Cyclone closet and return air ducting

[Michael Sapper]

I’m moving my 2HP Grizzly GO440(1354 CFM) Cyclone DC from inside my heated shop to another portion of my garage that is not heated during our cold Indiana winters. My ducting main is 6” reduced from the 7” cyclone intake. The reducer is attached directly to the DC intake.

I intend to build a closet surrounding the DC. The closet enclosure will be constructed as air tight as possible. One reason is noise suppression and the second being to be able to get the heated air returned to my shop as efficiently as possible. For this second purpose I am proposing to install flexible insulated ducting in the ceiling of the closet that will pass thru the unheated garage attic space and be returned to my shop area.

Simply put the DC will suck the (heated) air into the closet space and being an air tight area, once the closet becomes pressurized, the air will then be naturally “forced” into the return ducting back into the shop area.

Is there a formula to follow to figure the proper size return ducting? I will have the return ducting runs at least 15 feet long to minimize noise transfer. I’m guessing that since the incoming closet air is forced in via the DC and the outgoing flow is unforced, I’m going to need significantly larger return ducting over the 7” inlet size. So will two 8” return runs be sufficient? How about one 12” run or two 10" runs? Any help would be much appreciated, thanks!

[Steve Peterson]

2 to 4 times the cross sectional area is my gut feel. This would keep the air speed 1/2 or 1/4 of the inlet velocity. Two 8" or one 12" duct sounds about right.

It would seem good to allow a few extra feet of duct to create a gradual "S" shaped curve. This might help prevent a short circuit path for the sound waves.

Steve

[Michael W. Clark]

Agree with Steve, except I would err toward 4X (or more) the inlet cross-sectional area. You want minimal or negligible losses in the return duct. The higher the losses in the return duct, the higher the pressure in your closet to push the air, and therefore the tighter your closet has to be to prevent leaking (or to work).

You may want several return grilles on the ducts in the shop, keep the velocity low on them. I think recommended velocity is around 300 FPM or less to minimize the hissing noise from air flow. Titus HVAC has some literature on grille selection if you want to dig into it.

If your closet is adjacent to the workshop wall, you could put a louvered damper in the common wall. A baffle in front of louver should help reduce noise.

[Jim Becker]

My return is actually in the ceiling and uses the joist space. It also doubles back to prevent direct noise radiation.