Pit Building Ratios

[slidinstar]

Alien or anyone with answers....Currently my chimney is 6" diameter and 48" tall. Fire box is 24" diameter 36"long(pipe). Smoking chamber 30" diameter 90" long. By your recommended calculators I can reduce the height of the chimney to 30" and still stay within the recommended volumes. I'm trying to reduce how tall the unit is if possible. Certainly do not want to affect the cookers efficiency. Any help is appreciated.

Tim

[Alien BBQ]

Man that is one long smoker. How does she run? Do you have to choke down the exhaust any to keep it from running away? My suggestion is to cut the pipe in 8 inch increments and fire her up to see what happens. I would not however mess with a cooker that is running good. Kinda like killing the goose that lays the golden egg!
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https://www.linkedin.com/in/michaeloberry

[slidinstar]

She works very well!! My father-in-law and I built her...and I relied on his knowledge concerning all the openings, lengths, etc. Appears he knows a lot more than I realized. I'm wanting to add a vertical over the fire box with about a 12" stack and wanted things to be fairly uniform...appearance wise. I did some calculations based on the formulas and it seems my firebox intake is shy about 1.65 square inches...not sure if it's really a problem, but it's easy to fix if necessary. Here's pics http://usera.imagecave.com/slidinstar/

[podnah]

I'm building a 36"x36"x6'tall upright with a 24"x24"x24" firebox. Can I use these equations to come up with the intakes/outlets??
I saw that someone mentioned an upright but I was assuming that meant on the end of a horizontal.

Thanks for any help
Rodney
Podnah's Pit BBQ

[GameFix]

Hey guys,

Bought metal 2 days ago and will start build this week. This post andeveryone's response is a great tool for me. One question: Earlier someone mentioned/diagram of the smoke stack actually extending down into the smokin gchamber. I had planed to just weld it to the top of the box. Does the smokestack need to go down into the smoking chamber? And if so, what does the math look like (and the reasons), for how far it needs to extend into the smokeing chamber?


(Geez, I love this board!! Don't post much, but theis is EXACTLY the kind of no-nonsense information I am looking for..) Thanks all!


Michael Carter
Round Rock, TX.
www.GetGameFix.com

[k.a.m.]

slidinstar, I saw your pics and i gotta tell you that is a fine looking set up.Is that what is called a reverse flow set up?

[Michael B]

Can the firebox be too big?

eg.
If you have a 2:1 ratio instead of 3:1, would that be a problem?

For the firebox to cook chamber opening:
If you do have a 2:1 ratio, would you still use .008 x the actual firebox volume, or would you instead use .0053? (2/3 * .008)
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Michael B.
I have not yet begun to procrastinate!

[Nate_bone]

[Michael B]

[GameFix]

Nate-Bone.....Thanks!!!!!!!!!!!!!!!!!!!!!!! I appreciate it. Smoker should be done by the end of this weekend and I'll post pics.

[Alien BBQ]

The original post on this subject was looking for confirmation of the formula as it applies to properly working pits. Somewhere it became the hard and fast rules but I would say that there is some definite wiggle room as to the figures. In answer to your questions, there is a proportionate ratio between the air inlet, cooking chamber inlet and exhaust stack exhaust. The only one that is seldom adjustable is the cooking chamber inlet so it is the one that needs a margin of error. The air inlet is similar to the exhaust in that you can always adjust it down. But if you need more, you are out of luck. As far as the 12 inch pipe working, it should not. When you design an exhaust you need to consider the volume of air moved thru the cooking chamber and air inlet. Too much and you normally experience higher temps that are too hard to control. Too little and the heat in the fire box just sits in the box and radiates out the walls.
This is where the volume and velocity of the hot air comes into play (they are inversely proportional.) A larger diameter exhaust would give you the potential for greater volume but it would also inversely affect the velocity of the air moving thru the chamber. A smaller pipe would increase velocity but would limit the volume. To add another problem is the weight of the air as it cools. If you make your pipe too long, the air cools in the pipe and slows down the movement (velocity.) Similar to breathing from a pipe under water, the longer the pipe, the more difficult it is to move air thru it.
So where does this leave us? The original formula does work based on a 4-5 inch pipe and will get you in the ballpark as to flow and heat dispersal within the cooker. As I stated earlier, the post was a call for confirmation of the ratios. Apparently, they are close because I have not read anyone claiming that they did not work for them. As you increase the diameters of inlets and exhaust you increase the effectiveness of your cooker until you reach a saturation point at which time any increase would start to hurt your flow and effectiveness. Now mix in exhaust placement, pit air flow characteristics, humidly, altitude, pit elevation, weather, and you can see where a pit that is adjustable to work within a range of openings is needed to work in every situation. I hope this helps……………….
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https://www.linkedin.com/in/michaeloberry

[Michael B]

Yes it does, thanks Mike.

