Air intake size requirement question

[hpmer]

Anyone have a formula (or even a sense of how) to determine how much intake air is required for a given system size? I seem to remember Donkey thought perhaps 1/2 the volume or even less is required.

I'm thinking even less, as my current stove uses a four inch horizontal feed tube and a six inch heat riser and goes like crazy once the coal bed is sufficiently built up. So, subtracting the area blocked by the fuel, I'm probably operating at about 1/4 to 1/3 as a guess.

I'm thinking about building a 7" system and wondered if my old charcoal grill top with the four approximately one inch holes would be sufficient as a cover/air regulator for the feed tube.

In the book it looks like the opening in Ianto's feed tube cover is 2" (?) and is feeding an eight inch system if memory serves.

So, 4 one inch holes should be the equivalent of one two inch hole. Ianto indicates that the thing works best with the cover on and the bung removed, so I'm thinking I should be o.k.

Anyone with thoughts? I'm sure the oil burner crowd must have a formula for such a question. I know the whole, "try it and see," but if it is insufficient I'd prefer to scale down the system rather than build, tear down, build again.

[Donkey]

Err.. Ianto's stoves are all six inch. Though in the book the pattern was for eight inch stoves.

I don't think there is (as of yet) a formula for intake, etc.. Though 1/4 - 1/3 seems close.

Somebody's gotta do the testing and discovery to pin down an actual hard ratio. I don't have the time right now, so someone else is gonna have to do it..
You, maybe?

The oil-burner folks might have it though I suspect that each fuel type will have it's own ideal aperture. Also, likely as not, altitude will play a role.

[hpmer]

OK, well I'll build it as I proposed and see if it works. That will at least give some sort of pass/fail on at least one size pair.

[hpmer]

Ianto, p31: "Usually, there's a two-inch bunghole in the center with a threaded cap that you can screw in or take out. Most rocket stoves with eight-inch flue size get just the right amount of oxygen to sustain a clean burn with the lid on and the bung open."

Since the area of a circle is PI x radius squared, that means:

2": 3.14159 x 1" x 1" = 3.14159"
8": 3.14159 x 4" x 4" = 50.265"

3.14159" / 50.265" = .0625 or less than 10% !
-- or --
50.265" / 3.14159" = 15.9998 times the air intake opening

********
I have four 7/8 inch holes in my charcoal grill lid:
7/8": 3.14159 x .4375" x .4375" = .6013" x 4 holes = 2.405"

2.405" x 15.9998 times = 38.4795" flue size (or a 6.20"x6.20" square or 7" round heat riser).

So I guess that's where I'll start the experiment.

[Donkey]

Cool..
My intuition says that 10% is too little..
I could EASILY be wrong though, so start there.. You can always make more holes or enlarge the ones you got.
10% is the throat to face opening ratio in Rumford fireplaces, so the rule might carry over.

It's funny that you caught that in the book.. Good catch. I happen to know that ALL of Ianto's stoves with that arrangement are 6 inchers. Perhaps he's done the same with 8 inch stoves that I'm unaware of. I'll need to tell him about it, it could be a mis-statement.

[hpmer]

If you could ask him that would be great. As I said, I'd rather not have to tear the thing apart if I don't have to.

[Donkey]

If the issue is just the hole size for a cap, seems like the cap can be modified pretty simply.
Heck, an eight inch rocket stove without a cap works fine. You can use a brick or two to modify airflow, just slide 'em up alongside the wood and cover a portion of the feed.
It seems that there would be no need to rebuild anything major for a new cap.

Personally, I'd like to see you use the 10% measurement as a starting point. If it works, we will have learned something. If it needs more, it should be pretty easy to drill more holes in the cap or bore out the old ones, and we will have learned something.

I will ask him though, I'm supposed to call him soon anyway.

[hpmer]

Yeah, I just liked the elegance of the grill cover to give it a "finished" look, rather than a jury-rigged approach. Still, I guess from a testing standpoint, it's worth the experiment. Once the approximate relationship is determined a more finished cap can be found or fashioned.

And I think you may be right about it possibly being a mis-statement. A two inch hole feeding a six inch system works out to about 11% air which seems more likely than the 6.25% with the 2" vs. 8" combination.

[johnjmw]

I thought that 2" hole in the lid was for after the fire has burned down to coals and your letting it go out... I'll have to reread the book again.

Edited for lack of spelling

[Donkey]

Ok..
I just got off the phone with Ianto.
It IS a misprint!
The 2 inch bung has always been used with 6 inch rocket stoves..

I think it should be too small for an 8 inch system, though I STILL think it should be tried. I'd like to see the limits bracketed ASAP..

[hpmer]

I guess I get the gold star for discovering the misprint?

Probably makes more sense anyway. I like your idea about determining the opening with a brick(s). That way we can easily measure the ideal opening by length x width.

[Donkey]

Umm.. It's a good theory. In practice, for best burn, you tend to slide the bricks around throughout. Wood, down in the feed, chokes the fire automatically to some extent. Sometimes the wood arranges poorly and the fire chokes itself out, or down. It's part of the "fiddliness" of rocket stoves. With good wood arrangement and some luck, there's still no real constant aperture size as the fire will want more or less air throughout the burn cycle.
The brick method, so far, has been a favorite. It's cheap, easy to do and works well. Just be aware that the bricks can get hot.

What it seems we're looking for is the best all-round ratio for the bung-type air opening. It would have to be the maximum intake size, the one that let's in no more than the hottest burn needs. It would be great if it were a nice, clean, scalable, rule-of-thumb kind of a ratio that would fit every size stove.... Like the 62% height ratio for beehive oven doors.

I hope that was clear.. A head cold has me a bit muddled..