Pit building ratio calculator

Tom C · Posted: Mar 28 2009 Post subject:

Good luck on your build! Don't forget to post pictures of the build.
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Homemade offset smoker 24"x48"
Charbroil CB900 grill

skeels01 · Newbie Joined: 07 Apr 2009 Posts: 50 Location: Wendell, NC

I ran my numbers through the calculator, and it says I need a chimney 6" diameter by 28" long. Is the 28" a minimum dimension? Can I go a few inches longer, just no shorter than 28�?

Thanks!

John

Alien BBQ · Posted: May 06 2009 Post subject:

The calculator uses approximates based on prior reader input.
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https://www.linkedin.com/in/michaeloberry

skeels01 · Newbie Joined: 07 Apr 2009 Posts: 50 Location: Wendell, NC

snappybob · Posted: May 07 2009 Post subject:

I was just going to ask the same thing. Where do the formulas in the spreadsheet come from? Did someone do some sort of analysys to get the best flow through the pit and come up with these ratios?

SPM · Joined: 06 May 2009 Posts: 7

For some reason (probably I'm an idiot) I can't plug in the numbers. I'm using the google account. Does someone know where I'm going wrong? Also this site is fantastic. I've already told everyone I know that smokes about it. Thanks, SPM

Tom C · Posted: May 07 2009 Post subject:

SPM,

To be able to use the Google calculator you have to have a Google docs account, its free to sign up.

snappybob,

The ratio formulas are the work of Mike (Alien BBQ); I just put them in the spread sheet.

Good luck with the builds!
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

SPM · Joined: 06 May 2009 Posts: 7

Thanks Tom, I did sign in though. I'll try again. SPM

SPM · Joined: 06 May 2009 Posts: 7

Well it worked this time. Thanks again, SPM

Tom C · Posted: May 08 2009 Post subject:

SPM,

Glad to hear it worked for you. Good luck on the build.
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

snappybob · Posted: May 08 2009 Post subject:

I roughed out a design based on the spread sheet. Below is a pic of what I wound up with. Looks pretty good to me. Here are the numbers.

Cooking Chamber 24"Dia. X 36L
Fire Box Volume 5694 CI
Fire Box Air Intake Area 45.38 SQ In
Fire Box to Cooking Chamber Area 132 SQ In
Exhaust Stack 5" Dia. X 41.5 Length

Did I decipher all the numbers right. I made the fire box a little larger than called for because I figure I'll probably put a plate in the bottom that will take up some room.

I know, it's got a funky paint job. Rolling Eyes

The different colors make the components stand out a little better.

Anybody have any suggestions or comments?

Tom C · Posted: May 09 2009 Post subject:

snappybob,

Your firebox size looks fine but it looks like you used your cooking sections volume to figure your opening and smoke stack numbers.

You need to use the firebox volume in the calculator. I got numbers much smaller on the openings and stack length based on your firebox volume of 5694 ci.

Firebox air intake area: 17.08 SQ In.
Firebox to cooking chamber area: 45.55 SQ In.
Smoke stack 5� dia. X 14.51 inches in length.

I would use 4� dia. pipe for the smoke stack and then it comes out to 22.67 inches in length or even 3� for 40.3 inches in length.

Let me know if you have any questions.
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

Tom C · Posted: May 09 2009 Post subject:

Everyone,

I had to update the link to the calculator site on my original post. If you have saved the location as a favorite in the past the link will no longer work. Sorry for any problems this may have caused. The new link appears to work fine.

Also you will notice that the link to the MS Works spreadsheet is no longer listed on the web page. It was not working correctly so I removed it.

Tom C.
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

snappybob · Posted: May 10 2009 Post subject:

Thanks Tom, I thought the air intake to firebox and opening from the firebox to the main chamber looked rather large. That expains it. I'm glad I put my design out there for a sanity check. These changes are easy to make at this point. That's a great calculator. Thanks for sharing that with us.

Tom C · Posted: May 10 2009 Post subject:

You're welcome. Glad to help.
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

SPM · Joined: 06 May 2009 Posts: 7

Hey Tom, I have a few questions for you and whoever is familure with the calculator. Is it ok for my firebox to be half the size instead of a third? Also the calculator says I need a firebox to chamber opening of 57, when I scroll down it give me an opening of 157. Do I make it bigger or try to achieve 57. My dimensions are: fire box is 18"x22" dia, chamber is 36"x22" dia, and I have a 4" stack. Thanks, SPM

Tom C · Posted: May 11 2009 Post subject:

Hi SPM,

You can be bigger for your firebox. Mine is almost half and works fine. It just means more material and bigger openings to get airflow.

On the calculator remember that if you enter dimensions for a circular firebox you need to look in the blue cells for all your results.

If I'm following you correctly, you entered 22" dia x 18" length for a circular firebox.

When I enter this size I get the following:

Air inlet to firebox = 20.52 sq.in
Firebox to cooking chamber =54.71 sq.in
4" dia pipe length needed = 27.23

For the firebox to cooking chamber opening I entered 7.8" by 7" to get the volume as close to the 54.71 listed above. The calculator showed that 7.8" x 7" = 54.6 sq. in. the calculator indicated that I would only need 1 opening of that size indicated in the blue cell to the right.

You can change the dimensions of the opening to anything that will fit for you but you need for the blue cell on the right to indicate the need for 1 or slightly less than one for the opening to be big enough.

Example:

Instead of 7.8" by 7" you could enter 7" by 9.25" and the calculator indicates that you need .99 or one of these openings.
This allows you to change the opening size around to best fit your needs but still make sure you have the correct opening volume needed for airflow.

Hopes this helps

Tom
_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

SPM · Joined: 06 May 2009 Posts: 7

Tom, you're golden brother. You answered everything I needed to know. Now I can get to cutting and welding and maybe some smoking by June. Thanks a bunch, SPM.

Tom C · Posted: May 12 2009 Post subject:

Thank's SPM and good luck with your build. I'm looking forward to some pictures. Very Happy

_________________
Homemade offset smoker 24"x48"
Charbroil CB900 grill

snappybob · Posted: May 15 2009 Post subject:

Does this look a little better?

I think so. Here are the numbers I wound up with.

Fire box vol. = 5740 cu in
Inlet Air = 18 sq. in
Air to Main chamber = 49.56 sq in
Stack Dia X L = 4 X 22.83

I am assuming that the stack length is the circumference of the elbow center line and the straight section combined.

Also, how important is the position of the stack on the end cap. Is near the grill better than near the top or vice versa?