Pit building ratio calculator

dodis · Newbie Joined: 09 Mar 2010 Posts: 30

Once again, Tom C beat me to it... Smile

I was going to suggest you check for a missing decimal in your calcs, as I also get 25.75" for the 3" stack, and 14.5" for the 4".

And, after reading Alien BBQ's post above yours, I would think best to start with the 3" stack and see how well it draws in operation. If it doesn't you can always move up to the 4", just cut off the stack, gut a 4" hole and weld on the new one. Easier than welding in a patch to go down from the 4" to 3"...
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Mike "Dodis"
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UDS (55 gal & converted Old Smokey Electric)
Weber One-Touch
Broil-King Baron
Red"neck"stone Griddle

hotshot1 · Joined: 17 Jun 2010 Posts: 13 Location: St.Paul Or

hey guys i finally finished the build on my cooker. i put a fire into it and it took 2 -2.6 hours to get 225 degrees out of it. i know the weather isn't the best 35 to 48 degrees outside. is that normal for the temp outside. how long should it take to get to temp in normal conditions. i built it with the calculations alan and doids came up with. wondering if I'm doing something wrong. 500 gallon reverse flow build.

k.a.m. · Posted: Feb 11 2011 Post subject:

hotshot1, I do not come over here much anymore as I do most my work in the cooker section. To answer your question yes 2 to 2.5 hrs. in those outside temps is about normal for your size tank. That is a lot of iron to bring up to speed and stabilize. On the R/F I built which was a 250 gall tank it takes 1.5 to 2 hr. in those type of outside temps. I hope this helps.
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Always remember slow and steady wins the race.

Hybrid Cooker

Rooster's Rebel BBQ · Joined: 31 Mar 2011 Posts: 8

OK, just double me on my counrt math.
I am going to build trailer mounted pit 30x 66
The firebox will be 25x25x25
So if I did this correctly I will need the floowong
Chamber to pit opening 11.25x11.25
Air inlets 3 @ 3x5
Stack 4" 62.25 tall

dodis · Newbie Joined: 09 Mar 2010 Posts: 30

Rooster,

Yes, all your numbers are correct, however, based on Alien BBQ's recommendations on the stack length at 75% of the cook chamber length, it calculates to a 4 1/2" dia pipe at 49" long. It was mentioned that too long and the exhaust can cool off too much and not draw well, and too short doesn't seem to get the momentum of the flow to draw the smoke through properly (all this as best as I can remember without re-reading).

I see you did on the air vents, so, depending on the connection between your pipe and firebox, you may want to make the firebox to cook chamber opening rectangular, just keep the opening area correct (I just tossed in some numbers and got 14"x9" as an example).

Which reminds me, I never did continue to edit the calculator to calc the height and area of a chord on a pipe to use the semicircular opening between a pipe and square firebox. I need to get back to that (one day...) Rolling Eyes

_________________
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 · Posted: Mar 31 2011 Post subject:

Rooster's Rebel BBQ · Joined: 31 Mar 2011 Posts: 8

Rooster's Rebel BBQ · Joined: 31 Mar 2011 Posts: 8

1 more question {lie} I also am putting a warming box on this pit, will this affect the firebox size?

Tom C · Posted: Apr 02 2011 Post subject:

I'm not an expert on warming boxes on a smoker but I would say no.
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Homemade offset smoker 24"x48"
Charbroil CB900 grill

Rooster's Rebel BBQ · Joined: 31 Mar 2011 Posts: 8

1 more question warming box measuring
22.25 width x 23 deep x 34 height.
how will this effect the size of the stack on the pit.

Tom C · Posted: Jun 08 2011 Post subject:

If you're going to put the stack on top of the warming box then it doesn't need to be very long at all. My 6-12 inches.
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Homemade offset smoker 24"x48"
Charbroil CB900 grill

speed70 · Newbie Joined: 07 Jul 2011 Posts: 33 Location: georgia

i tried this chart and its confusing to me also i used a 250 gal propane tank with a 24x24x24 firebox with a 6"x24" pipe and a 6"x16" opening between the tank and firebox and 4 1"1/2" air holes

Tom C · Posted: Jul 08 2011 Post subject:

Alien BBQ · Posted: Jul 08 2011 Post subject:

Like Tom said, the ratios are just recommendations based on what has worked for others throughout the years. There really should be a diminishing effect curve applied to the calculations but it would be purely hypothetical and a sliding curve is beyond my math and programming skills. The bottom line still remains the same, fire boxes over 24x24x24 have minimal effects as they get larger and cooking chambers over 6 feet long, reverse flow smokers, smokers made from propane tanks, and verticals operate outside the original design of the calculator and require more thought in design.

Think of your smoker as an air compressor and your smokestack as an vacuum pump:

The firebox and fire is the compressor. Ask yourself what could limit the amount of hot air produced?

The size and heat produced from the fire are the critical factors.

Too big of a box and the pressure does not build adequately, too small and the amount of hot air produced will not be enough to fill the next chamber.
Next we consider what could affect the size of the fire. Air intake is the first limiting factor. Too little and the fire never builds to an effective temperature; too much and the heat gets out of control or burns the fuel source up too fast.
Once the fire is burning (compressing hot air) what would limit the amount of air moving into the next chamber?

Easiest to identify is the opening from the firebox to the cooking chamber . Too big and you will get fire lap (where a portion of the fire enters the chamber) which makes temperature control extremely hard. This is a common problem with the SnP type smokers around $200. Too small and the amount of heat (pressure) being produced in the firebox is wasted and cannot efficiently fill (heat) the cooking chamber. In reverse flow smokers we also have to worry about smoothing out the air flow as well but that is for another discussion.

