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Offset Fireboxes

herr tulpe

Fire Starter
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Joined May 21, 2007
Why are the fireboxes in offset smokers always at half the height of the meat box? Is there any good reason, other than minimising the amount of baffle required to get the smoke to the meat?

After my cold smoking bacon (which was great eating!) the ambient temperature gets to 35 C during the day at 60 - 70 % humidity, So I think not too much more cold smoking this summer! So now I want to build a hot smoker. I want to build somthing reasonably compact, and as such have an idea to mount the firebox next to the meatbox but add an extra long smoke baffle between them.. Once I get sketchup installed I hope a sktech will follow of what I am thinking, but roughly a square box about 75 by 30 cm, 25 cm thick. (Thats about 2 1/2 by 1 by 1 feet in US) with removable legs so its easy to store.. Big enough to do a couple racks of ribs, or a brisket or a couple of chickens.

Can anyone visualise what I'm thinking??
 

herr tulpe

Fire Starter
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Joined May 21, 2007
Here is what I am thinking:

Attachment 3021

Over all dimensions: Width 2.5 ft, depth and height both 1 ft.

Starting at the left, firebox with hinged top and bottom cover for easy loading of wood/charcoal, and unloading of ash.

Then 8 cm width for a baffle which will channel smoke downwards and hopefully reduce radiation of direct heat from the fire through the firebox wall.

Then the food box is about 1 2/3 ft wide. Big enough for a chichen, or a brisket or a couple racks of ribs..

This is not big, the idea of it being all on one level is that it will be small, light, and mobile. easy to through in the back of the car for example..

Then a set of tubular removable legs (not pictured)

Made from probably 430 stainless..

Any advice.. Will probably build in 2 or 3 weeks..
c5b6dacd_vbattach12918.jpg
 

triple b

Smoking Fanatic
OTBS Member
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Joined Feb 27, 2007
Wow!
That looks cool!(and portable)
I'm not sure how much heat you would get.
IF you do build it I'm sure we'd all like to hear all the details.
 

hawgheaven

Master of the Pit
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That's a very interesting concept... I also wonder about the amount of heat you will actually get into the cooking chamber.

Let us know how this works out!
 

biggieq

Newbie
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Joined Jun 6, 2007
Thats a neat idea but the big problem I see is that heat rises and you've got all your heat going out the bottom. That's the reason all the fireboxes are offset, so that the heat comes in aboe the charcoals.
 

squeezy

Master of the Pit
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I agree with BiggieQ on this ... heat rises and this design may smother your fire ... hope I'm wrong.
Do you have any other views showing where air enters, the planned circulation before the smoke exits?
 

herr tulpe

Fire Starter
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Joined May 21, 2007
BiggieQ & Squeezy - You are both right, this will be much more difficult to get direct heat passed through.

Airflow would come from 2 - 5 vents at the side, front & back of the firebox area. 'My thinking is that most heat carried through is by convection, conduction and radiation will make little difference for the cooking. Now I worked out today that the BTU needed to heat the air space in this small box is only 75 BTU, where charcoal puts out a massive 9000 BTU/pound. SO if you would burn 150 g of charcoal and 50 g of wood per hour, you would generate around 3750BTU, seems even if you take losses of conduction, radiation and bad seals into account there should still be enough left (75BTU) to heat the air??

Here is another view showing 2 airvents.

Attachment 3032

Have the following additional ideas:

angle the firebox lid down at about 30 deg. This would deflect some of the convection currents into the baffle.

angle the fire grate at about 10 deg towards the baffle wall, so fire is always against it.

add heat sink plates into baffle area, so metal heats by conduction and smoke passing over it can 'reheat.'

Finally what about turning the coal grate into tubing connected to a small water tank. The coals would presumably heat the water and turn it into steam, which you could then inject into the cooking chamber..

hmmm.. too many ideas!!
2b0e9264_vbattach12923.jpg
 

biggieq

Newbie
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Joined Jun 6, 2007
The long and short of it is this...The fire needs to be below the cooking chamber to properly cook and smoke the food. Thats why you see offsets and vertical smokers. What you need to do if you want to keep that design so that it's portable is flip it on it's side and make it a vertical smoker. I attached my own 3d mock up to give you an idea. (I am NOT very good at this 3d thing at all, but you get the idea.)
e4bc4dfb_vbattach12924.jpg
 

biggieq

Newbie
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Joined Jun 6, 2007
Here is a close up above the fire box. There is another vent in the back that I couldn't get a good shot of, but it's the same.
b5884508_vbattach12925.jpg
 

gypsyseagod

Master of the Pit
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not to get off topic but what program are you using for the 3dgraphics ?
 

deejaydebi

Legendary Pitmaster
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Joined Nov 18, 2006
Herr Tulpe -

They are below the meat box so that the smoke flows through the meat and not over it. Loks like and interesting design but I would move the stack to the side even with the grate so the smoke will be drawn over the meat and not up and away from it.
 

herr tulpe

Fire Starter
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Joined May 21, 2007
Biggie, This looks great! What kind of dimensions are you working with? At a guess about 1.5- 2 feet square with 3 or 4 feet high?

Have you built this, or will you build?
 

biggieq

Newbie
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Joined Jun 6, 2007
The deminsions I had in mind were 2 feet deep by 4 feet high but it would work with any dimensions really to keep it as portable as you like.

I havn't built it and probably wont, I'm an offset kinda guy.
 

pyre

Meat Mopper
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Joined Apr 19, 2006
I think the fire would go out in that design because it wouldn't want to ventilate very well. That design would be trying to push the hot air from the ceiling all the way down to the floor before it can escape. I'd be afraid that the fire would suffocate. Whats to keep the smoke from going out your air intake instead of the path you want it to go? Seems the air intake would be the path of least resistance. Your air vents are a little higher than the baffle into the cooking chamber. As the baffle between the two chambers fills, so will the fire chamber, until the top of the air vents is reached and you'll start losing smoke through the intake.

Please take my views with a grain of salt. I'm no expert on the subject.
 

squeezy

Master of the Pit
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I think you hit the nail right on the head with that analogy Pyre, I agree with your theory!
 

herr tulpe

Fire Starter
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Joined May 21, 2007
Thanks Pyre, that makes a lot of sense!!

I am thinking now maybe to eliminate the baffle, and have a middle wall that is solid at the bottom (so no ash can get through) but then highly perforated from the grate level up. I'll draw it over the weekend and post some pics.. Otherwise here in China they have some great induction hotplates. You can set temperature as well as wattage, or rolling boils, and lots of different cooking modes. I wonder how they would work on wood chips.. Then it might just turn into the square box that BiggieQ is referring to.
 

herr tulpe

Fire Starter
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10
Joined May 21, 2007
This is maybe another horizontal option.. No Baffle, and a big hole just like your traditional offsets..

Attachment 3060

any thoughts? Or still same potential issue of not enough heat, particularly circulating around the bottom of the meat?
7f8712c3_vbattach12944.jpg
 

msmith

Master of the Pit
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Joined Nov 22, 2006
Well to me the grates need to be above the the firebox opening since heat and smoke will rise.
 

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