Pit building ratio calculator

[dodis]

HOTSHOT1,

Just based on the calcs from the spreadsheet, you will need a bigger firebox. What you have listed comes out to about only about 35% of what is needed for the size of the cook chamber. As for the chimney, a 6" pipe calcs up to 73", but I don't think that the 5-footer you have will hurt to use.

And for the reverse flow, I haven't even begun to try to include any tips/comments/experience comments into the spreadsheet yet. Best bet on that is either more searching on the subject here in the forums, or maybe one of the other more experienced builders will chime in shortly.

Here is a list of the calcs from the spreadsheet, just FYI.

Your tank, 36"x120" = 122,083 cu. inches.
Firebox (1/3 of smoker) = 40,694 cu. inches (cube size of 34.5") I entered 36"W, 34"H, 34"D for the calcs.
Opening from firebox to cooker = 18.25" square (333 sq. inches) or any dimension to get the sq. inches
Air inlets to firebox = I entered 5"x5" square, shows needing 5 of these openings (125 sq. inches)
Chimney = calc: 5.5" dia at 90" tall. I used your 6" dia, for 73.5" (60" should be OK)

Remember, this calculator is based off of reports to ALIEN BBQ on what cooker dimensions are working for experienced users, so the spreadsheet is just using those values. I haven't even started acquiring materials to build on the 150 gal tank I have, so I don't have any hands on exp yet. (I'm just the local Excel guru at my office)
_________________
Mike "Dodis"
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UDS (55 gal & converted Old Smokey Electric)
Weber One-Touch
Broil-King Baron
Red"neck"stone Griddle

[hotshot1]

that helps me out but still confused alittle bit. I was looking in the fourms and on august 29 heavyd and tom c were talking. heavyd is building the same cooker and tom c suggested to use a 24x24'' firebox. so I was thinking mine has to measure out about that. maybe it doesn't. so I will look for a bigger firebox. could you explain the math on the numbers so I can understand it better. if I used a round fire box what would the measurement be or would it change? I appreciate all the help.

[dodis]

HOTSHOT1,

Well, all the explanation I have is the calculator spreadsheet that was built here as a collaboration between mainly Alien BBQ & Tom C. All I did was pretty up the spreadsheet, and add a few tweaks, but they came up with the formulas.

Also, I have been quite busy lately, so the only time I have read the forums is when I get notification of a post to this thread, so I have missed that thread you are talking about.

All I did was to input the tank size you listed, and the first calc says the firebox should be 1/3 the size of the cook chamber (tank). I added a calc to get the cube root of the firebox volume to get a cubic size for the box, then just tweaked the width to be equal to the diameter of the tank, and the other two dimensions to get back to the correct volume.

Now if there is some additional considerations that Tom C & Heavyd were using to determine the firebox size, I am unaware of it. Will need to get their input to answer that question.

My un-educated guess would be that once you exceed a certain size (volume) of smoker, then the firebox may not need to be close to 1/3 the volume. Another thought on my part is that the little I have read so far on a reverse flow is that it takes more fuel to maintain the heat across the entire smoker, so I would think that a larger firebox would be helpful to keep enough heat going, and also be able to keep some larger logs off to one side to preheat before being pulled into the fire. All supposition on my part... That is why I am trying to watch and learn as much as I can before I invest in my own cooker, only want to do it once!
_________________
Mike "Dodis"
------------------
UDS (55 gal & converted Old Smokey Electric)
Weber One-Touch
Broil-King Baron
Red"neck"stone Griddle

[dodis]

HOTSHOT1,

I searched a bit, and found this thread http://www.thesmokering.com/forum/viewtopic.php?t=19930&highlight= of HeavyD's big pit (Aug dates). He was asking about a 24" sq. box, but others also pointed out the calculator and that it would be more like 36" sq., as I also calculated. Looks like he started thinking 36x30x30.

