I'm Baffled....

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Forum topic by Jerry posted 03-27-2015 02:27 AM 1512 views 0 times favorited 6 replies Add to Favorites Watch
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2613 posts in 1616 days

03-27-2015 02:27 AM

Topic tags/keywords: question

Okay, I made this lovely Thien baffle for my new HF dust collector.

and my plan was to vent it to the outside, I’ve read that’s the best thing to do, especially since the HF filter bag is only good for 5 micron particles, and from what I’ve read, the most dangerous ones are in the 2-3 micron range. So I thought that if I built a cover to replace the top filter, installed a 4” port in it, that I could vent the overflow outside and there would be NO dust in the shop from the dust collector. This is what it looks like.

Here’s the problem, ( and I bet some of you are already hootin’ about what you see here ) the air pressure coming from the impeller is so powerful that I’m afraid that even if I screwed this cover in and sealed it up, that it might not hold, and that furthermore, the back pressure might damage the motor. It’s so strong that if I start the thing up and try to hold the cover down, I can’t get it down into the cannister, it’s like a hovercraft or something.

I know that people vent these things to the outside, but I’m confused as to how that can be done if the air pressure is too great to direct the flow.

Any ideas or insights will be greatly appreciated.


-- There are good ships and there are wood ships, the ships that sail the sea, but the best ships are friendships and may they always be.

6 replies so far

View OSU55's profile


1633 posts in 1957 days

#1 posted 03-27-2015 03:08 AM

If venting outside, you don’t need the bag portion you show. The outlet of the blower goes directly outside. The thien baffle is used in a container (trash can) prior to the blower.

View bbasiaga's profile


1202 posts in 1963 days

#2 posted 03-27-2015 03:50 AM

Yes, I think generally the baffle and collection can are put on the suction side of the blower. This helps to protect the blower from the chips impinging on the impeller, puts the dust collection bin under negative pressure to further help avoid fugitive dust, and allows a ‘clean’ air stream to be vented if desired.

Unless what I’m seeing in the pictures is two different things and that wood cap with the 4” port is not directly over the thiene baffle and garbage can. In that case, you have really choked off the outlet of that blower. It has something like an 8” hole in the separator above the bag, and then all that filter bag area to discharge over normally. Choking that down to 4” will put some backpressure on the system, reducing your air flow and therefore dust collector efficiency. See if you can double the size of that port, or just go ahead and put the whole mess with the filter bag on it outside. Or, if your separator is good enough up stream and you really don’t need a secondary step, just do away with it all and pipe the fan directly outside.

Note that with fans, too much back pressure is not as bad for them as not enough back pressure. Too little back pressure and they will move too much air, draw too much current, and overheat and ultimately destroy the motor. Sounds counterintuitive, but its true. The amps drawn are related to air pumped more than the pressure that air is pumped at. So when the pressure is low the fan moves more air, and draws more amps. Consider putting a blast gate in the system somewhere so you can slow things down after you remove the filter assembly and its associated back pressure.


-- Part of engineering is to know when to put your calculator down and pick up your tools.

View daddywoofdawg's profile


1028 posts in 1543 days

#3 posted 03-27-2015 06:36 PM

dust comes from say saw-trashcan/barrel out the outlet-to the DC bag; to vent it outside to would be the same but go out at where the bag collects you would have a “hose/duct” vented to the outside.the barrel is a pre-filter to collect the big stuff.

View firefighterontheside's profile


17961 posts in 1824 days

#4 posted 03-27-2015 06:48 PM

My understanding would be that the can and separator should be on the negative pressure side of the impeller. Bigger chips and dust drop into the can and the smaller stuff goes on and thru the impeller and then it vents to the outside. The way you have it, all of the debris and dust are going thru the impeller. One other thing to consider is that since you are venting to the outside you need makeup air coming into the shop. In a basement shop or some other connected shop you have to be sure your makeup air is not coming thru a furnace or water heater flu. Those would be the point of least resistance.

-- Bill M. "People change, walnut doesn't" by Gene.

View GregD's profile


788 posts in 3104 days

#5 posted 03-27-2015 07:08 PM

I have something similar on a DC of similar size except that I replaced the filter bag with a canister filter from Wynn. I have some pictures on my blog.

The turnbuckle/bracket thingies I use to hold the filter on the separator ring could also work to hold your discharge plate onto the separator ring.

I’m confident that you don’t have a pressure drop / flow rate issue. If you have any concern increase the discharge hose to 6” (you can find it and fittings online). Compared to a felt filter bag, particularly after it is dirty, what you have is likely to be much lower pressure drop.

-- Greg D.

View tomsteve's profile


772 posts in 1187 days

#6 posted 03-27-2015 07:13 PM

The then baffle in a separate drum/ seperator before the blower/ impeller. Then outlet on blower/impeller plumbed outside.

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