Need help calculating flow and fire box size for BBQ Bomb

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Need help calculating flow and fire box size for BBQ Bomb

Post by pitbull8151 » October 12th, 2011, 2:52 pm

I am building a smoker out if a sphere type propane tank. It has a Dia. of 40.75 in. I have a piece of 12x24 tubing to build my fire box. How big do I need my fire box and what kind of flow would I need? I will post pics of my material to give you a idea.
Thanks.



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Re: Need help calculating flow and fire box size.

Post by pitbull8151 » October 12th, 2011, 3:42 pm

Here is what I have so far.
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Re: Need help calculating flow and fire box size.

Post by Gizmo » October 12th, 2011, 5:26 pm

Frank or Smokertom are the guys with the calcs - they can tell you what size but it's prob going to be 30% firebox compared to the cook chamber or thereabouts. Looking at what you've got on that trailer I would suggest putting the firebox directly under the sphere and set a baffle (1/4"?) inside the bottom of the sphere and allow the heat to surround it as it comes up the ball and out the top - look inside a big green egg sometime that has the big hotplate accessory in it - that arrangement sure works well for them.


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Re: Need help calculating flow and fire box size.

Post by pitbull8151 » October 12th, 2011, 6:22 pm

I was thinking of putting firebox on back of trailer ( to be built) and running 2 pipes off top and into the middle of smoker. I also thought of adding a circular plate in middle to make it a sort of indirect cooker. The stack will come into middle down just off cooking grate.

What do you guys think? Will this work?

Also gonna build a spinning grate to access from one door.

Any thoughts?



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Re: Need help calculating flow and fire box size.

Post by Frank_Cox » October 12th, 2011, 7:25 pm

I think i would do something like this if it was me...
viewtopic.php?f=46&t=166
I would do a different counter weight system if it was going to be on a trailer.


If not that then i think what EK recommended would be best. however you build it, I think it would make one heck of a UDS! or a green egg clone.
If you do put a fire box under it and feed it from the center 30% would be a good number for that since you would not be able to go for a reverse flow very easy. I would not suggest using the pipe in between method really. that does not seem to work as well since there is so much restriction and heat loss in the pipes.
You could put the square tube you have at the bottom and feed it endways into the side of the sphere at the bottom and put the baffle above the opening a few inches above the top of the inlet. that would work well i think.


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Re: Need help calculating flow and fire box size.

Post by Tom_Heath » October 12th, 2011, 8:07 pm

I have to agree that the calc isn't gonna work well since its not gonna be a reverse flow.

Myself, I would take the firebox and set it on edge 12 wide 24 tall then have the heat enter right in the center bottom of the sphere.

That will be more then enough firebox for the setup. I also really like the diffuser plate idea. Possibly even have the heat plate mounted on a threaded rod and be able to adjust it from the top of the sphere when the lid is closed. Depending in the lid design, the top of the threaded rod could have a sort of a bolt head on it. Then the lid would have a crank and a socket on the inside. WaLaa.

For the rack or racks I would have a pipe over the threaded rod for the racks to mount to. When cool you could open the lid and lift the racks out by sliding the pipe off the threaded rod.

Sorry about my typing but the comp crashed and all I have is my phone.

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Re: Need help calculating flow and fire box size.

Post by pitbull8151 » October 12th, 2011, 8:41 pm

If I pipe in from the firebox into the lower middle of tank and put a diffuser plate to make heat go to outside, then bring stack down the middle. Will it work kinda like a reverse flow?

I want to have the grating work like a turnstile so I only have one door to open and can spin around to work it.

I will look into the Green Egg idea.
Should I cut the fire box down? The way I figured (probably wrong) is that the smoker area is about 34900 cu. in and if my fire box should be about 10500 cu in. I fugured 12x24x36 fire box....am I close or waaaaaaaaaay off? :D



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Re: Need help calculating flow and fire box size.

