Putting impeller "after" baffle and filter

[mxc106]

I recently saw an article and accompanying youtube video from John Heisz at ibuildit.ca (I can't post the external link, but I'm sure you can find it) where he adds a Thien baffle to the dust collector ring as many have done before, but he still uses the filter bag, and the impeller is "after" this setup pulling air through the system.

This seems to have some good ideas in it (impeller is protected, small footprint), but typically you want the output port of the separator to go down inside closer to the baffle right?  Would his setup decrease separation efficiency?  Could a "pipe" be added to such a system to increase the separation, but keep the other aspects of the system?  He mentions for his shop this works well mainly with table saw and chop saw dust.  Do you think this won't work very well for planer/jointer shavings?

Thoughts?

[galerdude]

With planer shavings being considered different than regular sawdust, or so it seems, I did a search on the main page of this forum using "planer shavings". Came up with 23 threads that address it. Might be interesting reading while you wait for other responses.
Wish you the best with your design and build!

[jdon]

Caveat: I do not have any formal training in air flow technology, nor am I an engineer of any sort, so as they say, "my advice is free, and worth every penny."

That being said, by putting the entire collection system upstream of the impeller, you're basically turning it into a vacuum pump. With the conventional location of the impeller, it works two ways: 1) as a vacuum pump, drawing air (and dust, etc.) through the inlet, and 2) as a blower, forcing air through a filter bag/canister, or venting outside.

My gut feeling- again, not a professional!  :P  is that dust collector impellers work more efficiently, i.e. achieve more air flow, when they act more as blowers (pushing air against a resistance- such as a filter) than when acting purely as vacuum pumps, "sucking" not only through the ducts, separator, etc., but also through the filter.

Somebody with an air flow meter should be able to tell pretty easily  in a system, identical in every way (ducts, separator, filter, impeller),  except impeller location (traditional, or at far end), which has better flow.

[mxc106]

My gut feeling- again, not a professional!  :P  is that dust collector impellers work more efficiently, i.e. achieve more air flow, when they act more as blowers (pushing air against a resistance- such as a filter) than when acting purely as vacuum pumps, "sucking" not only through the ducts, separator, etc., but also through the filter.

Hmmm.  I didn't really think of it like that.  I'm not a professional either, but if I think of the system as a continuous loop, air comes in somewhere, goes out somewhere else, there's a mechanism to move the air (the impeller) and some resistances (ducts, filters, etc.). In the loop concept the inlet and outlet are where the loop comes back together but it is obviously a large reservoir of air to make that loop.  I would not expect moving pieces around to different positions in the loop would affect the flow (i.e swapping the filter and impeller) any more than putting the separator before or after the impeller. 

Also, the folks that vent outside aren't pushing against any real resistance, but they have removed the filter resistance from their loop, so they might get more flow. In that case their systems are only sucking air with less resistance in the path.

[Schreck]

I recently saw an article and accompanying youtube video from John Heisz at ibuildit.ca (I can't post the external link, but I'm sure you can find it) where he adds a Thien baffle to the dust collector ring as many have done before, but he still uses the filter bag, and the impeller is "after" this setup pulling air through the system.

Thoughts?

This arrangement encases the filter in a wood cabinet that will be under negative pressure;  it will probably experience more air leaks than the standard arrangement having the filter downstream from the blower.  This will reduce the system CFM at your tools, so any benefit that might exist would probably be lost.  Requires more material. Hides the filter, making cleaning more difficult, less likely to occur....

[mxc106]

This arrangement encases the filter in a wood cabinet that will be under negative pressure;  it will probably experience more air leaks than the standard arrangement having the filter downstream from the blower.  This will reduce the system CFM at your tools, so any benefit that might exist would probably be lost.  Requires more material. Hides the filter, making cleaning more difficult, less likely to occur....

All valid points, but let's separate the implementation from the concept.  I agree with you that this could use more material, be leaky, hard to clean and the like, but if things were ideally sealed yet could be reached for maintenance, is there anything wrong with this setup?  He even points out the need for sealing and questions how the top chamber "door" is attached for cleaning purposes.

I think it's valid to say putting a box around the filter bag could increase the "resistance" of the bag just as adding more turns to the ductwork hurts flow, but I don't know if that's true. Could that be abated with the proper sized upper chamber? 

Before this I hadn't seen a setup where the air was "pulled" through the whole system.  Are there fundamental reasons for that, or are the reasons simply that it's difficult to implement in practice for some of the reasons stated?

[Schreck]

Before this I hadn't seen a setup where the air was "pulled" through the whole system.  Are there fundamental reasons for that, or are the reasons simply that it's difficult to implement in practice for some of the reasons stated?

Implementation issues aside, what do we gain by filtering air before it enters the blower?  The separator will have removed ~95% of the wood waste before the filter, so only fines would be removed by the filter.  What impact do fines have on the capacity of the system?

The enclosure around the filter will add some pressure drop that you would not have in a typical system; I would bet this is a larger negative than the positive from having no fines enter the blower. 

So who has seen tests on blower capacity with and without sawdust and without fines?