Duct collection return air baffle

[Bob Borzelleri]

I'm pretty much done with my DC closet and newly piped drops. I built the return air baffle with dimensions that I got from an article that spoke to the size opening needed to handle the volume of air moved by a DC of my size/rating.

While there is no question that the DC sucks up a storm, it appears to draw a bit better when I leave the closet door ajar a bit. Obviously, leaving the door ajar also allows the noise I built the closet to avoid to enter the shop so that seems to be a poor way to improve the draw.

So far, my method of measuring draw is the hand in front of the open blast gate. I'd like to use a more sophisticated method to experiment with and, see if I can reach a good balance between draw and noise reduction. Does anyone know of a reliable tool (hand held anemometer?) that will measure draw efficiency of a cyclone?

[Alan Schaffter]

I'm pretty much done with my DC closet and newly piped drops. I built the return air baffle with dimensions that I got from an article that spoke to the size opening needed to handle the volume of air moved by a DC of my size/rating.

While there is no question that the DC sucks up a storm, it appears to draw a bit better when I leave the closet door ajar a bit. Obviously, leaving the door ajar also allows the noise I built the closet to avoid to enter the shop so that seems to be a poor way to improve the draw.

So far, my method of measuring draw is the hand in front of the open blast gate. I'd like to use a more sophisticated method to experiment with and, see if I can reach a good balance between draw and noise reduction. Does anyone know of a reliable tool (hand held anemometer?) that will measure draw efficiency of a cyclone?

Any restriction, ducting, filters, return gratings, etc. will affect CFM depending on the size of the opening. The restriction is not only the size of the opening but also the path the air must follow and turbulence created to get to the opening- just like bends in your ducting

The only way to measure the affect on CFM is with something like a Dwyer magnehlic manometer which measures and adjust for static pressure and velocity pressure and can be used to compute CFM. Two other ways to compare, not actually measure, CFM are to use a simple water tube manometer to measure static pressure, as SP increases CFM decreases, or put an ammeter on the blower. As CFM goes up so will the current draw.

Remember, static pressure resistance on the outfeed side is also present on the infeed side. Input = output, restrict one, you restrict the other.

[Bob Borzelleri]

Any restriction, ducting, filters, return gratings, etc. will affect CFM depending on the size of the opening. The restriction is not only the size of the opening but also the path the air must follow and turbulence created to get to the opening- just like bends in your ducting

The only way to measure the affect on CFM is with something like a Dwyer magnehlic manometer which measures and adjust for static pressure and velocity pressure and can be used to compute CFM. Two other ways to compare, not actually measure, CFM are to use a simple water tube manometer to measure static pressure, as SP increases CFM decreases, or put an ammeter on the blower. As CFM goes up so will the current draw.

Remember, static pressure resistance on the outfeed side is also present on the infeed side. Input = output, restrict one, you restrict the other.

Thanks Alan. Sounds like the ammeter on the blower is the easiest way to go.

...Bob

[Jim Becker]

I used the joist space to construct a return air baffle for my cyclone closet...it's detailed here at SMC "somewhere" including a drawing. It does the job with allowing free flow of air while still impeding noise. I used Homasote as the duct material to further dampen noise.

[Jim O'Dell]

Here, http://www.sawmillcreek.org/showthread.php?t=89822 post 16. That only took about 15 minutes to find!!! :D Seems we have this same problem every time this subject comes up. I'm bookmarking it for future questions!;)
As you can see, I was one that thought this info had been on Jim's web pages also. Still not sure where I originally saw it.
Think of this type of baffle as a muffler on a car. It works by changing direction of the noise. Sound waves bounce and different angles canceling some out. If you get whistling noise using this design it's not big enough for the air that is moving. Make a second or third one to slow the air down. Jim.

[Alan Schaffter]

The more things change the more they stay the same!!!! It is interesting that a majority of folks who posted in that 2008 thread at Jim's link recommended putting the DC outside and/or discharging outside with or without a cyclone or other separator. Also, many said the loss of heated/cooled air wasn't bad as long as you didn't run your DC continuously.