Once the heat (pressure) enters the chamber Boyle's law of expanding gasses (I slept thru this one in physics class) applies. In a nutshell the expanding gas will move into and fill the void in the cooking chamber. This movement causes the energy contained within the gas (heat) to lose its potential as it expands and tries to heat the surrounding metal and food. The more ambient air, food, or metal in the chamber; the longer it takes to heat. Different types of wood burn at different temperatures but for the sake of this argument let's average it at 1200 degrees. So as you can see, there is a lot of heat loss thru absorption, transfer, and the heating of the surrounding areas.

So basically I have built a POS cooking device that I am having trouble with keeping the temperature constant. ...... Enter the vacuum pump (smokestack). To this point my firebox has been kinda passive in its design and function. Sure I used my air intake to control the amount of fire and heat produced in the box but the law of expanding gasses is the only thing controlling or limiting my air flow into the cooking chamber. When I add my exhaust stack to the equation.... now I have a active control.

Discussion of heat rising, expanding gasses, or the differences in molecular weight between hot and cold air does not change the fact that the diameter, length and configuration of your exhaust stack makes a difference in the amount of vacuum (draw) your smoker has. If you have ever snorkeled underwater as a kid you surely had the bright idea that if you just breathed thru a garden hose you would never have to come up for air. Well after about 30 seconds, you discovered that that idea does not work. The reason is your lungs cannot create enough vacuum to pull enough air thru the hose to keep you alive. Air has weight and hot air weighs less than cold air. As the hot gasses expand in the cooking chamber they find an outlet via the smokestack. The air enters the enclosed stack and tries to expand. Eventually it reaches the end and exits the smoker. That movement of expanding gasses creates a vacuum. Too short of a stack and enough vacuum is not created, too long of a stack and the air cools and (back to our garden hose snorkel) weight too much to leave the stack. The secret here is air flow and volume to control the amount of heat entering and leaving the cooking chamber.

So why did I break into this long discussion on expanding gasses and smoker design? Well I like to know how things work and I do a lot of experiments on how to make them better. Some of my ideas are great, some are not. The difference is I have time and equipment to monkey with these things so others can learn from my mistakes. Others on this site have time, materials, and experience that I don't have so we all learn collectively as a group. The pit builders calculator is not a "punch in your numbers and get the only answer" device. It is based on observations, averages, and best guesses. Over the years it has evolved thanks to Tom and a lot of others that added to the original observations. Think of it as a guideline for general development. However, remember that it was developed for average style/size/shape smokers . For specialty applications, you need to know the physics involved and then share your triumphs and failures will all of us so we can all learn.
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Tacket · Posted: Jul 12 2011 Post subject:

I've used the calculator to get the desired volume for my smoke stack -- would this volume be only for the portion of the stack that protrudes from the cooking chamber or would that volume also include any smoke stack mods I've made inside the cooking chamber itself (ie Char Griller mods)?

Thanks and I've learned so much from lurking on the forums the past few weeks! Will make the formal introduction soon.

... Tacket

bones64 · Joined: 12 Sep 2011 Posts: 1

Thanks for all the great information. I have a commercial stainless oven with gas burner I want to remove the burner and add fire box this is a very well insulated cabinet all welded seams.what I would like to know is should I still follow the 1/3 rule for the fire box? the cooking chamber is28x28x55 that would make my fire box aprox. 20x24x30 this seems over kill for this. any thoughts?

ihuntbear · Joined: 25 Jan 2012 Posts: 15 Location: eastern canada

on a reverse flow does the opening under along the bottom where the heat and smoke go have to be the same sq inches as the opening from the firebox to cooking chamber??it makes sense that it would be but im a nubie at this..The calculater says i need 60.5 sq inches from fire box to cooking chamber.so do i make a plate 16 inches wide and 5 inches off the bottom that would give me a 1/5th moon shape .My tank is 24x48 and my fire box tank is 20x24

Rgrigg77 · Joined: 18 May 2012 Posts: 1

Hello everybody, I'm new here and already you have probably saved my smoker building project from doom with the smoker calculator. Great tool !!
Question: when it gives you a specific length for your stack, is that total length vertically or does that include the bend on a 90 degree part coming out of the smoke chamber???
Or is it the length of pipe before any bends are added?
Thanks everyone.!!

feldon30 · Posted: Sep 12 2012 Post subject:

I've updated my BBQ Calculator based on Alien's research and Tom C's excel sheet. I added a note about RF, cleaned up some of the text, and added labeled diagrams of Offset and Reverse Flow smokers.

Please let me know if you spot any mistakes or have any suggestions on how I can improve it.

http://www.feldoncentral.com/bbqcalculator.html
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22" Weber Smokey Mountain + Weber Spirit

dodis · Newbie Joined: 09 Mar 2010 Posts: 30

Feldon30,

Very nice work on the web page version! And the illustrations should help quite a bit as well.

Since I am not a programmer, best I could manages was the small excel tweaks I did. And I have since been off track with any grill construction and spreadsheet improvements.

The last item I was hoping to add, and would be a suggestion for your calculator as well would be a method to calculate the required offset of two pipes when building a round pipe smoker and cutting off part of the pipe to build the firebox. The correct offset would use the area of the two chords/semi-circles of the diameter to make the firebox to cook chamber opening the correct ratio. I just could never get my head around backing in to the calculations. I suppose a diamond shape approximation may be sufficient for the opening area calculation though.

Once again, great job on the web page!
_________________
Mike "Dodis"
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UDS (55 gal & converted Old Smokey Electric)
Weber One-Touch
Broil-King Baron
Red"neck"stone Griddle