Shop made air filter advice

[Bryan Bear]

This is one of those things where, I know there is more to than my understanding of the parameters, so I’m Turning to the experience here.

I have one of those shop air filters that does a great job but the filter only filters down to 1 micron. I started thinking about how to vent it through a HEPA furnace filter without restricting the airflow too much. I was planing on venting it through a box of some sort that would allow me to use a filter(s) with a much greater surface area to minimize the restriction of airflow the filter would add.

While thinking this through, I realized I have an overhead cabinet that I really don’t use because it is not very convenient. It occurred to me that I could put an air mover in the cabinet and replace the doors with furnace filters “in series” (a large particle filter, then a medium and finally a HEPA filter). The whole thing could vent out the side with the added bonus of keeping the air (dust) circulating so it can make it to the filters.

I found this fan for really cheap and it seems too good to be true. It says it can be used to exaust rooms and CLAIMS 420 CFM. 420 CFM would turn over my tiny shop 20 times an hour. Obviously, filters would reduce the amount of air moved, but by how much. Would a fan arrangement like this be able to pull enough air through a filter box or would I need a squirrel cage or impeller style blower?

https://www.amazon.com/gp/aw/d/B01C82T0 ... psc=1&th=1

With my cabinet, I would be able to use two 14”x20” filter sets (a total of 14”x40” filter area). That would be 5.5 times the area of the 8” fan duct. Would this be reasonable? If I could get that setup to move 300ish CFM, that would still turn my shop air over more than 12 times an hour.

[Bob Gramann]

It certainly won’t cost you very much to run the experiment with the price of that fan. I tried stacking a coarse and a fine filter once. It turned out that most of the particles were fine and the coarse filter didn’t extend the life of the fine one much so I abandoned that strategy. What did extend the life of the fine filter was vacuuming it occasionally. I have been using the 3M 1900 filter. It’s not quite HEPA, but by the time the same air goes through it a few times at 93% capture per pass, the air is pretty clean. My particle counter confirms this.

I would expect the duct fan you are looking at to lose a bit of throughput from the resistance of the filter. A squrrel cage fan might have a bit more static pressure potential. I have nothing but opinion to back that up—I am in no way knowledgeable about airflow. Certainly, the bigger your filter surface, the less resistance to airflow you’ll get. I took apart one of the 3M 1900 filters—the filter paper in my 20x25 filter was 8’ long across the 20” dimension. At Lowe’s, where I buy filters, the filters are all the same price regardless of size, so you get the most filter for the buck in the larger sizes.

[Eric Baack]

The course filter is nice for anytime you are spraying. It picks up the liquid particles so they don't clog up your finer filter as quickly

[Barry Daniels]

I used to size blowers in engineering applications. That fan is nothing more than a glorified blow dryer. It is listed as 42 watts which is a tiny motor. It would definitely lose the majority of its flow rate trying to push air though a filter. You need a real motor to move significant air though filters. I would look for a quarter horsepower blower or something close to that.

[Bryan Bear]

Thanks Barry. That is kind of what I expected to hear. I’ll keep an eye out for a used furnace blower. I see people putting filters on box fans and claiming they are happy with the results. I’. Dry skeptical. I know box fans are rated to deliver big CFM numbers but I can’t imagine them pulling or pushing through a filter. I figured this would be similar. Is there another parameter I should be looking for other than CFM to help me compare?

[Barry Daniels]

Look at the power rating of the motor.

[Bob Gramann]

I used a 20x20” filter on a 20” box fan for quite a while before I built my box around a furnace blower. The box fan worked quite well. The furnace blower works better.

[Bryan Bear]

Bob, I’m curious. Do you have any data from your particle counter from your box fan days? I would expect to see clean air right in front of the fan but what about the room in general. Did it seem like it was having a useful effect on the whole space? Is it a case of it working but just taking a long time?

[Bryan Bear]

Barry, is it more complex than just motor power? How does fan type, size or any other parameter I'm not considering factor in. Using this fan as an example, lets say that I have a motor that will spin at the same speed but is 1 horsepower. It would seem that a big motor would be lost on a wimpy fan. Or would it really pull significantly more air? What about the same fan and larger motor but at a higher RPM? Somewhere I have an old motor out of a vacuum cleaner (obviously not a good choice for this application), this small motor and fan can create quite a bit of suction and was even designed to pull through a HEPA filter. It has lots of suction but doesn't move a lot of air. It would seem that these two small motor/fan arrangements have very different strengths and weaknesses.

In the end, my answer is to find a better air mover, one that is designed for a similar purpose. I get that. But now I'm curious about how all of this is related. Mind you, not curious enough to spend a month studying up, but I'd like to have a better understanding for understanding sake.

[Bob Gramann]

I didn’t keep written records. What I recall is that it would take about 20 minutes to get my 30x15xalmost 8 shop down from around 100,000+ .5 micron particles per cubic foot to less than 30,000 which I considered adequate. My bigger fan can do it in less than half that time. With the bigger fan in the box, I have the exhaust directed to the ceiling which pretty much stirs all of the air in the shop so that eventually it all has to go through the fan. I used to use the box fan on the floor (particles fall). I keep the meter on my workbench a bit of a distance from the fan.

All things considered, the Dylos Pro 1100 particle counter doesn’t cost that much and lets you measure what you’re dealing with. It lets me know when it’s time to put on a mask and turn on the filter.

