DIY Air Filter Plan

[DeanJackson]

I have a 3/4 MDF cabinet under my drill press that holds a scrap bin and and a backup router table. Thinking of rebuilding it into a dust collector. The plan:

Two 3.5 amp attic fans, 1300 cfm and $50 each:

http://www.amazon.com/Ventamatic-CX1500-Power-Gable-Ventilator/dp/B000LNII4Q

Filters, 3 stages, all 20x25:

One washable first stage, $70:

http://www.amazon.com/20x25x1-Boair-Air-Conditioner-Filter/dp/B001EUGNWY

One cheap disposable second stage, $7.50 each:

http://www.amazon.com/gp/product/B000P7BJXU

One expensive third stage, that should be MERV 14 (0.3 micron) for $20 each:

http://www.amazon.com/gp/product/B002WC83EO

That comes under $200 for parts, plus wood, screws, and something to seal it. Might build or buy shutters for the fan, so the cats don't go exploring. If I read all of this right, that should give me equal filtration to a $500+ commercial unit, but have *really* cheap filter changes, at $27.50 for a full new set, and have sub-micron filtering, which most filters don't seem to do.

Am I reading that right?? Will this be loud enough to rattle teeth? Is it really that much cheaper *and* better to DIY, in this case?

[VitalBodies]

One comment I would add is a fan like you have envisioned vs the squirrel cage type vs say, 10-20 or so computer (120+ CFM) fans.

Not really sure what the difference would be other than noise and the angle (90 degrees for the squirrel cage) the air moves through the box.

The computer fans would be low noise and take up little space being only an inch thick. They would move the air through a rectangle better perhaps. Additionally if you shop for them at say Newegg you can see the noise, RPM and size.

Another thought, you might consider a simple duct that moves the air IN away from the OUT.

Just some thoughts not expertise...

[DeanJackson]

One comment I would add is a fan like you have envisioned vs the squirrel cage type vs say, 10-20 or so computer (120+ CFM) fans.

Not really sure what the difference would be other than noise and the angle (90 degrees for the squirrel cage) the air moves through the box.

The computer fans would be low noise and take up little space being only an inch thick. They would move the air through a rectangle better perhaps. Additionally if you shop for them at say Newegg you can see the noise, RPM and size.

Another thought, you might consider a simple duct that moves the air IN away from the OUT.

Just some thoughts not expertise...

I don't think they'd move the volume; the attic fan I linked uses 3.5 amps @ 120 v; for a 12vdc case can, they draw 1.5 amps. Hrrm. Maybe 20 of those *would* work to replace an attic fan, and would be quieter, I think.

[Beechwood Chip]

The plan:

Two 3.5 amp attic fans, 1300 cfm and $50 each:

When you put filters (and maybe ducting) on them, they won't pull 1300 CFM anymore. I don't know how whether the difference will be minimal or huge. Depends on how much resistance the filters create, and it sounds like you are piling on a lot of filters. I'm thinking of doing something similar, but using a high efficiency low resistance cleanable filter from Wynn, without a pre-filter.

[DeanJackson]

When you put filters (and maybe ducting) on them, they won't pull 1300 CFM anymore. I don't know how whether the difference will be minimal or huge. Depends on how much resistance the filters create, and it sounds like you are piling on a lot of filters. I'm thinking of doing something similar, but using a high efficiency low resistance cleanable filter from Wynn, without a pre-filter.

No ducting; this is an air filter, not a dust collector.

That said, if they pulled 2600 CFM, I'd be clearing the air in the room every thirty seconds or less. If they get over 500 (combined), I'll be just fine. :-)

[jHop]

There are plans on the web to build an air filter set up using a simple $20 box fan available from pretty much everywhere, and simple filters that can run $1 if you know where to look. True, it's more disposable than reusable, but it's far cheaper.

I'd suggest not keeping the twin fan set up. You only need one at the exit to pull the air out of the box and back into the room. The other fan pulling air into the box is only going to add havoc. And if you want to adjust the air flow, I'd suggest something similar to the plantation shutters, with adjustable louvers, for both inlet and outlet vents.

Don't forget to either seal or add weather stripping around the filter openings, so that the sawdust does not leak out those ports. And if you are planning on washing these, I'd recommend some sort of pool or bag to carry them to the rinse location.

