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 ........
Edited by DaveOmak - 5/15/14 at 9:50am
This is the shape of the throat and why the area is 27.68 sq in
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?
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"
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!
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.
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?
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....
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.
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.
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...
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...