[Barry Daniels]

All pumps (including fans) have two relevant and interrelated specifications: pressure and flow rate. Different types of fans and pumps have different combinations of these parameters. They are like spray guns. Some are high volume and low pressure. Some are the reverse. And then there are some that have both high volume and high pressure. So it depends on your application. And a filter motor needs to have some ability to push air through filters that will get clogged up, but still move an adequate volume of air. And to simplify things, you need a fairly strong motor to do both of these things. If you have a fractional hp motor mounted to a standard fan blade, it should move a bunch of air and overcome your fine filters. But you are correct, you don't attach a big motor to a tiny fan blade. The power would be wasted.

[Steve Sawyer]

Bryan - I'll leave it to others to recommend how to push the air through the filters, but want to toss in a few general comments.

I have a Jet AFS-500, which was marketed as a "benchtop" filter. Really just a shorter version of their gigunga AFS-1000, which was way too large for my shop. The AFS-500 is sadly no longer available, but measures about 24x18x12, has three speeds and is rated for a max 500 cfm so plenty for my little 500 sqft shop. I added a timer switch to it so I could leave it running when I leave the shop. Bit of a pita to have to go over and re-start it every hour when in the shop, but...first-world problems. I have it mounted to the I-beam that runs through my shop, just above my jointer and planer where I won't walk into it (the bottom would hit me across my forehead).

I replaced the original outer electrostatic filter with a washable custom-made unit from an online vendor, as the original cardboard, chicken wire and spun filter media had taken a number of "hits" over the years, and was getting pretty beat-up (this used to sit on a cart at floor level). I've never replaced the 4" thick pleated inner filter.

It has a small squirrel-cage blower inside.

I have this running on low speed all the time I'm in the shop, and usually crank it up to full and leave it running when I leave the shop for the day.

It is amazingly effective. With no other collection running, at full speed it will bring the fine particle count in the shop from 800-900 down to under 200 in about 20 minutes (and yes, Bob - I did finally bite the bullet and bought a particle counter).

If I ever need to replace the inner filter, I might again buy a custom filter, but it appears that the filters for the AFS-1000 are the same size, and only run about $45. I'd recommend either finding one of these units used, or if you build one, design it around these filters.

I have to admit I didn't expect this filter unit to be this effective.

[David King]

Squirrel cage blowers generally have the potential to push air harder than a simple fan. If you really need to push air hard then you add a second, third, and forth stage (squirrel cage) in series. Eventually you end up with a rotary screw compressor. What you pay for is a motor that puts more energy into pushing air and less into heating that air up. Leave it to pot growers to do all the research, find the best fan for the purpose, and then get the price down as low as possible. They are pushing air through large activated charcoal canisters which probably have a lower impedance than a dust filter but it's easy to match the filter area to the capabilities of the fan but acknowledge that the filter will gradually put more back-pressure on the fan as the media gets clogged. Oh yeah, 400CFM is about what a 1-2HP dust collector can squeeze through a 4" pipe. Let's not pretend that 42W can do what 748-1496W can do.

[Mario Proulx]

Somewhere in the archives, you'll find my air cleaner setup. All these years later, I have yet to modify it, as it works perfectly. Quiet, too....

[Mario Proulx]

Here, I found it. And seeing those photos now, I have modified it a bit. I now have two filter banks. Photos to follow...

http://www.mimf.com/library/Mario_Proul ... -2008.html

[Mario Proulx]

Here are my filters. There are three filters in each bank, with the final one inside being a very fine filter.

[Mario Proulx]

Having re-read the entire old thread, it appears incomplete, so here goes. I did close off the opening across from the blower, but I didn't cut the wall to add ducting. What I did do is to remove the diffusers at the top of the wall and enlarged the area of each one. And while I originally had a 4 hours timer on it, I now have a simple switch. It runs so quietly that I don't mind it running all day if necessary.

[Bryan Bear]

I remember that thread! That is actually fairly similar to what I was thinking about doing but using an overhead cabinet instead of interior wall space. The difficulty would be finding a blower that fits in the cabinets properly.

I have an air filter similar to Steve's and am very happy with it. I would like to go from 1 micron filtration to 0.5 or 0.3, which is what got me started on this thought experiment. The high speed on that unit is rated as just over 400 CFM and my shop is about 1,300 cubic feet (minus the volume of tools, wood and cabinets. . .) so it is more than enough flow for the small room. I worried that adding a finer filter would drastically reduce flow so I started thinking of ways to increase the surface area of the fine filter. My filter unit is just a little bit too big to put into the cabinet so I started thinking about other fan options. I have space on the wall next to the cabinet and am currently considering mounting the unit to the wall, getting rid of the 5 micron and 1 micron filters on the intake, gasketing the intake to the cabinet and replacing the doors with 2 14X20 filter arrays.

The intake of my unit is 10x15 designed for the 5 and 1 micron filters. Removing those filters and providing air to the unit through the doors would give me close to 4 times the area but with more restrictive filters. I'm thinking this will give me close to the air movement I get from the stock arrangement. Or am I way off base?

I'd prefer to have the fan in the cabinet for a cleaner look but I don't hate this option. I could also consider taking my unit apart and getting the fan/motor to fit in the cabinet but then I might not be able to change my mind later. This project is a little down the list so I'll keep my eyes open for a small enough furnace blower in the mean time.

[Barry Daniels]

Well, it is certainly true that with more restrictive filters, you need more filter surface area in order to achieve the same flow rate with the same motor, all things else being equal.

I use to have to calculate air flow using charts and tables of friction factors for pipe and common fittings. It was tedious. Now apps do it.