I kind of figured that the diminishing returns on the stack diameter would start kicking in as it approached the area of the air intake to the firebox.
I am still trying to figure out where skybob got an optimum of a 5.5 X 5.5 square tube for Kosmos Q’s pit. 5.25 square would equal the air intake and 8.57 square would equal the transition into the cook chamber.

[Tom C]

Hi Mike,

I don't know if you had a chance to see the post of my finished smoker but I would like to report that your ratios worked out fine for me. As you may remember we had went over the air inlet size on my firebox and the size of the firebox in general. I did decide to decrease the size of both but not too drastically. On my air inlet I ended up using .0025 vs. .003. This helped me fit the opening on my door with room for the slide opening I ended up using.

I just wanted to report to you that it all worked out great and that I'm grateful for you help. Your efforts and willingness to share your knowledge I'm sure made the difference with my project!

Thank you!

[Alien BBQ]

I am glad it all worked out for you............. Now where are those action shots of the Q?
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https://www.linkedin.com/in/michaeloberry

[Tom C]

We did our first cook the other day:

http://www.thesmokering.com/forum/viewtopic.php?t=14517&highlight=

I'll be trying babyback ribs today!

[HillbillySmoker]

[Tom C]

HillbillySmoker,

Are you going to have a way of isolating the grill section so that you won't have to be heating that section while you're trying to use the smoker(s) section? Other than that, I would use the ratios to make sure that the opening for your tuning doors are the correct size and that all of your stacks are the correct length. Also, that the air intake size on the firebox is correct.
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Homemade offset smoker 24"x48"
Charbroil CB900 grill

[HillbillySmoker]

Thats just it i dont know how to get my sizes for all that using the calculator, lol im lost. To answer the part of isolating the grill part when not in use, yes im gonna have plates that i can lay in that will be just above the hinge to the door going to the smoker and will completely isolate the grill area. im getting ready to post a few pics on the work i done this evening on the vent openings.

[Tom C]

Here is how I would approach it.

Fire box = 24” cubed

24” x 24” x 24” = 13824 cubic inches

13824 x.003 = 41.47 inches.
This is the total square inches the air inlet into your firebox should be. So if your air vent is 2” tall it would need to be about 20.75” wide.
2 x 20.75 = 41.5 sq.in.

13824 x .008 = 110.59 inches.
This is the total square inches that the opening between the firebox and the smoker section should be. I would use the same size to go between the two smoker sections as well. The opening you have directly above the firebox for the grill section should work fine as long as you can block that off when not in use.

13824 x .05 = 691.2 cubic inches. This is the volume needed for your chimney using 4 inch pipe.
I would build the stacks as if you would want to be able to use each section by itself. In that case each chimney would need to be 55 inches in length. If you’re going to have smoke enter the vertical section from the bottom and the section is 36 inches tall, I would place a 19 inch stack on top.

Depending on how you plan on placing your stacks in the horizontal cooker and grill section would determine how high the stacks would stick up above the cooker.

With all the stacks being 55 inches, if you were going to use both smoker sections at once, you would damper both stacks by about half. That is of course with the grill section closed off.

I modified your drawing to illustrate what I’m talking about.

Hope this is helpful
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

[HillbillySmoker]

You the man Tom, thats exactly the info i was needing, ive already made my vent on the firebox and full open its 20sq inches which is half of what you have figured so im thinking i can add another vent below that one on the face of my slideout ash tray or i can just pull the ash tray out an inch because its 22 inches wide that would give me 42 inches of vent if needed.
Im not gonna be building the upright portion at the moment i will be adding that later and the pipe i have at the moment is 2 pieces of 4 inch 30 inches long, so looks like im lacking in the vent and the stack. so im just gonna go with what i have and see how it does, then if it doesnt perform very well ill add the extra length to the stack and make another vent. Thanks again for that info Tom.