Low Loss Stack Cap for industrial and forge use as recommended by Tony Bardol 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, ect.. 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 bringing 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. - Alan-L 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. Maybe a funny story.... Some industries in some states are regulated by the "opacity" of the stack discharge. Opacity is how much it blocks the view of a blue sky. You have to get your eyeball calibrated by the government in order to decide how opaque a stack discharge is. And you have to check stack opacity on a calibrated blue sky day. So one thing you can do to reduce the opacity of the discharge is to add dilution air to the stack. That concentric gap at the overlap of the main stack and the low loss stack cap DOES let in some clean dilution air as Alan suggested. Not that anyone would do something so low down as add dilution air to get below the opacity limit......... Yes, it worked. And yes, I had my eyeball calibrated. I think it's out of calibration now though... Low loss stack cap: Now here I go extolling the virtues of government again. Grin. According to the American Council on Government and Industrial Hygienists (as well as my own experience) The low loss stack cap has far less exhaust restriction and lets less rain into the stack on average than a coolie hat with or without reverse cone or any of the other hats mentioned. A turbine only draws well when the wind is blowing. I have seen the rotating tilted hat work OK on high heat industrial vents. But they always seem to bind up and not follow the wind. Maybe they had cheaper bearings. I don't know why it wouldn't be less expensive or easier than the other methods either. - Tony - Saturday, 10/09/04 00:08:28 EDT