Post by Tom_Heath » October 13th, 2011, 11:49 am

If it was me. I would have the friends intake center bottom the size of your rectangle tube. Down about a foot. Then 90 it and have aprox 2 feet running horizontal on edge. Then a door at the end. Save the space on top of the firebox for warming pans of water or beans. This way you can change the location of the fire depending on ambient temps. Closer to the center the hotter it will run. Build the fire by the door and it will disapate heat before the smoker. This would give you plenty of room for adjustment.

Entering the fire center bottom really cut down on how big you need the fire box. And yes with the diffuser it will act somewhat like a rf. And yes to having the stack right in the center.

Nice idea on having a smaller door on one side with a carasol. I would still try and incorporate an adjustment on the damper/diffuser plate. Just like working with a drum. When you open it up your fire will build from the extra air. So if I was gonna have it open for a length of time, I would close the difusser to maintain my fire as well as slow the smoke in my face.

Just saying...

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Re: Need help calculating flow and fire box size.

Post by pitbull8151 » October 27th, 2011, 12:39 pm

Ok yall. I need more info. How far apart do I need my divider away from cooking surface. After some discussion with my buddy I have scrapped the idea about the turn table grate and going with the double barn door opening. Just wondering how far I need to drop plate down and maybe still have some room to throw some charcoal incase may want to grill.

I have already cut my fire box and end caps. The cook chamber is 150gal and my fire box is 20X12x23 ( I thought it was 24) is there any other thoughts on my piping and stack size?

Thanks



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Re: Need help calculating flow and fire box size.

Post by Gizmo » October 27th, 2011, 8:17 pm

I just put Edgar's grate 3 inches above the baffle plate and the baffle is 6 inches off the floor of the cook chamber/firebox roof - and that one is a center feed rf. HOWEVER, your cook chamber is a sphere so my numbers are not too helpful. You need to send Frank a PM and work out the calculations. Since this is a sphere - it's a little trickier to "rule of thumb" it.


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Re: Need help calculating flow and fire box size.

Post by Tom_Heath » October 28th, 2011, 8:33 pm

I think E kid is right on for the grate levels.

As stated the sphere is a tough one.

I think you are plenty good on the fire and for the stack.
I think I would go with a 4" stack about 30 inchs from the top of the cooker.
I would even go 6 inch if I could get it cheep, you can always damp it down.


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Re: Need help calculating flow and fire box size.

Post by Gizmo » October 29th, 2011, 7:51 am

Ya but you can buy 4 inch off the rack chrome tips for the 4 inches pipe! - ...I'm just sayin'... :)


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Re: Need help calculating flow and fire box size.

Post by Tom_Heath » October 29th, 2011, 2:35 pm

Very true on the tips. I forgot about that.


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Re: Need help calculating flow and fire box size.

Post by Gizmo » October 30th, 2011, 5:46 am

If the women can't find you hansom - then they should find you handy!


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Re: Need help calculating flow and fire box size.

Post by Frank_Cox » October 30th, 2011, 8:51 am

Ah, the great Red Green

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Re: Need help calculating flow and fire box size.

Post by pitbull8151 » November 5th, 2011, 11:53 am

More questions....The stack pipe is 4 in. and I am going to bring it down to the grate on the inside. Can it be too long? If I have it 30 in. above the top of cooker and it goes down 20in on the inside is 50 in of 4 in pipe going to affect the draft?

Also. I have cut the doors already and went with a barn door style. I have also decided to put a "lip" on the baffle plate to have the ability to add water to be a sort of steam effect. Any thoughts on this?



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Re: Need help calculating flow and fire box size.

