one like this and I don't want to steer you wrong.


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Re: Top down style offset design help

Post by norm » February 13th, 2020, 8:57 am

I am interested in building one also. Baffle design drawing not clear. Did you find out a solution? Also chimney looks short. Is it measured from top of box? I found that an inscribed circle at 20 inch dia. Works. Still very interested in heat deflector detail.



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Re: Top down style offset design help

Post by Dirtytires » February 14th, 2020, 2:48 am

Check out the plans on the home page....they might lead you to a project.

This post Is 18 months old.....



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