# Designing a Shop Air Filter



## Tango2 (Jan 2, 2017)

So although this is my first post here, I've been directed to these forums many times over the years when looking for information on various woodworking ideas. Now that I'm getting more serious about woodworking, I figured it was time to join ;-).

My current project is dust collection. I've just invested in a HF dust collector (on order), and have some mods planned for it. I know that out of the box, the 5 micron filter is less than ideal. I also have several tools that create dust that will either be hard or impossible to connect to the DC. Because I have an HVAC blower that I've been using for air circulation for almost 20 years, I figure I should take it to the next level.

My idea is to build a square table on wheels to house my blower/air filter. This way, it can double as a mobile workspace. I've planned to build it to tuck under the wing of my tablesaw, as shop space always seems to be a problem. I'd like to get inputs in two areas before I move forward with the build, since many of you have air filtration units already:

1. I was considering stacking filters for effectiveness and cost. Ideas were 1×5" filters and 1×1" filter. The 1" filters are cheaper and could be changed more often. Original plans called for filters on two sides of the table, so 2×5" and 2×1" filters total.

2. I'm also considering having the output of the blower point straight down. Because this will be on wheels, pointing it any one direction may make the table roll on its own, which I don't want. Also, because of the airflow that the blower puts out, it can be annoying to have to walk in front of the output. Blowing it straight down would give 360° airflow, and would also kick up dust from the ground, which in the long run would get it out of the shop.

Any thoughts or inputs on these two items?


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## JBrow (Nov 18, 2015)

Tango2,

I see nothing wrong with your plan other than perhaps directing exhaust air against the floor. I could image that dust even on a freshly swept floor would be a constant source of annoyance. The finer dust could be tossed into the air and could linger for hours where it can be breathed in and later settle on everything. It seems that directing exhaust air out the side could mostly reduce this problem but when parked out of the way while ensuring the exhaust is not blocked would, as you suggested, be important. But as you note, stepping in front of the exhaust would also be annoying.

These problems might be solved by building a housing and mount the unit on the ceiling. Exhaust air flow could be directed to provide some ventilation that could be appreciated especially during the warmer months.

I did something similar to what you described several years ago. The table featured a slotted top with a router mat glued to the top. The air intake to the furnace blower was from above through the slots. The filters were placed between the slotted top and the furnace blower. The blower exhausted out the side. I then used this contraption as a downdraft sanding table. In this configuration, a lot of dust was collected and the filters required frequent cleaning. I have since converted the down draft table to one where the dust collector replaces the furnace blower. Had I kept the original design, I would have probably added a pre-filter made of course canvas set in a frame. I image this would have protected the subsequent filters better and the canvas pre-filter could have been easier to clean.

As for filtration, your plan sounds ok. The quality of the exhausted air would depend of the filter specifications. Another option would be to build the air cleaner to accept filters designed for wood shop air cleaners. These filters may offer superior performance; better filtration and longer between cleaning time. As an example…

https://www.pennstateind.com/store/air-cleaners.html


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## Tango2 (Jan 2, 2017)

JBrow,

Thanks for the reply. You offered some good points. As for the direction of the blower output, my thoughts were that if the fine dust was made airborne by the fan, it could also be filtered by the fan. At the same time, if the fan was able to filter all dust out of the air, it would still be airborne until it was filtered, so it may be counterproductive.

I also considered hanging this like you mentioned. While not out of the question, I was trying to accomplish two things at once (a workspace and filtration). I'll give this some more thought, as I could always use the table as a downdraft connected to the DC, as well as a just a regular table.

It sounds like such a simple thing, but there are so many approaches to this!

I'm not too familiar with the wood shop-specific filters. How do they compare to home HVAC filters? Examples of the filters I was considering are below.

https://www.amazon.com/Nordic-Pure-20x25x5-Honeywell-Replacement/dp/B004LURI4Q
https://www.amazon.com/Nordic-Pure-20x25x1-Furnace-Filters/dp/B004QL764M

Thanks again for the inputs!


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## JBrow (Nov 18, 2015)

Tango2,

I am a long way from an expert on dust filtration so I am not sure that I can offer sound advice. But what I do suggest is that looking into the filters sold for wood shop units since these could offer benefits in performance, longevity, and/or cost. I suspect that a wood shop filter is more likely to be designed to withstand cleaning whereas HVAC filters tend to be disposable. I am sure there are replacement filters for these various shop filtration units probably available from Grizzly, Woodcraft, Rockler, and Penn State Industries and perhaps elsewhere.