Post by Frank_Cox » November 5th, 2011, 8:56 pm

Sorry for the late reply pit bull. I consider the length of stack on the inside of the cook chamber to be part of the cook chamber volume. One thing I would recommend is ending the stack 1" inside the cook chamber, then using a chunk of 16ga rolled up like a newspaper or even as thin as 22ga. And sliding it up inside the stack and by doing that it will be adjustable up and down. Use a nut and bolt to either set screw it in place or weld the seem and drill holes in it for a pin.
As far as the water in the baffle plate, you need to make sure and keep the pan full or the temp will go sky high all the sudden. I have not really noticed a difference using water in the plate tho.

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Re: Need help calculating flow and fire box size.

Post by Frank_Cox » November 5th, 2011, 8:58 pm

Yes on the lip tho! It keeps the drippings from going under the baffleplate which is impossible to clean

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Re: Need help calculating flow and fire box size.

Post by Gizmo » November 6th, 2011, 8:47 am

You could put a water dripper on your design pretty easy though... especially if you're running the stack clear down to the grate... just a thought :)


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Re: Need help calculating flow and fire box size.

Post by pitbull8151 » November 6th, 2011, 9:41 am

This is what I have done so far. Changed the door layout.
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Re: Need help calculating flow and fire box size.

Post by Gizmo » November 6th, 2011, 12:43 pm

That is going to be the Quooolest smoker in the free world! LOVE that ball - The BBQ BALL! wow - I gotta go sit down... :)


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Re: Need help calculating flow and fire box size.

Post by Frank_Cox » November 6th, 2011, 1:01 pm

that sounds like a perfect name for it LOL! "The BBQ Ball" :-bd


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Re: Need help calculating flow and fire box size.

Post by k.a.m. » November 6th, 2011, 4:49 pm

pitbull8151 wrote:More questions....The stack pipe is 4 in. and I am going to bring it down to the grate on the inside. Can it be too long? If I have it 30 in. above the top of cooker and it goes down 20in on the inside is 50 in of 4 in pipe going to affect the draft?
pitbull8151,I dance to a different drummer on stacks and placement. I have built cookers with stacks down low and up high to be honest it does not make a difference in my opinion. If the cooker has the proper intake, firebox to cooking chamber opening, exhaust and well sealed it will perform its your job as the pitmaster to learn her. I also figure the stack protruding into the chamber as total length of stack and not volume of chamber. The reason being is the stack is the chimney and once it enters the smaller volume the taller the stack the harder the draft pull becomes. These are just my thoughts. :)



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Re: Need help calculating flow and fire box size.

Post by Frank_Cox » November 6th, 2011, 5:43 pm

k.a.m. wrote:pitbull8151,I dance to a different drummer on stacks and placement. I have built cookers with stacks down low and up high to be honest it does not make a difference in my opinion. If the cooker has the proper intake, firebox to cooking chamber opening, exhaust and well sealed it will perform its your job as the pitmaster to learn her.
Like I always say, "it's 50% the smoker and 50% the guy runnin it!" :mrgreen:
k.a.m. wrote:I also figure the stack protruding into the chamber as total length of stack and not volume of chamber. The reason being is the stack is the chimney and once it enters the smaller volume the taller the stack the harder the draft pull becomes. These are just my thoughts. :)
I've calculated this both ways and it seems the difference is marginal at best. So i tend to recommend putting the entire calculated length outside the cook chamber and then if the grate height is desired making it adjustable as i mentioned earlier. but just as Kam said. it doesn't seem to make much of a difference.
there was an argument that kam and tom may remember hearing at one point about "stale smoke" from having the stack at the grate height, but i think it is mainly due to operator error.
ok now that being said ... %-( ...
=)) =)) I always wanted to use that smiley :mrgreen:


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Re: Need help calculating flow and fire box size.

Post by k.a.m. » November 6th, 2011, 6:03 pm

Its good to have choices my Friend. :mrgreen: A lot of times I get emails and P.M.'s from builders that are killing themselves trying to stay on track with the paper. They get so involved in the numbers they sometimes forget their ability to overcome little obstacles. I would have chopped all your wood today to gladly give you these pain killers and possible kidney stones. I can honestly say I did nothing all weekend.



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