First reverse flow offset build

[JonnyJoe]

Long Time lurker first time poster. I've decided that the weber kettle isn't cutting the mustard anymore so I have decided to build a lang 48 clone. Firebox is already built and it's 18 by 18 by 18 inside dimensions and 3/8 plate. I know according to the put calculator it's slightly undersized but it's the same dimensions as the lang. Cook chamber is a 24 by 48 by 1/4 cylinder. Planning on a 4 inch stack about 24 inches in length. The firebox will protrude into the cook chamber about 4 to 5 inches.

So here's where I'm stuck. Pit calculator calls for 46.6 square inches between the firebox and cook chamber. Then I read it's actually needs to be increased by 50% because of the increased friction loss on the RF plate. Ok, increased the second drawing to 68 square inches. Then I found a post that stated the throat size multiplier is different for a reverse flow and the new calculations put it at 82 square inches.

So what's the deal? I seriously don't want to build a 750 pound lawn ornament.

Lastly, my RF plate will be 20 inches wide, current drawing, and the top of the bottom grate will be 4 inches above the RF plate. Is that a good distance?

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[Big T]

That's what the forum is here for is to help anyone build a smoker that will perform just as you would expect. I'm not sure what pit calculator that you're using but I used the pit calculator on the forum to build my 24''x48'' and the throat (FB to CC) is 63 square inches. I also used a 4'' stack and it's about 32'' tall, just an inch or so longer than recommended. The 4'' between the BP and bottom rack is the same as mine, the range is 2''-6'' so you're right in the sweet spot. I ran your numbers through the pit calculator and it's only 80% of the recommended size, that's a huge difference. I'm not trying to discourage you but if the FB is too small it makes it very hard to maintain a constant temperature.

[JonnyJoe]

I just ran the calculation on here and same thing. 46.6

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[JonnyJoe]

It's the size of the firebox that's making it a smaller throat. But I'm not changing it. The lang is set up like this

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[Pete Mazz]

If you want to keep the same sized FB at least use the throat size of a recommended FB size, which would be ~60 sq in. That will at least maintain the airflow needed. Also, if you build it and then decide to mod a larger FB, the throat is good to go.

You should really use interior dimensions in the calcs.

[Squiggle]

Sound like it will be a sweet unit when it's finished dude, can't wait to see it in action.

Also, you mentioned earlier that you don't want a lawn ornament, & please don't take this the wrong way, but there's one thing I've learned on this site is that when these guys suggest a change then it's only cause it's going to make the smoker work better. I know you said that this is how the Lang is built but there might be other factors in play with those ones, for example, a Jambo is built way different than most smokers & they usually out perform most smokers but that's cause Jamie understands convection & airflow & there's a few little things on his smokers that enhance those things. That being said, it's a good idea to enlarge the FB to 105-110% on the pit calc, just extend what you've already built, & the length of the stack is critical to the draw of the smoker. Trust me when I say you won't be sorry you did.

By the way, freakin sweet beads dude!!

[JonnyJoe]

Thanks for the replies fellas. I agree with the 60 square inches for the throat. As far as enlarging the firebox I'm gonna stick with what I have for now. I'm wondering since part of the top of the firebox is actually inside the cook chamber does that aid in the heating of it.

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[Rodcrafter]

I agree with everything I have heard about making things according to the pit calc. I have made one with too small of a firebox, I found that it was very hard to get up to temp, and if I wanted to bake a cake it was almost impossible to get to 350. I do also believe the FB being into the CC area some will help heat it, it will probably also make a hot spot on that end. The radiant heat pouring off the top of the raging FB will be very hot.

I have found that the smaller fire is much easier to control and keep steady. The larger FB is a box full of hot air that is easier to move than trying to run a big fire in a small box.

jm2cw

[mp4]

It does - primarily thought if it's in too far it creates a hot spot. I have this issue with mine so I'm thinking about installing a shadow plate.

Beating a dead horse here...fire box size has more to do with providing the right volume and maintaining proper air flow than providing more space for a bigger fire. This is another lesson I learned with my build. When I learned how small a fire it actually takes I reduced my fire basket size by probably 1/2 and I've had way better performance.

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[JonnyJoe]

Maybe I should make the cook chamber 42 instead. Then my firebox is actually sized properly.

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[JonnyJoe]

I was also thinking about using the cutouts from the throat to make a shadow plate too

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[Squiggle]

That is some nice lateral thinking right there dude, I like that plan with the CC.

I'd run the new numbers through the pit calc again & make sure the throat is the right size.

I've run my new baby RF build through the pit calc about a dozen times & then again when I make any changes, just to be sure, lol.

My shadow plate is going to be about 10" & my pit is only 32" long, I think yours should be about 12" so the cutouts probably won't be big enough.

[JonnyJoe]

You're right. The cutouts won't be large enough. I shouldn't cheap out and I should just buy another plate, it's just money. I'd post a pic of my drawing but it's hard to see. I did a full size drawing on a piece of plywood. Let me try and get a good picture.

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[JonnyJoe]

A few pics

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[Big T]

I don't blame you for not wanting to rebuild your FB because it's super nice. I think your suggestion of shortening the CC is a great idea.

[JonnyJoe]

Thanks! I'm thinking that's what I'm going to do. I can use the cutoffs for the cook chamber door seals.

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[Rodcrafter]

Yep you will be glad you did


Making memories!

[JonnyJoe]

Should I stick with the smoker calculator numbers or go slightly larger. 46.6 is the throat size it's calling for. I'm at about 55 on the drawing. Thanks for all the help fellas

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[Big T]

I would stick with the pit calculator and make it 47 square inches.

[Squiggle]

Yeah, the pic calc is tried & tested numbers.

[JonnyJoe]

I'm planning on cutting out the door and vents today. How's this look?

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[Squiggle]

Lookin good dude!

[Big T]

What's the dial in the top of the door ?

[JonnyJoe]

Upper intake

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[ajfoxy]

If its the upper intake it must be the uptake...

OK, I'll leave now...