For example the pre-filter I posted from Penn State Industries is washable, which would allow the pre-filter to last longer before replacing. It is also offered at a lower cost than those you posted from Amazon for the HVAC filter, which I doubt can be washed.

As far has fine filtration, I am not sure which would be the better direction. The design of the fine filter I posted from Penn State Industries offers a lot of surface area but I did not see how well the filter removes fine particles. I am not sure that it can be effectively cleaned, other than dumping out the dust and shaking it.

The pleated 4"or 5" HVAC filter may offer more surface area and there may be a better range of fine filtration. I have cleaned 1" pleated HVAC filters in the shop with a shop vac and that did restore some air flow. The thicker HVAC fine filters probably could also be cleaned, but cleaning could conceivably reduce the fine filtering capability offered by the filter when new (just a guess).

If you can find wood shop fine filters that meet your requirements, then it would be a cost based decision (wood shop filters versus HVAC filters) for me.


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## Nick424 (Aug 24, 2016)

This is what I threw together one day to help out. I was at the Goodwill Restore and got the two speed blower for $12.50. Made the box fit the same filters I have for the house, so I only have one size to stock. 16 by 25 in my case. I made it the same height as the table saw so if I need support for a long board I can put it there. It has a solid bottom so the exhaust is directed in all directions. I tried it out by blowing off my shelves, and it cleared the air very quickly. There was still a light coating on stuff, but the dust was not suspended long enough for the filter to get it all. On low it is quiet and the exhaust is not bothersome, but on high the air on my ankles is not pleasant in cooler weather. I think this summer it will be fine.


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## Allman27 (Jan 2, 2017)

That's a good 12 dollars spent!


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## OSB (Dec 16, 2016)

You are going to generate the same amount of fines no matter what your dust collector will do so you are going to be clogging filters with fines at the same rate unless they can be removed from the air before they get to the filter which is why I think cyclonic makes sense for an air cleaner. If the cyclone is effective the outlet could be filtered with a HEPA filter and not clog.

I will probably try a small cyclone with a high velocity duct booster fan (or similar) and a good filter, on wheels so I can more it to where the dust is.


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## Tango2 (Jan 2, 2017)

JBrow: I didn't realize the woodworking specific filters were washable. That changes my original thoughts on them, but I'll have to see how performance compares. My thoughts were that the 5" ones were more expensive, but using a similar grade 1" filter prior would capture a large amount of the dust and be cheaper to replace when dirty. Also, having two in series seems as if it would filter better than the rating of just one, but that is just my non-scientific hypothesis.

Nick424: That design is very similar in a lot of ways to the design I had in mind. In your opinion, would you prefer all of the air venting from one side rather than a 360° output?

OSB: I'm not sure I'm tracking what you are saying, but it may be my ignorance showing through. I do realize that dust will be generated by a dust collector, and my plans for my HF unit are to do a Thien baffle mod as well as a pleated filter (i.e. Wynn Enviro) to replace the bag. As for the cyclone-style dust collectors, from the little bit of research I've done, it seems like the cyclones require more power, not to mention are more expensive out of the gate. If you're referring to something else about the cyclones, I'm all ears…

My main goal with the DC is to keep a cleaner shop, but health is a big concern to me as well. I normally wear a quality respirator when doing dust generating tasks, and may continue to do so even with the best dust collection. I would like to move around in my shop without kicking up so much dust that I have to wear it all the time. I'm hoping a combination of these two concepts (DC and air filtration unit), I'll achieve my goal.

Here's a quick sketchup of what I was thinking:


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## Nick424 (Aug 24, 2016)

Tango2,

I like it the way it is, my hope was to keep air moving in all directions, and therefore have it pass through the filters in a better circulation pattern. This way any one side is not blasted with air.


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## woodbutcherbynight (Oct 21, 2011)

I recently was given a ceiling mounted WEN air filtration system. Lots of interesting tips they give in the box and on the website. Mounting it in the ceiling takes the dust from the air while not dragging it through your breathing room. So they they say. I did note that when the unit is on it blows upwards not down so no draft or blast of cooler air on you while working. Nice in the winter not sure how well that will do in summer. In the 3 weeks I have had it I have checked the filters and each week taken them outside to use a shop vac to clean them out.

Like you I was about to build a unit from a furnace fan when this dropped in my lap.


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