Best bet, I still think is wait for the experts to chime in, I wanna see too! I helped some years ago on a 30" dia but as a grill (no firebox). That was a monster in itself. But, 10' x 36" I'd like to see that up close when it's done!
_________________
Mike "Dodis"
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UDS (55 gal & converted Old Smokey Electric)
Weber One-Touch
Broil-King Baron
Red"neck"stone Griddle

[Tom C]

HOTSHOT1,

dodis is correct in that the calculator indicates that your firebox needs to be larger if it is to fall with in the 1/3 rule. People have been successful in making smokers that defy the rule but my guess is that you would have a tough time obtaining temps with the firebox s and cooking section combination you list.

I don't remember the specifics of the conversation you mentioned with heavyd. A lot of times people ask for calculations based on the size of firebox they have on hand even if it might be off from the size the calculator indicates.

If you like you can pm me with any questions.
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

[hotshot1]

hey tomc being a newbie at these smokers its kind of making sense. maybe I'm researching the whole thing to much and getting things mixed up? so I will make or rework the firebox I have and use the measurements dodis worked up and give it a whirl. if I switched to a round firebox would that change or still looking at 1/3 of the size of the cooking main cooking chamber? I really do appreciate all the help I'm getting. I hope I can put this thing together shortly.

[hotshot1]

hey tomc being a newbie at these smokers its kind of making sense. maybe I'm researching the whole thing to much and getting things mixed up? so I will make or rework the firebox I have and use the measurements dodis worked up and give it a whirl. if I switched to a round firebox would that change or still looking at 1/3 of the size of the cooking main cooking chamber? I really do appreciate all the help I'm getting. I hope I can put this thing together shortly.

[Tom C]

Hi hotshot1,
The 1/3 rule applies to round or square firebox. I actually went larger on mine based on what I saw on commercial built smokers. My firebox is closer to 1/2 the cooking section. Basicly the calculator is a guide but not set in stone.
Take a look at some of these comercial rigs that are close to the size you are planning. you might get some ideas. Based on the size fireboxes they have, you can go with 1/3 or a little smaller. One of the links is to a reverse flow smoker.

http://www.peoriacustomcookers.com/new/30X96/index.html
http://www.bbqpits.com/mobile_cookoff_and_catering_rigs/36x10_mobile.htm
http://www.horizonbbqsmokersstore.com/servlet/the-398/36%22-Triple-Door-Trailer/Detail
http://www.pigroast.com/108pages/model108_orig.htm
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

[hotshot1]

hey guys I looked at those links that you posted. so I got a 30''x 36'' tank for the fire box. so I should be good with that size?

[Tom C]

hotshot1,

It sounds like you are on your way! Good luck with the build.
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

[dodis]

HOTSHOT1,

Sounds like lots of good brisket in your future!

Good luck!
_________________
Mike "Dodis"
------------------
UDS (55 gal & converted Old Smokey Electric)
Weber One-Touch
Broil-King Baron
Red"neck"stone Griddle

[hotshot1]

i hpoe the build will turn out ok. i need to get going on this bbq. want to do a test run on it in mid july for a birthday. the guy that's doing the cutting and fab work. i trading a bbq for his party in august that should be interesting. how much fuel will this thing burn far as charcol and wood? pulled pork is on the menu

[smittybuilt]

ok guys i need some help, my wife bought me a smoker about a year ago and i love this thing but the more i play with it the more i think it needs a little help. i have played with the calculations and just want to see what you all think, the problem i am having is i can't get rid of the heat without closing my air inlet copletlely off and it still gets 300 in the the smoker. of course this causes lots of smoke, i have the stack damper wide open like i have read on here and tried less wood and it still will run hot until the coals begin to die out. here are my dimensions on my smoker, i am just wondering if i need a bigger stack to get rid of the heat. my smoker is 30 in by 56 in long. fire box is 30in by 22 in long. my stack is 4 in by 26 in long. also, before you get to the stack on the inside they have welded a piece of plate down to within approx. 12in of the bottom of tank, i guess to keep the heat or smoke from just going straight through. this is a offset smoker with the stack at opposite end of firebox. any suggestions would be great. thanks.

[day_trippr]

Sounds like a firebox with an air leak.

If you can't reliably throttle down the heat source, I'd fix that problem first...

