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Equip Your Shop With An Air Filtration System
you can make this necessary yet inexpensive shop accessory in a single afternoon



During the seemingly interminable winter of '97 I made a trip to my local shopping mall which, to my chagrin, is the very same place that some people refer to as the landfill.  On this day, I was shopping for a squirrel cage fan that would eventually play the lead role in my air filtration system.

Upon arriving I immediately spied a fan sitting there alone and beckoning. Being the ever cautious and conservative shopper, I made sure it worked properly before loading it and thanked the nice lady at the gate for the fine selection as I pulled away. Back at the shop I put the fan aside and turned my attention to gathering whatever information I could find concerning the fine art of air-handling by tapping the knowledge of my local HVAC contractor.  Through him, I discovered this fan would work as efficiently in a woodshop air filtration environment as it did in its normal air-handling situation, i.e., home heating and air-conditioning.

So, for your health and certainly for a cleaner shop let's get started...



There are basically two places where you can easily acquire your fan: either your local landfill or a HVAC contractor.  Whichever your source, the condition of the fan and whether it is the proper size are the two main concerns but more so in the case of the former while in the latter you will have a better chance of asking for what you'll need.

Before settling on any fan, be sure the bearings are not rubber grommets. These types of fans are not capable of withstanding the higher RPMs necessary for adequate air exchange.  Some other things to check for are:


  • Does it squeak as you slowly spin it?  If so, can it be corrected with simple lubrication?
  • Is there any side-to-side play in the bearings?


  1. Any play here will increase the noise effect once the motor gets the cage up to speed.
  2. If the bearings are worn, can they be replaced? Usually, these bearings are of standard manufacture and can be acquired easily but the actual replacement of them may be difficult.
  3. Is the cage itself in decent shape? If not, can it be straightened out? Any odd shape here will interfere with the efficiency of airflow and may allow finer dust particles to settle in them.




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.  Therefore, if we take the average 2-car garage to be 24' x 24' x 8', the volume of air within that building is 4608 Cubic Feet.

Using the table below, suppose we have a fan with a diameter of 12", a width of 12", and the blower is turning at a rate of 745 RPM and the overall volume of 1,600 CF.  Using the example of our building from above, dividing the total CF of the building by the total CFM capacity of the fan, we determine that the exchange rate is 2.30 minutes.


Blower (dia x Length) Motor HP Blower RPM CFM
10 x 4 1/4 690 600
10 x 4 1/4 890 800
10 x 6 1/3 830 1000
10 x 7 1/3 790 1000
10 x 7 1/3 950 1200
10 x 8 1/3 900 1200
12 x 9 1/2 795 1600
12 x 9 3/4 850 2000
12 x 12 1/2 745 1600
12 x 12 3/4 800 2000

(Table taken from Handbook of Air Conditioning Heating and Ventilating, 1965)


By comparing the values in the above table, we see where the total CFM can be changed drastically by either increasing the HP or the RPM.  So, the size of your fan is not exactly important, however the fan's ability to withstand any increased RPM is.



Using the fan by itself isn't an option because air is drawn into the fan through the side of the cage.  Since the fan's bearing system is usually a bracket over the sides and top of the cage, there is no flat surface to place the filters without the bracket interfering with them.  So we need to surround the fan's cage with a box which has three main functions:


  • It acts as an airflow chamber where air enters the rear of the case, through the filters and out the front.
  • It provides a way to safely tuck away the belt and motor while muffling noise to an almost unnoticeable level.
  • It gives a much more attractive and cleaner appearance.


The material I used throughout the project was ¾" and ½" particleboard. I chose it mainly because of it's cost and because it didn't need to be anything more substantial than that.  Actually, by the way it's mounted to the ceiling, strength isn't much of a concern, but we'll get to that later on.

I used ½" material for the top of the unit.  I decided that since the system would be attached flat to the ceiling, the top didn't need to be so stout.  However, for the sides and bottom I used ¾" material in an effort to assist in nullifying vibration and assisting in noise abatement.

The air output sections of most cage fans have either a square or rectangular frame and a flat area or flange, I used this to build the opening around.  Since air leaves the cage through this orifice, I simply bolted the cage near the end of the carcass bottom and then closed off the outer case from the cage.  Leave enough clearance between the fan and the inside of the carcass for ease of assembly and future maintenance. My filtration system ended up being 2' x 2' x 4'.  It didn't need to be that long but it allows plenty of room for any future changes.

