Exhaust duct details...

[mlo1]

I am planning to install a 12" round duct 20' long with a Continental inline 840cfm centrifugal fan. This will be a straight duct run through the wall with a simple pressure activated backdraft damper and exterior hood. Shop size is 26'x30'x11'. Duct run will be at 10' high and 4' from the shops rear wall. Only wall openings are the doors on the shops front 26' wall, which is also the direction from the weather (pressurizing) often making fume evacuation slow.

Desiring the most even scavenging from the ducts length as possible, I am leaning toward 4 equidistant 3"x12" return grilles on the trunk. This is just based on equal sq/in numbers for the trunk and intake grilles. Would be very interested on any input before I made the purchases.

I have attached a couple images depicting my idea. Thanks for your time.

 

[yeldogt]

What are you trying to exhaust?

Why not use an exterior fan ... like the one pictured ? Fantech

How did you size the duct ? a typical hood w/ 1200cfm is 10 round for about 15' max .. if my memory is correct

 

[metlmunchr]

For 840 cfm you should have about 250 square inches of net free area in the grilles. You'd need to look at the net free area in the grilles you plan to use to determine the nominal size of the grilles. IOW, a 10x10 grille for example will have some net free area that's less than 100 sq in, but you have to look at the manufacturer's data to determine the actual number for the specific grille.

Graph of required net free area vs cfm at https://www.engineeringtoolbox.com/air-return-intakes-sizes-capacities-d_1592.html

Undersizing the grilles increases the velocity and resultant pressure drop as well as causing irritating whistling type noises.

It can be difficult to get a good appearance on grilles mounted on the curved surface of conventional round or spiral pipe. Even if you have a set of slip rolls, its really not practical to attempt to roll the grille to fit the pipe contour as the grille ends up looking sorta like a potato chip. On return or exhaust grilles where you're not looking for a particular air throw pattern, its often better from an appearance standpoint to just use some 1/2" galvanized hardware cloth over the openings. It can be formed to the contour of the pipe by hand and held in place with a narrow strip of sheet metal across the top and bottom, screwed to the duct and clamping the hardware cloth in place. Holes in the strips for the screws should be clearance holes as opposed to running something like a tek screw thru both the retaining strip and the pipe. IOW, let the screw pass thru the strip and the hardware cloth and screw into the pipe.

Its also good to avoid cutting long slots into the pipe unless you have the facilities for forming angles to reinforce the edges of the hole. The residual stress present in the pipe from rolling will make it want to bulge outward along a long cut and angle reinforcements are about the only way to pull it back straight so it doesn't look like hell.

 

[mlo1]

Thanks for the input.

yeldogt...I never thought of an exterior fan, but any exterior noise would be a slight concern as this is a zero lot line build with neighbors and I would prefer to be as discrete as possible. Duct size was simply the maximum diameter that fit between framing and siding ribs when steel standing seem metal siding was being installed. So the opening is flashed with a cover plate ready for duct install. Was a last minute detail as siding was going up. My exhaust needs will run from solvent fumes or typical occasional shop chemicals, to exhaust from occasional engines.


metlmunchr...Thank you for the tips and link. I may be making more out of this than needed considering the small size of the shop. My little experience with duct work is that flow dynamics and pressure's need to be addressed for maximum performance. Seems if I want to try and make the entire 20' duct scavenge equally, maybe a stepped trunk with undersize initial (first stage grilles) may be in order. Or maybe consider the size of the shop one single radial intake in the center is best...this is where field experience will play huge I know, and I lack.