Cheers!
_________________
Save the planet: it's the only one with beer!
FrankennBrinkmann ECB
Char-Broil Commercial Gasser

[SoEzzy]

[smittybuilt]

[smittybuilt]

[Alien BBQ]

Well this post has certainly received a lot of interest. It might be a good idea for a number of us to pool our respective knowledge and come up with an article addressing the finer points of pit building. Any takers! Until that happens then maybe this will help a little.

Firebox size

The size of the firebox in proportion to the pit is important up to a point. Too small of a box and you won’t get a big enough fire built that will generate heat to cook your food. Too big of a firebox and you cannot control the fire and heat generated in such. So the proportions work well up to a point. It has been my experience that anything over 30x30 is too hard to handle. Once you get into a super large cooker, you are really dealing with more of an air flow (read heat distribution) problem than a “how big can I make my fire” problem. Bottom line on super large cookers… air flow is more important than size of your firebox (within reason.)

Air Flow

One of the biggest (if not the biggest) problems pit builders have is with air flow. Too much and you have fire lap entering your cooking section and uncontrollable heat. Too little and you either smother your fire or it only heats one end of the cooker. The first place to start thinking about air flow is with the air intake on the firebox. Too small and you will never get enough fresh air to correctly control your fire. Too much and it becomes hard to keep the heat down. Regardless of either, the air intake should be adjustable and more than what you need. In other words, it is easier to throttle back too big of an intake than need more air and not have it.

Placement of the air inlet from the firebox to the cooking chamber is also critical. Too small of an opening and you are again choking the fire because you are preventing sufficient air flow. An inlet with large hard angles at the connection between the firebox and the cooking chamber will also cause problem (especially on bigger cookers.) What happens is hard angles cause eddy currents and actually creates a vacuum, thus slowing down the sir flow in the cooker.

Too small of an exhaust pipe will restrict air flow and too short will prevent the establishment of a correct draw. Remember that in smokers, pressure and suction means everything. Too long of a pipe will result in the cooler gasses (still in the pipe) falling back down the pipe and ruining your draw. In exhaust pipes, it is the movement of air up the pipe at a fast rate that creates a negative pressure in the cooking chamber and subsequently in the firebox. Slow down that movement and it will back up everything.

Radiant Energy

Radiant energy coming from the steel used in the construction of the smoker as well as any plates and or baffles can greatly affect the temperature consistency within the smoker. Thicker steel takes longer to bring up to temp but it also takes longer to cool. Adjustable plates or channels used to direct heat within the cooking chamber can add to the smokers’ efficiency but can be overdone. A long channel (as in a reverse flow cooker) can help to create a cooker that has a very small temperature variation (from one end to the other) but it also calls for a lot more fuel to be burned as well as a superior air flow design. Unique to the reverse flow design is gravity. By raising and lowering the non-firebox end of the cooker, you can speed up or slowdown the airflow within the cooker and actually adjust the temperature.

Final Thoughts

What is the best way to build a smoker? Well you can ask any of the 1000 manufactures in the world…….. well maybe not because they will all give you different answers. It really is personal preference. Everyone always says that they want a set it and forget it cooker, but I don’t think that it is necessarily true. If it were true then we would all just splash some liquid smoke on our meat and toss it in our electric oven. What I really believe is the case is most of us like the technical (and art) aspects of BBQ and the fact that most people can’t (or won’t ) do it. What we like better is the ability to complain about how hard it is to other people just like us
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https://www.linkedin.com/in/michaeloberry

[Agrund]

Alright, I have figured this thing out pretty well except the exhaust. I need help there- I'm either entering something wrong, or not understanding.

I have Circular offset with a circular firebox-- Measurements are
Firebox- 16.5" dia by 17" long
Cooking chamber- 16.5" dia by 34.5" length.

I would like to use 3", possibly 4" exhaust. How long should It be? Somehow I keep coming up with like 300 something inches for 3in., which just doesn't seem right.

Thanks all!
Adam
_________________
Weber EP-320
Yoder Witchita
Junk charcoal grill, mainly used when I want to Scream obscenities. Looking to upgrade.

[Tom C]

I come up with 25.71" for 3" and 14.46" for 4"

Good luck with the build.
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill