# exhaust diameter question

Discussion in 'Reverse Flow' started by rib150, May 7, 2014.

1. ### rib150Newbie

can I run two 2.125" exhaust at 40.02" instead of one 2" exhaust at 80.04"

Feldon's calculations

cook chamber

height  18"

width    25"

length   43"

fire box

17" diameter

length 25"

size differential  99%

chimney

2.125" inside diameter

length 80.04"

intake

at 2" diameter need 5.42

five 2" holes and one 1" hole all on bottom of burn box.

cylinder is enclosed in a box and adjustable vents on 2 sides that are both 8.5" x 2" - just one of these  would equal 1 vent if it was a square fire box. box around cylinder is just .125" stainless steel to make mating it to the cook chamber easier. I ran two so wind is not a factor. one will probably be always closed.

fire box to cook chamber opening

2" x 22.75" which equals 45.5

calculations call for 45..39

the entire smoker is stainless- so coming across 4" or larger stainless pipe for my chimney is not a normal scrap item where i work. This was made and altered off an old tilt skillet from our kitchen at work, it has a nice 1/2" thick steel plate that has a stainless steel skin on it for the bottom of my cook chamber. I just welded stainless on the lid, to lift and gain more cooking room.

I have not fired it up yet, and want to get everything right before doing so, so i have a fighting chance of getting it right the first time.

2. ### lendecaturalSmoking Fanatic

Where are you Rib? I have some 4" stainless pipe here in North Alabama.....

3. ### rib150Newbie

Thanks for the offer, but i live in michigan.

Can I, cut the space between the two 2" exhaust, half the pipe and then welding sheet metal to to join everything, this would allow a solid chimney that would be 2" x 14". What i"m trying to do is create more cooking area, and a smaller chimney foot print inside the cook chamber. I have to plunge the chimney through the lid. Not enough room on the back, or the side- and it would look silly in the front. Chimney will plunge 12" into the cooking chamber.

4. ### rib150Newbie

lid is 1/8" thick stainless

5. ### lendecaturalSmoking Fanatic

That would work much better than double 2" pipes.

Feldon also has the following note about chimneys:

• 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.

6. ### rib150Newbie

maybe, i"ll run it in 4" and be done with it.

7. ### rib150Newbie

when i do the volume calculation for a tank, they don't convert to the feldon calculations with out making it real short. I can use one hole for one of my temp gauges and increase the other to 4".

8. ### rib150Newbie

lid will need gussets, connected to the pipe for strength to keep the chimney straight when opening the lid.-- Change on thing, and deal with additional problems. Thanks for the help. by the time i'm done, I should have built it on a trailer.

9. ### lendecaturalSmoking Fanatic

If the lid is 1/8", it might be fine since the 2" pipe and sheet metal won't be that heavy. You can always add gussets to the top if you notice much flex.

10. ### rib150Newbie

I punched out both 2" exhaust. I increase the diameter of the exhaust. One is 4" at 22" and the other is 3.5" at 35". I fired it up last night. It ran very well. With starting with charcoal that were white hot before I put them in the fire box, here were my temps.

450 at fire box

210 on both sides of cooking chamber

180 at the tip of the exhaust.

I warmed unit up for an hour and when I closed everything, it started chugging right off the bat.

I could not be happier. I have to put one 4" square x 14" log in the fire chamber every two hours to maintain temp. Not bad.

I'll send some picks later-- this thing is some kind of frankenstien. Didn't cost me a dime. - well I bought 4 gauges at 4 dollars a piece.