chimeny pipe

[Johannes]

hey guys,
As you may know from previous post i'm in the process of building my new shop, I'm about to build my chimeny hood and chimeny. A side draf is not an option (structural issues) so its gonna be a pipe right up, with a sheet matal hood (i'm thinking aboud making it hight adjustable), but now I'm having doubs about the pipe's I'm using, theire about 6.7 in (18cm) in diameter, would this do.; i'm not sure.. s i was thinking of putting two of them next to eachother, would this work ? or would this just create a draft between the pipes and thus lower the total draft of the chimeny instead of largening it..? what are you guys opinions about this?
thnx!

[Jymm Hoffman]

I don't understand the structural issues. A side draft chimney can be (as were my last several chimneys) simply old sheet metal rectangular heating duct run down to the fire with a hole cut in the side, placed next to the fire. These were approximately 8 inches by 12 inches. The best chimneys I ever worked with were side drafts built with brick around 12 inch square flu liners. The brick forge I built with a double forge. Only problem I ever had with it was remembering to block off one side while starting only one fire. It was fine to fire both at the same time. I do like larger fires than some, but not huge fires. I used both side blast and 8 inch by 12 inch fire pot for these fires. 6.7 will only work for very small fires and very good, no smoke fuel. But I don't have to worry about chimneys any more since switching to gas.

[yesteryearforge]

side draft relates to the hood and not so much the configuration of the stove pipe or chimney.

Mike Tanner

[Jymm Hoffman]

The best chimneys I ever used were hoodless side draft, as described above. The worst were hoods hanging over the fire with the chimney pipe in the center of the hood.

[Johannes]

i'm well aware of the advantages of side draft or hoods, my question actually is; would it work to use 2 stove pipes next to eachother, instead of one big one (be it either for side draft or hood) cause i hove allot of the 7 inch stuff lying around.

[dablacksmith]

ide use one larger... ive not seen anyone use 2 tho...i would go 8 in minimum...

[Glenn]

There have been a lot of recommendations for 8", 10" and 12" chimneys, followed by bigger is better.

8 inched diameter = 50.2654 square inches
10 inches diameter = 78.5398 square inches or 1.56% increase from 8 inch
12 inches diameter = 113.0973 square inches or 1.44% increase from 10 inch

Is there someone out there that can provide numbers to explain the air flow rates through the chimneys, and why one size is better than another? It must be related to the size of the fire and the air flow needed to move the smoke. Also why is it always round and not square or rectangular in shape?

We need some hard data and numbers to use as a guide.

[eyrian]

Glenn,

I'm not entirely sure about your numbers - isn't 78 a lot more than 1.56% bigger than 50?

The hard numbers for a passive chimney are derived from the stack effect, which indicates that air flow will be directly proportional to the cross-section of your pipe (because it's constant pressure).

Now, at some point, I think there's going to be a dropoff, as the higher airflow reduces the temperature difference (which is also a factor in increasing flow), as you gradually approach what might as well be working outside .

Round is better because it causes less turbulent flow, which causes drag which reduces the flow rate.

[Jmercier]

Glen just used % instead of times, 78 is 1.56 times bigger than 50, or 156%

I have no idea what is being talked about with structural issues preventing side draft, because the chimey for both an overhead chimney and a side draft chimney is identical. Personally I like a good side draft forge, they seem much better for drawing off the smoke from a coal fire from what i've used.

[Glenn]

Thanks Jmercier, for putting the decimal in the proper place.

If your using standard stove pipe, you can assemble two pipes together to make one larger pipe.

[Steve Sells]

I would use the IN HAND 7 inch pipe you have BUT: put 2 sections together and make it a 14 inch diameter flue. Assuming you have the type that is made flat and connects down the side.

I have seen two of the 5 inchers used for a one 10 inch flue... my new side draft is 12 x 12... brick ala JeremyK

[ThomasPowers]

I recently went down to the Habitat for Humanity Re-Store and bought 2 10" dia 10' long spirial seam vent pipes for US$4 a piece to use for my chimneys.