Low Loss Stack:

Smoke Stacks, Chimneys and Flues


  
I have been having problems getting my chimney/hood to draw the smoke from my coal forge in my studio.

I have a side draft forge hood based on an article in ABANAs hammer's blow. The article is by Brian Gilbert and based on one in Loonie Farmer's shop.

I draw air from out side the building to provide the blast for the fire. I dont really want to put a fan in the chimney or hood. The building is not air tight and also at the base of a hill. I could make it air tight but i dont know if that will make much difference.

I was hoping you guys would have some good ideas.

Hayes - Wednesday, 06/02/04 00:03:30 EDT

Side Draft Hood Problem: Hayes, You cannot assume that we have specific back issues of the Hammer's Blow or know the details of anyone else's plans.

Here are some guesses. Your stack is too small. 10" is recommended. 8" works SOMETIMES but is NOT recommended. 12" will generally suck up the entire shop.

If your air is coming from outside AND the shop is tight then you do not have enough fresh air for the 80% cold air sucked up the stack. Having air for the fire only is NOT enough.

If your stack is not high enough above the roof line OR you are in a wind pocket (surrounded by large buildings, trees, hills) it may not matter HOW big or fancy a stack you have. Two feet above the nearest roof within five feet (I think) is the rule. That may be ten feet. The BEST practice is to have it higher than the peak of the roof no matter how far away that is.

Drawing (c) Jock Dempsey Click for detail Stack caps can screw up the works. Thinking cap types MUST have a minimum of one diameter clear height under them AND work MUCH better with an inner cone pointed downward to reduce turbulance. The best high efficiency caps are the turbine type but I worry about their life exposed to coal ash. The next best is the low-loss type posted here by Tony and with a drawing I've burried somewhere. . . and need to post where I can find it. . .


Send that $90 to Dempsey's Forge, 1684 Mitchell Mill Rd. Gladys, VA 24554.

- guru - Wednesday, 06/02/04 02:21:21 EDT


Jim I was in the archives anyway, so here you are.

Alan L's first post replying to you below:
For Tony 's low-loss stack cap to the guy who wanted a good draft... If you're using round duct for a chimney, say, 8 inch (in my case) take off your old cap and get a section of the next larger size duct, 10 inch for 8 inch chimney, etc. This bigger duct goes atop the chimney with some overlap. How much? The bottom must extend at least one diameter below the top of the smaller pipe, and the top must extend at least 3 diameters above the top of the smaller pipe. It's held in place by brackets, blocks, or whatever. The point is to have an equal space between the sides of the smaller pipe and the bigger one. Picture a telescope tube to get the idea. It works in at least three good ways: It eliminates any stack pressure loss since the rising column of smoke doesn't have to turn, I suspect it actually increases draft when it gets warmed up due to convection bringning air up between the two ducts, and it keeps out rain better than a pointed cap, believe it or don't. This is because rain doesn't fall straight down. It will go in the top, but it will hit the sides of the bigger duct and run down to drip out the bottom on the OUTSIDE of the smaller duct. I tried it and my shop was immediately smoke free, and the neighbors thought I was stoking a boiler, there was so much more smoke coming out at such a higher velocity! Tony explained it to me better, but that's the gist of it.
End of Alan's post, my clarification below:

Low loss stack cap: Alan, I think you explain it better.

The over lap between the cap section and main stack only needs to be 6 inches. And the cap length should be 4 times the diameter of the main stack. So if the main stack is 12 inch diameter, you should use a 54 inch section of 14 inch diameter for the cap. 4 times 12 = 48, plus the 6 inch overlap = 54. See, I said you explained it better. I must have told you three times the diameter by mistake! Grin.

There ya go Jim. Guru, maybe you should sell these in the store. Grin.

Tony - Wednesday, 05/23/01 23:08:09 GMT


Stack Cap: Jim, you will have less problems with birds if you use the low loss stack cap we talked about. I gave dimensions for a 12 inch stack and see Alan's good description of operation. Most birds are not attracted to an open top pipe unless it is small enough in diameter for them to build a nest on. Birds like a hat section cap because it keeps the rain off their nest. The low loss cap keeps the rain out of your stack, but not off a birds nest. Don't put a screen on top of the low loss cap. No need for birds, and it will reduce the draw considerably. But if you have lots of sparks going up and out the stack, you may need a spark trap close to the forge. A smoke chamber should act as a reasonable spark trap for coal forges.

If you want to use a higher loss, lower draw, hat section cap instead, you will want to have something around it to keep the birds out.

Tony - Wednesday, 05/23/01 16:39:00 GMT

Low loss stack top: Jim, You've got it mostly right. Iíll try to clarify. If your stack through the roof is 12 inch, you will want a 54" long section of 14" diameter duct. Your 12 inch stack can be cut so that it is at least 3 feet above the roof or more. Make 4 spacers that are six inches long and 1" by 1".

1" by 1" by 14 gage steel tubing would be ideal for the spacers. Slide the 54" long section of 14" pipe down over the 12 inch stack coming through the roof. The overlap should be six inches, so you will be adding 4 feet to the height of the 12 inch stack. Slip the 4 six inch long spacers in between the 12 inch and 14 inch pipes at the overlap, space them equally at North, South, East and West and screw the two pipes together through the spacers. Support the stack and cap as required.

Yes, you will now have a 14" hole that the exhaust comes out of. And it will be 48 inches higher than where your 12 inch stack ended. 54 inches minus the 6 inch overlap.

Make more sense? Donít think of it as a cap. Think of it as a stack extension.

The total height of your stack above ground should not be more than twice the height of your building. So if your building is 12 feet from the ground to the peak, your stack should not be more than 24 feet from the ground. With the low loss cap, the gap between the two pipe diameters should be at least 3 feet above the roof, so with a 12 foot building height, and a 12 inch main stack, the top of the stack would be at least 12+3+4=19 feet above the ground.

Let me know if it is not clear. Yeah, a picture would be worth a thousand words. I could fax a sketch if the guru wanted to post it. Or you could buy the book it came out of for $90. (SEE DRAWING ABOVE) Grin. Hope this helps.

Tony - Wednesday, 05/23/01 23:08:09 GMT


References and Links


Copyright © 2004 Jock Dempsey, www.anvilfire.com

Counter GSC