This is also theory; I haven't built one yet, but I'm planning on it later.

[Vic]

First, attic fans in an attic, not the best idea. Proper passive ventilation is much better. That goes toward the need for air sealing. If you feel you must have an attic fan, a solar one is better as they do not pull as many cfm and therefore are less prone to create a negative pressure situation in your attic.

All that said, this would work but would be quite loud. Also, I'm not a huge fan of the air filtration systems. I only have problems collecting dust properly when I'm using a router, which can be mitigated by switching up equipment. That will be coming in the future. Remember, if there is dust in the air filters, it's also in your lungs. Now, if you're not getting the DC you need and are wearing a respirator on a regular basis in your shop, then this is a good way to prepare the space for finishing. Other than that, it seems a bit of a waste of money that could go for a better DC system.

Again, all just my take on things.

[DeanJackson]

First, attic fans in an attic, not the best idea. Proper passive ventilation is much better. That goes toward the need for air sealing. If you feel you must have an attic fan, a solar one is better as they do not pull as many cfm and therefore are less prone to create a negative pressure situation in your attic.

All that said, this would work but would be quite loud. Also, I'm not a huge fan of the air filtration systems. I only have problems collecting dust properly when I'm using a router, which can be mitigated by switching up equipment. That will be coming in the future. Remember, if there is dust in the air filters, it's also in your lungs. Now, if you're not getting the DC you need and are wearing a respirator on a regular basis in your shop, then this is a good way to prepare the space for finishing. Other than that, it seems a bit of a waste of money that could go for a better DC system.

Again, all just my take on things.

You put a vent on one end of the attic and a fan on the other, so you don't create any negative pressure (which sucks cool air out of the house). Works great, and saves quite a bit of loot.

I'm working in the house, and generally wear a respirator if I'm making dust. Problem is that without a good dust collector, I'm still bombing the house with dust, and I'm more worried about the stuff I can't see, than te stuff I can?

That said, I have a tiny shop; would love to figure ways to *use* a better dust collector; trying to figure out a way to rig up an overhead on the table saw to pull dust and chips away from the top side of the table. Problem is that the table saw needs to be moved around to cut various sizes of wood, and is folded up in a corner the rest of the time. (Bosch, on a folding cart.) That makes quite a bit of dust, as does the router and sander, but the table saw is used most often.

[Vic]

You put a vent on one end of the attic and a fan on the other, so you don't create any negative pressure (which sucks cool air out of the house). Works great, and saves quite a bit of loot.

I'm working in the house, and generally wear a respirator if I'm making dust. Problem is that without a good dust collector, I'm still bombing the house with dust, and I'm more worried about the stuff I can't see, than te stuff I can?

That said, I have a tiny shop; would love to figure ways to *use* a better dust collector; trying to figure out a way to rig up an overhead on the table saw to pull dust and chips away from the top side of the table. Problem is that the table saw needs to be moved around to cut various sizes of wood, and is folded up in a corner the rest of the time. (Bosch, on a folding cart.) That makes quite a bit of dust, as does the router and sander, but the table saw is used most often.

Yes, as long as you introduce the same cfm in as you're pulling out it shouldn't be a problem. But still, proper ventilation, air sealing and a good amount of insulation is a better solution.

Check on Half Inch Shy. I know he's made his own overarm blade guard dust collection. He's mentioned it in here before, and knowing Paul-Marcel, he's got the build on his site.

[DeanJackson]

Check on Half Inch Shy. I know he's made his own overarm blade guard dust collection. He's mentioned it in here before, and knowing Paul-Marcel, he's got the build on his site.

Thanks for the tip! Yeah, I see a picture of his, looks just about right.

[mpatrick]

Having built 3 of these air filters, I have learned some things. A squirrel cage fan is the most effective, just semi loud. computer type fans are the smallest but unless you use a lot of them will not move enough air through the filters, when you add more fans the noise does become quite loud. What I have now is a duct fan and I am still not happy, It moves plenty of air but it is just too loud. I am at the point where I might just buy a commercial one.

[usacomp2k3]

That said, I have a tiny shop; would love to figure ways to *use* a better dust collector; trying to figure out a way to rig up an overhead on the table saw to pull dust and chips away from the top side of the table. Problem is that the table saw needs to be moved around to cut various sizes of wood, and is folded up in a corner the rest of the time. (Bosch, on a folding cart.) That makes quite a bit of dust, as does the router and sander, but the table saw is used most often.