After the carcass is surrounding the fan the next step is some form of finish.  I decided on paint but not before applying a good primer as the first step in my finishing process.  Because particleboard is an inherently thirsty product, a primer coat is recommended but several coats of paint may work just as well.



Now that your air filtration system is assembled and completed, selecting the optimum location for mounting the system to the ceiling is next. Obviously, it should be near where 90% of the work is done but, not in the way of the work being done.  For instance, if you placed it directly above your workbench, then you won't be able to set or turn taller objects while they're on the bench.  Likewise, if you place it above a piece of equipment, you may be limiting what size material can be used on that equipment.  So, careful consideration should be allowed for the location of your fan.  The direction in which the air will blow is also important.  Blowing dust away from the unit isn't constructive nor would you want a constant draft blowing on yourself.

For the mounting brackets, I took two pieces of 1 ½" x 1 ½" x 30" angle iron and drilled holes near each end to allow a length of 3/8" all-thread to pass through.  I then drilled the corresponding holes through my ceiling.  The weight of the system is fairly heavy so cribbing should be placed up in your attic, spanning across several of the trusses or ceiling joists.  Drill the same hole pattern in the cribbing so the all-thread can also pass through.

Double nut one end of each piece of all-thread.  Slide a washer onto the all-thread then place each section through the hole of the cribbing then through the hole in the ceiling.  Back in the shop, take a piece of angle iron and slip the all-thread into it's holes then follow with a washer, lock washer and a nut.  Run up each nut, allowing just enough space for the filter system to slide in.  When your system is resting on the framework, tighten the nuts. 



Once the system is mounted to your ceiling, you have to consider not only how to power the unit, but how you intend on switching it on and off.

I reflected on the first requirement and arrived at the conclusion that since the system could be running at any time someone was in the shop that a circuit dedicated solely to the system was required.  So, I drilled a hole through the top of the carcass and the ceiling to allow the motor's cord to pass through the carcass and terminate in the attic space of my shop. Directly above the fan is where I placed a dedicated outlet into which the cord from the motor is plugged. The next step was to figure out how to switch it on and off.

I thought about simply placing a switch near the area where the most work would take place, however it occurred to me that when it was actually needed, I most likely wouldn't be standing anywhere near the switch. It was obvious the switch needed to be controlled remotely - so I purchased another module to go with the controller I had previously installed on My Spiffy ShopVac™ and plugged it into the outlet then plugged the fan into the module. The transmitting device has two buttons; one controls the vacuum system and the other controls the air filtration system. To keep the transmitter close at hand, I fastened it to my leather shop apron.

I highly recommend installing a switched outlet and not hard wiring the system because having an outlet in your attic can be very handy when it comes time to perform any work up there.



The focal point of any air filtration system is obviously the filters and since filters is a subject that has filled volumes, I shall leave it to you to determine what your respective requirements are.  However, I will pass on some general guidelines.

Most systems should use two filters: the pre-filter and the particulate filter.  The pre-filter is responsible for collecting the larger and denser matter while the particulate filter traps the finer.  I used a standard furnace filter as my pre-filter - about 89¢ at hardware stores and went to my HVAC contractor to purchase a particulate filter, good down to 5 microns, for $20.00. I clean them both at about the ratio of 6 to 1 respectively.  When cleaning the pre-filter lay it flat on a piece of plywood then hit it with the flat of your hands.  Do this outside because you'll be amazed at the amount of dust it collects.

Strips of ribbon or pieces of yarn can be tied to the front of the fan to indicate when the pre-filter should be cleaned, but I simply look up at the unit to determine this.

The final step is to turn the suck...err, blower on.  Since I installed mine the fine layer of dust I saw on everything the next morning has been eliminated, if not, dramatically reduced.


So, what are you waiting for? Go get a fan then get one of your woodworking buddies to come over and spend a fun day making this accessory in your shop!


The total cost rang up to about $30.00 (I found the motor on a previous shopping spree).


That's it, now go clear the air!


Robert Methelis took my system to another step by installing a timer - much like the one commonly found on bathroom heat lamps - to control the fan. He simply sets the timer for thirty minutes and can leave his shop knowing it'll turn off automatically.