Pit Building Ratios

Alien BBQ · Posted: Sun Dec 03 06 7:19 am Post subject: Pit Building Ratios

Hey guys, I have bounced these ratios off all of my smokers and they hold fairly constant. Could you all check your properly working smokers and see if they are within an acceptable range. We get so many questions on this very subject that I thought I would produce a graphic to illustrate a GENERAL rule of thumb for pit building.

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mrcustomsteel · Posted: Sun Dec 03 06 9:36 pm Post subject:

Good idea, Alien, but do you really want volume measurements? If so, maybe someone can remind me how to calculate cylinder volumes so I don't have to go look it up. pi r length something? Shocked

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mine · Posted: Sun Dec 03 06 10:40 pm Post subject:

Try this page for calculations, it's about as easy as it gets.
http://www.1728.com/diam.htm

zilla · Posted: Mon Dec 04 06 5:09 am Post subject:

How/Where did you derive these numbers? This is great stuff.
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mine · Posted: Mon Dec 04 06 9:58 am Post subject:

Alien, I ran your numbers on my pit today before I started cutting the chimney out.

OLD air inlet ratio .0015
exhaust ratio .038
opening ratio .008
chimney length 5/8

The first ratio I guess would explain sluggish fire recovery when I get forgetful.

Along with my new chimney, I can easily add more air.

Thanks for the insight, I'll let you know how it turns out.

skybob · Posted: Mon Dec 04 06 12:52 pm Post subject:

You know, I've calculated areas and volumes, diameters and a whole bunch of other stuff since college. BUT, I've never ever had any kind of problem that the fool College Algebra was useful for. Took 4 tries to get through that dam class Twisted Evil

and have never used it in over 35 years. What a waste of very valuable beer drinking time!

SoEzzy · Posted: Mon Dec 04 06 8:52 pm Post subject:

BoilermakerFan · Posted: Wed Dec 06 06 12:20 pm Post subject:

Alien

Is the chimney length ratio for the entire length of the chimney, including the length that is down inside the smoker?

I want to see what the ideal length chimney would be for my Char-Griller, but I will be installing the aluminum flashing on the inside to lower it to the grate level as well as extending it on the outside.
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sheddcanyon · Posted: Thu Dec 07 06 1:39 pm Post subject:

If I'm using two stacks, does each stack need to be 3/4 the length of the cooker, or can the combined length of both stacks equal 3/4 the length of the cooker?

And while on the subject, can you have too long a stack?

skybob · Posted: Thu Dec 07 06 1:43 pm Post subject:

I'd say that yes you can have too long a stack. If the stack is too long, the temp will drop and the draw will go away. Or, conversely, you may not be able to get enough draw to get the smoke moving in the first place.

Rubit · Posted: Thu Dec 07 06 8:21 pm Post subject:

The taller the stack the stronger the draw with the exception of Skybobs comment.

The bigger in diameter a stack is, the higher volume it will suck.

A square tube even though it has more area than a round pipe of the same inside diameter. will suck the same as the round pipe.,
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Alien BBQ · Posted: Fri Dec 08 06 1:35 am Post subject:

Kosmos Q · Posted: Fri Dec 08 06 11:40 am Post subject:

If someone could explain this to me please.

Ok. my tank is 20x70 and my fire box is 18wx20dx20t. What should my stack be if I'm using 5" pipe? Rev. flow
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skybob · Posted: Fri Dec 08 06 12:17 pm Post subject:

Hi there, using Alien's calculations, your air inlet to the firebox should have 21.6 sq" of opening. Now with the 5X5 exhaust, using the area of a 5" circle because of the turbulence from the corners, it comes out to 19.6 sq" for the exhaust. Using the optimum, a 5.5X5.5 square tube would be just right, or go on up to a 6" sq tubing for a little extra exhaust capacity. Once again using his rule of thumb, the length of the exhaust pipe should be 52.5" long.

skybob · Posted: Fri Dec 08 06 3:26 pm Post subject:

Ok Alien, got a question. I went back over my figures and realized that the exhaust VOLUME multiplier is .05 rather than the .005 that my eyes first saw. Is it correct that you're trying to get the volume of the entire stack, rather than the outlet area? If that's the case, and Kosmos is going to use the 5" square tubing, the exhaust stack length would be approximately 18". The volume of his firebox 7200 Sq" X .05 = 360 (volume of the stack) / 19.6 (the area of a 5" circle) would equal 18" length.

Alien BBQ · Posted: Sat Dec 09 06 12:51 pm Post subject:

This is one of the reasons I was asking for input from others to verify to averages and figures. As I figure it:***All of these numbers were rounded***

 Your Fire box opening should have an area of approx 22 inches. A 4.5x5 inch opening should be about right. It does not mean you will run this way all the time; it is just a figure that will give you enough air to allow for higher temps.
 The chimney volume for that size fire box needs to be 360 sq. inches minimum. Based on a 4 inch chimney that would be approximately 23 inches tall. You have a 5 inch pipe so that throws the figures off (there should be an exponential component, but I haven�t figured it at this time) so the MINIMUM length should be approximately 15 inches tall (for that size fire box.)
 The fire box opening (area) should be approximately 58 sq. inches. (6x10 should be minimum.)
 Here comes the tricky part. The length of your cooking section is 70 inches, which would indicate a 52 inch long 4 inch exhaust. (I based all of my original calculations on a 4 inch square pipe) because you have a 5 inch square pipe the length of pipe comes out to approximately 33 inches in length.

So it would seem that there is a conflict between the fire box volume chimney figures (15 inches) and the cooker section figures (33 inches). It would seem so but not really. The 15 inches is the MINIMUM needed for a fire box of your size. The 33 inches is a ratio for the cooker section. So the happy point is somewhere in between.

When I started this, I asked for other working ratios so we could develop a formula. Other things not considered in the equation are the volume of the cooking section. Some smokers may have the same length, but are wider or narrower than what was tested. Only by collecting data from smokers that work efficiently can we come up with a formula that we can use in building.

I have almost the same exact dimensions you have on one of my smokers. The exhaust is 30 inches long and I can get it to cook at 200 degrees and go all the way up to 475 degrees. I am currently helping a guy with a smoker that can not go past 225 degrees with a full load of wood in the box. His smoker has a very little fire box and just can not kick out enough heat to keep it going good.

If your smoker is working well with your numbers then we will add them to the list of variables. If it is giving you problems, then we need to know that as well!
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Alien BBQ · Posted: Sat Dec 09 06 1:02 pm Post subject:

skybob · Posted: Sat Dec 09 06 1:02 pm Post subject:

Glad to see that I was on the right track and following your directions. If you get some free time, you could do the Pitmaster U here in Wichita Smile

Probably be able to give you a tour seeing some fine machinery that's built in the area. Not to mention the number of "Q'ers" in the neighborhood.

Alien BBQ · Posted: Sat Dec 09 06 1:14 pm Post subject:

SoEzzy · Posted: Sat Dec 09 06 4:55 pm Post subject: