Is Feldon's Calculator really flawed?

daveomak · May 15, 2014

Bob, morning....... If the FB/CC opening is as drawn, 3.75" fully cut out of the bottom of the CC, that number is ~ 38 sq. in. .... for the end of the CC, if your test the smoker and the FB overheats due to lack of air flow, Ribwizzard came up with a curved tunnel in the end of one of his smokers that worked awesome.... that tunnel is in red on the square drawing.... something similar, to move heat/smoke, will work without cutting into the RF plate....
EDIT: In most cases, the opening at the end of the RF plate can be oversized as the FB/CC opening will regulate the heat/smoke just fine...

.... click on pic to enlarge ........

Bob in Mo.jpg

daveomak
May 15, 2014

bob in mo · May 15, 2014

This is the shape of the throat and why the area is 27.68 sq in

cross_2.jpeg

bob in mo
May 15, 2014

Here's the approach I'm taking to come up with the correct numbers:

- Set Feldon's calculator to the correct cooking chamber volume. This would be cooking chamber volume minus baffle plate volume.

- Set the calculator to 100% of the calculated firebox volume. My understanding is that this should be the volume above the firebox grate.

- Take the Firebox to Cooking Chamber Opening value and multiply by 1.5. This is my new throat area.

- Adjust my End-Baffle clearance to equal of be slightly larger then the new throat area.

- Adjust the length of chimney as required

Am I on the right track?

daveomak · May 15, 2014

Is your FB round ??? Where did you come up with the ideas "bold and in pink"

Bob in MO said:
This is the shape of the throat and why the area is 27.68 sq in

cross_2.jpeg

bob in mo

May 15, 2014

Here's the approach I'm taking to come up with the correct numbers:

- Set Feldon's calculator to the correct cooking chamber volume. This would be cooking chamber volume minus baffle plate volume.

- Set the calculator to 100% of the calculated firebox volume. My understanding is that this should be the volume above the firebox grate.

- Take the Firebox to Cooking Chamber Opening value and multiply by 1.5. This is my new throat area.

- Adjust my End-Baffle clearance to equal of be slightly larger then the new throat area.

- Adjust the length of chimney as required

Am I on the right track?

bob in mo · May 15, 2014

LOL - no idea of how or why that is bold and pink - That Aint Me

But yes, the firebox is round.

daveomak · May 15, 2014

Answer the question of how you came up with the ideas that I put in bold and pink.....

daveomak · May 15, 2014

Bob, morning...... Looking at your "ideas"... You are on the wrong track.... Looks like you read several threads, comingled the calculations, and have confused the calculating process..... Dave

Here's the approach I'm taking to come up with the correct numbers:

- Set Feldon's calculator to the correct cooking chamber volume. This would be cooking chamber volume minus baffle plate volume.

- Set the calculator to 100% of the calculated firebox volume. My understanding is that this should be the volume above the firebox grate.

- Take the Firebox to Cooking Chamber Opening value and multiply by 1.5. This is my new throat area.

- Adjust my End-Baffle clearance to equal of be slightly larger then the new throat area.

- Adjust the length of chimney as required

Am I on the right track?
[/quote][/quote]

bob in mo · May 16, 2014

So this is where I'm at on my calculations for my RF smoker.

Could someone check my numbers.

True Cooking (round) Chamber volume:

8924 cu in

Firebox (round) volume above the grate:

2971 cu in

Firebox Intake max area (below grate)
20 sq in

Firebox-to-Cook Chamber Opening
36.14 sq in

Baffle Plate End Clearance

38.26 sq in

Chimney Length 22.5"

lendecatural · May 16, 2014

Those numbers look good except for the chimney length. There is a note on Feldon's as follows:

Enter the diameter of your chimney pipe to find out how long it should be.
• A chimney that is too short may produce insufficient draft (drawing of air). A chimney that is too long may cause the air to cool before it exits, reducing effective draft and worse, dripping of exhaust materials onto food!
• Many horizontal smokers have an exhaust between 30-40 inches in length, but there is no hard and fast formula.

I assume you are using 3" pipe, run it up 30 inches it would be better.

You may also want to consider taking some of your intake area, say 4 sq-in and place it inline with the fire instead of having it all under the grate. Some guys report smokers running much better when you balance the two allowing easier flow across the fire and into the cook chamber.

You are ready to make some sparks!

bob in mo · May 16, 2014

Thanks Len for the comments.

I agree and will be modifying the air intake.

Curious, how did you determine the 30 to 40 inch length on the chimney?

lendecatural · May 16, 2014

There is a note in small print under the "Chimney Size" block on the Feldon's page. I quoted it above and tried to color the font in red but wasn't successful. All chimneys need a column of warm air to create a draft and they recommend it be around 30 to 40 inches to get good draw and not have the smoke cool too much. Cooling of the air column isn't an issue in a fireplace chimney since they are usually made from stone that isn't much affected like a bare pipe in the breeze. It makes sense to me.

txsean · May 16, 2014

So what do you think is more important, getting the required volume (ex, 5" pipe at 24-ish" tall) or using a smaller pipe at a longer length (ex. 4" pipe at 30-ish" something tall)? And just to clarify, this length is also based off of 100% sized FB numbers?

lendecatural · May 16, 2014

4" at 30-ish" tall would be better, it would have a bit more length to create a good draw. If the height is too tall, the pipe would tend to cool the smoke column which would reduce the draw. Realistically, I don't think you could tell much difference between the 4 and the 5 using the heights you have listed, the pressure difference between the two over 6" is a really small number....

bob in mo · May 16, 2014

Thanks Len. I'll stretch that chimney out a bit.

have a nice weekend.

rikun · Aug 20, 2014

LenDecaturAL said:
Taking your numbers for area, it equates to a cylinder of 36" diameter and a length of about 51". Your fire box size is on the money and I would recommend using the 32" dimension as the cook chamber width end. Make it as tall as you want, the Ribwizard has some clever designs on firebox floor and ash dump. The segment height will need to be 9 3/4" since Feldons gives a 147 sq-in recommended size. 147 x 1.5= ~220 and 9 3/4" will result in 223 sq-in opening and area under the RF plate. Here's your Feldons link.

http://feldoncentral.com/bbqcalcula....61&fi=6,2,0,55.30,4.61&fc=13.71,19.38,147.46

If you plug in the radius of 18" and segment height ED of 9 3/4" into the circle calculator, you get the 223 sq-in. It also shows the segment chord AB as 32" to match up to the width of your fire box. http://www.1728.org/circsect.htm

If you cut your door at 9 o'clock and 12 o'clock and install your grate at the 9 o'clock, you will have 8" above a 1/4" RF plate since you are using 9 3/4" from the bottom of the tank. That's a good distance.

Your tank is big enough for two grate levels so you may want to put the stack down 3 to 4 inches into the cook chamber, the top grate will run a little hotter than the lower grate but you can use that to your advantage once you experiment and learn the differences.

Hope this gets you off and running and feel free to ask questions, this is a great forum with lots of guys willing to lend a helping hand.

Len

Wait,

I'm still confused. You guys said calculate with 100% firebox and multiply by 1.5x, but this example is calculated with 106.6% firebox and multiplied by 1.5x?

I've always calculated with my FB's real size and multiplied with 1.5x like in this example...

lendecatural · Aug 20, 2014

100% in Feldon's then multiply by 1.5 is correct. I didn't keep messing with until it came out to exactly 100%. Within 6% is in the noise for these calculations and on the conservative side.

coon8448 · Nov 1, 2014

hey keny do you have pictures of the two smokers you made out of one tank I am wanting to do the same thing. I would like to see what you came up with.

thanks

keny · Mar 27, 2015

I'm sorry I haven't been on here for a while...no, I still don't have any pics. I'm afraid there has just been too much going on for me to work on the smoker.(car repairs, home repairs etc. etc. etc...) I'm hoping to get working on it soon. I've acquired an axle and wheels for free, and have started getting some pieces around to get started, but I work for an oilfield service, and with the price of oil being low, my hours have been cut so my paychecks are smaller for a while, which also isn't helping...

daveomak · Mar 27, 2015

Morning Ken..... when you can get back to the build, we will be here...... :110: ... big popcorn time......

nick pass · Jul 18, 2017

Here is a vertical design where the cook chamber is stacked directly over the firebox; I believe this design offers better heat distribution and smoke flow than similar stacks because the cook chamber and firebox are better separated (dimensions are for example, but I think this is a good size; easy to insulate and hand-truck). Maybe y'all could tell me if I'm missing something.

smokem.png

nick pass
Jul 18, 2017

Is Feldon's Calculator really flawed?

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