Dust collector return air with duct fan

[Bob Michaels1]

I have had an Oneida 3 hp DC for about 13 yrs in a 700 sf shop. Great unit. The noise has always been an issue. I'll be building a closet around the DC within the next week but am concerned about return air. As I understand it, the duct is used to REMOVE air from the closet so the closet doesn't get pressurized from all the air being sucked through the DC and blown out the filter. I called Oneida and they said my DC produces about 1300 cfm and I need 3 sf of duct (1 sf/ 500 cfm). After thinking about it, I asked if I could use a 1300 cfm intake fan but I think it will still require 3 sf of duct to move 1300 cfm of air so no sense using an intake fan. I'm trying to reduce the size of the duct but I don't think I can, even with a fan. Is that correct? Any other ideas to reduce duct size. Thanks.

[Bob Michaels1]

Thinking about it again, since the intake piping on the cyclone is 8" why wouldn't 8" round duct work.

[Paul F Franklin]

Using a return air blower could let you reduce duct size (the blower on the collector is moving 1300 CFM through an 8 inch duct (or whatever) after all, but it's just going to add to the noise. Most folks just use baffled openings with furnace filters or the like. If you have a baffle between the collector and the opening (with enough clearance so the 3 sq ft is maintained) it will cut the noise a lot. Ideally the path for return air would make a right angle turn; that will help reduce noise too, as will insulation in the walls of the closet. If you can move the whole thing outside....even better.

[Roy Petersen]

You may be better off building a passive return, with baffles to mitigate the noise. There have been a few examples here on this forum, with claims of being basically silent and no chance of overpressure (pulling more air out of the enclosure than you're putting in due to piping and bends and so on) in or out using it.

(post above mine happened while I wrote this out)

[Jim Becker]

A passive return path is just fine...just be sure it's "bent" so there's no direct sound transmission through it. I did that with a baffled setup in the ceiling joists, but others have used flexible HVAC "duct hose" to accomplish the same thing. Just up-size it a bit from what your cyclone's outlet to the filters is to insure there's no pressure from restriction. While mine return is rectangular, it's probably about the equivalent in area of 96 sq inches which is larger than the ~50 square inches of the 8" outlet on my Oneida system.

This is a graphic of how my return baffle is setup....

[Roy Petersen]

Jim, is that very quiet, or did it need something lining it like some carpet or something?

[Bob Michaels1]

Thanks for the responses. Jim Becker...it appears that your return path is about 10" x 15" and that is cut in half by the baffle. Also, it appears that your return "duct" is only about 3 - 4' long. Is that correct.

[Mike Kreinhop]

Thinking about it again, since the intake piping on the cyclone is 8" why wouldn't 8" round duct work.

The volume of air leaving the closet must equal the volume of air entering the closet. While you could use the same ducting for input and output, the velocity of air for an 8-inch duct at 1300 CFM would be about 3,725 feet per minute (about 42 MPH), and likely quite loud. If the output was a 22x22 inch opening (just over 3 square feet), the output air velocity would be about 387 feet per minute (about 4.5 miles per hour).

I'm going through a modification of my dust collection closet. The closet has double doors and is totally contained in my shop, but I installed a 23x46cm vent on only one door, using material that was locally available. I now know I need a larger vent opening, so I will either add a similar vent panel to the other door, or replace the existing vent panel with one that is twice the size.

With only the DC system running, the noise is low and it is easy to have a conversation in the shop. However, I always wear hearing protection when the DC is running, so I'm not concerned about any noise.

[Jim Becker]

Thanks for the responses. Jim Becker...it appears that your return path is about 10" x 15" and that is cut in half by the baffle. Also, it appears that your return "duct" is only about 3 - 4' long. Is that correct.

The joist space is ~ 12" x 16" (I joists), so the "duct" is ~6" x 16" and is three feet long making for about 5' of return duct from the closet. Unless I have a gate open, the sound level of the DC is almost "not there". (same for the compressor in the same space) The folded return is not lined with anything but the baffle layer is 1/2" Homasote. The closet itself is "regular" 2x4 construction with a layer of Homasote on the shop side covered with 7/16" T1-11. The joist bays are filled with R15 un-faced fiberglass insulation and the interior surface of the closet is 1/4" perforated pegboard with the rough side showing to the room. The door is a typical exterior, insulated, plain steel door. The total closet size is about 4' x 8'. It could be smaller, but the electrical panel is also in there and that requires unrestricted wall space by code.

[Bob Michaels1]

The joist space is ~ 12" x 16" (I joists), so the "duct" is ~6" x 16" and is three feet long making for about 5' of return duct from the closet. Unless I have a gate open, the sound level of the DC is almost "not there". (same for the compressor in the same space) The folded return is not lined with anything but the baffle layer is 1/2" Homasote. The closet itself is "regular" 2x4 construction with a layer of Homasote on the shop side covered with 7/16" T1-11. The joist bays are filled with R15 un-faced fiberglass insulation and the interior surface of the closet is 1/4" perforated pegboard with the rough side showing to the room. The door is a typical exterior, insulated, plain steel door. The total closet size is about 4' x 8'. It could be smaller, but the electrical panel is also in there and that requires unrestricted wall space by code.

Thanks, Mike and Jim. Very helpful. Jim...do you have a 3 horse DC.

[Carlos Alvarez]

MDF is a great sound absorber and doesn't have its own resonance, so it's a great thing to attach motors to. My DC is outside with no return air. Before I decided to do that, one plan had been to make an enclosure closet with a full height return that made a bend. It would use dead space (just outside the circle of the filters). And being 5' tall, it wouldn't have to be very wide to flow a lot of air. Just a slot along the side, and two MDF panels inside arranged so noise/air would have to make two turns.

[Jim Becker]

Thanks, Mike and Jim. Very helpful. Jim...do you have a 3 horse DC.

Mine is the 2 HP Commercial (predecessor to the Gorilla) but my setup would be perfectly effective for the 3hp system, too. They are not all that dissimilar.

[Tom Bender]

I have designed quite a bit of industrial ductwork and done some noise control as well. As usual Jim has hit the nail on the head. Rectangular ducts transmit less noise than round, soft materials absorb higher frequencies. Use heavier for the lows. At 96 sq in he is on the tighter side which might take a couple percent off his collector volume but not enough to notice. You might shoot for 120 sq in.

Another thing to consider is leakage into or out of the shop. I am guessing that you have a basement shop. If you distribute some return air to the living space your shop will be negative and air will leak in under the door etc. This will keep the dust in better. However if you use anything with an odor it will quickly perfuse the house.

[Bob Michaels1]

Tom, thanks for the reply. The shop is not in the basement, it is a 14' x 32' third car garage with a 10' x 22' "L", all wrapped around the existing 2 car garage. The walls of the 2 car original attached garage are still in place which makes the shop a separate entity. Shop has it's own gas hung heater. Totally separate from the house. All comments are welcome and appreciated.

[Bob Michaels1]

I built the DC closet and pretty much followed what Jim Becker did as far as wall construction and reurn air route. The improvement in noise level is good, probably reduced by 50%. I used an 8" x 30" plywood plenum to route the 180 degree change in return air path; that is, the entire plenum is 16" x 30" with a "shelf" separating the air path. That gives me 240 si of outlet but it still blows the door open (custom made door same construction as walls, no lockset to latch). I can obviously just install a barrel bolt on the outswing door but is there a concern that the motor life will be shortened? Thanks in advance for any advice.