On most big tools like a table saw or router, you want to pull the dust/chips from the top and the bottom. Does your tablesaw have a port in the lower section? If you, you might want to think about adding one. A zero-clearance insert helps with that too, I've heard. For a router table, you ideally want one below, and then it's common to have one on the fence.

[DeanJackson]

On most big tools like a table saw or router, you want to pull the dust/chips from the top and the bottom. Does your tablesaw have a port in the lower section? If you, you might want to think about adding one. A zero-clearance insert helps with that too, I've heard. For a router table, you ideally want one below, and then it's common to have one on the fence.

I'm already pulling chips from the bottom on both tools; just need to pull dust from the top, as well. :-)

[APierce]

I built a dust filter from an old attic fan and used some cheap furnace filters. I attached a dimmer switch to the fan to allow for speed and thus noise control. The fan pulls air through the filter rather than pushing it through(already tried it!) and it collects a fair amount of fine dust. Your idea sounds much more heavy duty and I think its a great idea, I might add a timer to the outlet its plugged into so you could clean the air even while your not in the workshop.

[DeanJackson]

So, I changed plans, and built this into the bottom 24" of a cabinet; the top of the cabinet is the new base for my router table, just moved the top over.

Two 3.5 amp attic fans, 1300 cfm and $50 each:http://www.amazon.co...r/dp/B000LNII4Q

Filters, 3 stages, all 20x25:

Two super-cheap disposable filters stage, $3ish each @ Home Depot.

One expensive third stage, that should be MERV 14 (0.3 micron) for $20 each:

http://www.amazon.co...duct/B002WC83EO

3 cans of window & door gap-filling foam ($5 each); took the dead space in the cabinet that wasn't between the fan and the filters and filled that up as well.

A 2-to-12 hour timer that is OK with 7 amps, $30

http://www.amazon.com/gp/product/B00313JXBA

Outlet box to put that in, a single-pole switch to turn off one fan if needed, cover, cord, wire, electrical nuts: $25ish, Home Depot.

Four casters: $7.50 each

http://www.amazon.com/Steelex-D2608-3-Inch-220-Pound-Polyurethane/dp/B0000DD1EE

MDF and screws. $50?

$50 worth of dust collection doodads at Rockler, so I can just plug it into the rest of my dust collection system.

$375. *Ow*.

On the plus side, filter changes for a 0.3 micron-ish filter are $25... instead of $100+, and I got a *lot* better dust collection on the router table while I was at it.

[dmorrison]

First. Call the local AC installers. I got a 5ton blower from them for free. They were just throwing it out anyway. So they allowed me to remove it before the air handler went into the dumpster. Did not use it, gave it to someone else because I really needed a 2-3ton blower setup. They were going to call me when they were scheduled to replace a 2-3 ton unit.

Second. Check craigslist often. I bought a PSI woodworking AC620 air cleaner locally for $100.

Otherwise your plans may seem fine. As posted consider a squirrel cage type blower.

Dave

[DarrollReddick]

I built an ambient air cleaner/purifier system like you are talking about a couple years ago for use in my 24’x24’ garage. I've had great luck with it so far but am now looking to update it some since I have a much bigger shop now and need more filtration. I am a welder and use it to get rid of the smoke, gases, fumes, dust, etc.

I have researched many sites and many existing machines and come up with what I think is a great solution but am looking for someone with more knowhow to tell me if my calculations are correct. It can be very daunting trying to understand all that is involved in creating an efficient air cleaner/purifier. There are so many calculations and variables involved; it is hard for me to wrap my head around it all.

I will post some pics later. For starters I built a box with a 13.5”, 1/10^th HP, 1320CFM attic fan. The fan is mounted in the top facing the ceiling. This way the clean air is distributed using the "Coanda Aerodynamic Principle." The air hits the ceiling and is distributed across the shop ceiling and away from the intake/return filters. I made my box so that all four sides have filters. Leaving a space between the fan and the filters assures that the entire area of the filters will be used. Good filters require significant backpressure, which means tight sealing to prevent bypassing. There are currently 4 stages of filtering. Each side of the box has four 20”x20” filters in series. That is a total of 16 filters. I do not remember the micron rating or the cost for all of the filters.

These are the filters:

4" merv 12 pleated filter with 226.66 Square Ft. of filter media, and an initial resistance of 0.15 @ 300 FPM,

1" merv 9 carbon activated filter with 33.33 Square Ft. of filter media, and an initial resistance of 0.218 @ 300 FPM,

1" merv 8 electrostatic washable filter with 33.33 Square Ft. of filter media, and an initial resistance of 0.32 @ 300 FPM,

1" merv 2 cheap Fiberglass furnace filter prefilter, (I will not use this type again, it doesn’t do enough good to justify using it. I am replacing it with 1” MERV 4 Cut-N-Fit Blue Natural Fiber furnace filter with 33.33 Square Ft. of filter media, and an initial resistance of 0.125 @ 300 FPM.

I plan to make a 5^th washable prefilter out of fine mesh fiberglass window screen, just to catch the large debris.

I know that this is a lot of filters, but it is my understanding that you want to have a low static pressure for the machine to work properly. And in my opinion, the more filtering, and the more types of filtering, the better.

My combined total Square Feet of filter media for the 16 filters is = 1306.6

My total initial resistance of filter media for the 16 filters is =? I am not sure how to add these up!

The idea behind a proper air filtration system is to exchange the greatest amount of air in the shortest duration while at the same time keeping the velocity slow enough so as not to disturb settled dust in other parts of the space. Another important issue is to avoid negating the 'arrestiveness' of the filter by having too much air being drawn through the filter and thereby drawing the dust through the filter. Arrestiveness is defined as the ability to retain particulate without it being drawn through the filter. So, the idea here is to move enough air to trap the dust at the source by using the proper amount of airflow and, of course, using the proper filters.

In the grand scheme of air handling, any fan is able to exchange a certain volume of air in a certain amount of time and is expressed in (CFM) or cubic feet per minute.

All of the air which enters the air cleaner/purifier system enters through the return air filter. The filter must be theoretically sized according to the cfm (cubic feet per minute) requirement of the system. A filter that is too small will cause a number of problems. The filter will clog very rapidly if undersized and reduce air flow. Air velocity also becomes critical with reduced size and dirt will pass through instead of staying in the filter. The system is trying to draw in the designed air quantity and if the filter will not allow flow, air will be drawn in from any possible crack. Every system, no matter how air tight you construct it, will have small cracks through which dirty, warm air will enter the system as static pressure increases. A large surface area filter will negate the effect of these cracks by having a lower static pressure and lower air velocity.

The airflow resistance of a filter and the fan pressure required to overcome it depend on how fast the air is moving and how long and narrow the paths are. Friction along air paths creates resistance to airflow. Fans must develop enough pressure to overcome this resistance and move air through the filter.

Airflow resistance and fan pressure are usually expressed in inches of water column (in. water, or in. H₂O). This term comes from gages called u-tube manometers that are sometimes used to measure pressure.

Propeller fans normally can't generate more than about 2 in. water pressure. They are most commonly used for exhausting air from attics or overhead spaces, or general air circulation.

Tube-axial and vane-axial fans are the most common types used for HVAC. They are relatively inexpensive and fairly efficient when static pressure is less than about 4 in. water. The main disadvantage of these fans is that they are very noisy.

The maximum allowable filter velocity is 300 feet per minute (fpm) on disposable filters. For best results, the recommended minimum filter surface area is 2.00 cubic feet per minute (cfm) per square inch of filter area. Using 2.00 cfm per square inch the velocity of air across the filter will not exceed 300 fpm.

Example: 2000 (CFM) ÷ 200 in. water = 1000 square inches of filter media.

Fan Performance

Because of the way fan impellers (blades or rotors) are designed, the amount of air they can move decreases as the pressure they are working against increases. The airflow vs. pressure information for a particular fan is called the fan performance data. Performance depends on the size, shape, and speed of the impeller, and the size of the motor driving it. Performance differs widely among brands and models, even for fans with the same size motor.

Access to fan performance data is essential for selecting fans and for determining airflow provided by existing fans. Most manufacturers sell fans that have been tested using procedures specified by the Air Movement and Control Association International, Inc. (AMCA). The manufacturers can provide you with performance data in the form of tables or graphs. Avoid fans for which AMCA data is not available.

