Air intake size question.

Poindexter

Minister of Fire
Jun 28, 2014
2,287
Fairbanks, Alaska
I am building a outdoor BBQ cooker. I need to figure out how big a hole to cut in it to let in enough air for the fire to burn while the cooker is otherwise closed.

Proposed firegrate is 12x16x16 inches- cordwood fuel. For stack I currently have 4" single wall pipe, 4 feet long- with a 3 inch restriction at the top. I could go to six feet of stack easily and cheaply, I could go to 6" chimney if I have to and take it up six feet. Does anyone have a wood stove about that size and are you able to measure the air intake?

That was the short version. If you want the long version you might choose to internet search "Knaack parilla" or "Knaack parrilla", the latter of which is the correct Argentine spelling of parrilla. By the time you read my second post on the second page of the discussion you find on a truck forum you will have all the information I have.
 

Corey

Minister of Fire
Nov 19, 2005
2,509
Midwest
Guess the short version of an answer would be if you have a 4" outlet, make a 4" inlet and the ability to stop it down to zero, and you can't go wrong there.

Technically you probably need less inlet than outlet... I typically burn my wood stove with an air inlet about 5% of the flue area, and close down the inlet from there to control temps. Though by 'control', I mean keep the inside cherry to orange-red heat, which is much hotter than you would cook with (I suspect!)

But that is with strong draft from ~15ft insulated flue. With 4ft of single wall pipe, you wouldn't have that strong draft and you probably also have a lot more air coming in through doors / openings than a wood stove - unless you're sealing them with fiber rope?

Either way, if the inlet = outlet, + the ability to shut down either / both, you should be set for control of the cooking temps.
 

Poindexter

Minister of Fire
Jun 28, 2014
2,287
Fairbanks, Alaska
Have a few pics how. I started with a 4824 Knaack box, removed the powder coat, been to the fabricator twice.

Tonight the fool thing is exterior sanded with HiTemp paint on the bare metal, the interior has three coats (burns) with baked on Saflower oil (after sanding), and I have it up on 3 courses of cinder block at (near) final working height, currently lined with free concrete pavers from CL.

This is how big a fire I need to build to have enough glowing red coals to cook on were I to light a fire when I first get home from work and later serve dinner at a reasonable hour.

20201001_192810[1].jpg

My main question is air flow. I want to latch it shut (enclosed cooker not regulated by Division of Forestry) while the cord wood burns down to live coals so I can go chop vegetables and so on. I suspect I may need to make the cimney pipe 6", but I still haven't the foggiest notion how much air intake I need. Because it is a cooking fire "greater than 3 feet in any one dimension", while open, I am subject to DoF regs while it is open.

Imma tag a few people who probably know, with malice towards none, since this thread got moved to the DIY section. @begreen @Highbeam @Ashful @BKVP

TIA (to all ) 4 y2c
 

BKVP

Minister of Fire
Expected peak burn rate in kg/h and FBV?
 

Highbeam

Minister of Fire
Dec 28, 2006
17,996
Mt. Rainier Foothills, WA
I’ve burned small wood stoves in wall tents. The intake holes were small maybe two 1” holes plus an imperfect door seal and a short 4” chimney. I would design your intake in such a way that it can be adjusted to avoid having to get it perfect at the start.
 

semipro

Minister of Fire
Jan 12, 2009
3,884
SW Virginia
Guess the short version of an answer would be if you have a 4" outlet, make a 4" inlet and the ability to stop it down to zero, and you can't go wrong there.
+1
If you want to be geeky you can go with a simplified version of the Ideal gas law and calculate a flow volume using:

V/T (in) = V/T (out) where T is the absolute temp and V is volume. The volume can then be used to calculate a cross-section (diameter).

I did this calculation once to better understand why my stove with a 6" diameter outlet functioned with a 4" diameter fresh air inlet.