Wasn't that what I was saying in a thread a week or so ago when everyone started beating up on me! :D:D:D

Anyway to get back to this thread. Remember, any duct, filters, air returns, mufflers, etc. etc. you add to a DC increases resistance (static pressure) and decreases CFM. There is just no way around it, it is physics and a fan curve fact. If you keep adding resistance overhead to your DC system you reduce the CFM everywhere in the system, but most importantly at the source. I do not know what they based their numbers on, likely some arbitrary maximum acceptable reduction in CFM based on a typical system, fan curve, and one of the recommended machine CFM values, but regardless, Oneida recommended a minimum DC closet vent opening of 1 sq. ft. per 500 CFM, but also 6 square feet for a filtered vent opening. (larger is better- or more correctly larger is less bad!). The larger number is puzzling since I have yet to see a home DC that sucks 3000 CFM, unless they were compensating for the vent filter. Anyway, if you compute Oneida's 1 sq. ft/500 CFM figure for a modest 1500 CFM DC it yields 3 sq. ft. = 432 sq. in.. Compare that with Jim B's 6" X 14" joist cavity return. There is a big difference in area!!!! 432 sq. in. vs 84 sq. in. If you use the larger 6 sq. ft. figure, it is double that- 864 sq. in. vs 84 sq. in.!!!! You don't even need to do the math to know the CFM capacity of each are drastically different!!!!! While the flow won't stop, the closet vent adds much more resistance (static pressure) that will affect the entire system, but most importantly collection at the source!

One last comment- I thought I would throw in about CFM and duct size- Most have heard or read the blowing a balloon with a straw analogy- I find that while I understand it, it is not appropriate in this case. When questioning the need for CFM at the source and adequately sized duct work, think of it this way-

Assume you have just created a beach ball sized cloud of dust, the fine hazardous stuff you can't see, around your machine. That is easy to do especially if you don't have a DC. Now lets model what happens on a smaller scale- create a similar, but much, much smaller cloud of (cigarette) smoke. First, try to suck it up with a drinking straw- pretty impossible. Then try to suck it up using the same DC blower with same hp and impeller (your lungs!!!) using a cardboard tube from a roll of paper towels- still not easy but certainly doable! (unlike a DC, you have a limited lung volume so will need to suck it up in multiple attemps)

[Jim Becker]

Thanks, Jim! For expediency, here's the graphic...

http://www.sawmillcreek.org/attachment.php?attachmentid=57221&d=1170798863

[Leo Graywacz]

That turn around you have in there is going to create a lot of static pressure. A lot.

[Jim Becker]

That turn around you have in there is going to create a lot of static pressure. A lot.

Not really. It's sized to be much larger than the air intake to the cyclone and the air flows quite freely. There is no performance difference whatsoever with the closet door open or closed with my Oneida 2hp Commercial cyclone which has a 7" inlet. The baffle is nearly twice the capacity of that.

[Leo Graywacz]

Glad to hear that.

[Kerry Wright]

I'm just finishing up my return ducting. My goal was to keep the noise and return air velocity as low as possible.

I started with plan A:
Duct the air up and through the wall space, exiting at the top into the shop area. I also built a homosote duct running up the inside of the closet to force the air to run past the motor eliminating any heat issues caused from stagnant hot air at the ceiling. Just picture something like Jim Becker's (thanks Jim) idea only running from the floor to almost the ceiling. This provided about 51 sq. inches of return. After testing I decided this was not acceptable. The velocity of return air was much more than I wanted and there was a noticeable decrease in the CFM of the collector.

Plan B:
I had already anticipated this before starting so continuing on with Plan B was no biggie.
Using small strips of toilet paper taped to the return air duct as an indicator of velocity (and restriction) I opened the DC closet door, fired up the collector and started closing the door until the velocity of air at both the door and the duct were acceptable. I then measured the crack in the door (about 3") multiplied it by the height of the door and came up with needing an additional 240" of return air duct size.


The DC closet sits next to the bathroom and both have 10" ceilings so utilizing the wasted space in the bathroom I grabbed another sheet of homosote and built another baffled duct 14.5 x 16.5 that exits into the shop within 2 feet of the other duct. All total, 290 sq inches, low velocity, reasonable sound dampening and best of all little reduction in collector capacity.

Conclusion; Oneida's 1 sq ft/500cfm of return duct could be considered a bit high, but when it counts, conservative is certainly a good rule of thumb.
BTW, I have a CV1800 (1442 CFM)