Selecting Fans

Calculate total airflow needed

The first step in selecting a fan is to determine the total airflow it must provide.

Estimate static pressure

The next step in selecting a fan is to estimate the pressure the fan will be operating against.

Estimating fan power requirements

Fans are usually described by the horsepower (hp) rating of the motor used to drive the impeller. It's helpful when selecting fans to estimate the power requirement first so you know where to start looking in the manufacturer's catalog.

Fan motor size depends on the total airflow being delivered, the pressure developed, and the impeller's efficiency. Impeller efficiencies generally range from 40% to 65%. If we assume an average value of 60%, we can use the following formula to estimate the fan power requirement.

Fan power (hp) = airflow (cfm) x static pressure (in. water) ÷ 3814

Example, Fan power = 7325 cfm x 2.4 in. water ÷ 3814 = 4.6 hp.

Selecting the best fan available

The most critical factor is whether the fan can provide enough airflow at the expected operating pressure. Start by looking at performance data for a fan having a motor rated just under the power value you calculated. If this fan provides more than enough airflow, look at the next size smaller. If your first pick is too small, try the next size larger.

These are some great links for info & examples of DIY ambient air cleaner/purifiers:

http://www.kevinsbra...llection.html#9

http://www.woodcentr...s/shot522.shtml

http://www.bt3centra...ead.php?t=24144

http://lumberjocks.com/projects/392

http://www.twistedkn...m/airfilter.htm

http://webcache.goog...=www.google.com

http://www.ncwoodwor...ead.php?t=23055

http://www.dustcolle...ts.com/ambient/

[DarrollReddick]

Questions I have:

1. Do I need a gap between each filter and if so, how big?

2. How can I determine the Merv rating of my filter unit with all of the filters combined?

3. What is the combined initial resistance of my filters? (AKA: Airflow Restriction, Airflow Resistance, Negative Pressure, Pressure Drop, etc.)

4. Do I have enough or not enough initial resistance for my fan?

5. Do I have too much area of filter media for my fan?

6. Is my fan adequate?

7. Any advise at all?

Things I want to implement on my new ambient air cleaner/purifier:

1. Build it out of 24 gauge galvanized sheet metal. This will cut down on weight and make a nicer enclosure.

2. Add a fluorescent light to the bottom of the box to better illuminate my work area.

3. Install a pressure gauge on my new box to tell when to clean/replace the filters. As the filters get clogged more negative pressure builds inside the box. Here is the type of gauge I plan to use. http://www.amazon.com/Dwyer-Mark-Model-Inclined-Manometer/dp/B003NUVVV6/ref=pd_sim_sbs_hi_1.

4. Add plug in receptacles for easy access at my work area.

5. Add power switch with timer.

6. I may use furnace filter gasket tape to better seal the filters.

[Beechwood Chip]

The idea behind a proper air filtration system is to exchange the greatest amount of air in the shortest duration while at the same time keeping the velocity slow enough so as not to disturb settled dust in other parts of the space.

That's one approach, and it makes sense if you're running the air cleaner while you are working in the shop. The other approach is to run the air cleaner after you leave the shop, with a timer to turn it off, and try to stir up as much dust as possible. That way, the settled dust gets stirred up and trapped in the filter, rather than sitting somewhere waiting to get into your lungs when you stir it up.

I'm not an expert, but I was thinking of running the air cleaner after I leave the shop, and also having a couple oscillating fans to try to get the dust airborne so it can get caught by the filter.

[DarrollReddick]

I plan on using it while I am working and setting the timer to run for an hour or so after I leave. I also use the oscillating fans to stir up the dust so it can get caught by the filter.

[jHop]

the problem with running it after you leave is when you are finishing. I'd suggest a low-volume setting for times when you have finish curing or drying in your shop.

(For some reason, I have an image of one of those big old Frankenstein knife switches to change modes between high speed and low speed settings... Along with some sort of maniacal laugh... Too much time watching _The Great Race_ recently....)

[DarrollReddick]

Here are some pics of my prototype that I've been using for a few years. Its very dusty because Ive had it unhooked and on the floor for a while.

[DarrollReddick]

[DarrollReddick]

[DarrollReddick]