# air cleaner motor from box fan?



## WWWorker (Jan 12, 2011)

Hi guys I have a 12 x 20 foot shop with a peaked roof. It is completely open inside all the way to the roof. I have calculated the cubic footage to 2400 cubic feet. This is not a large area. 

Currently for an air filter I use an 18' box fan with a filter attached to the back. Pretty low tech I know but better than nothing. As I am preparing to build an air cleaner I am wondering why I can't use the same fan and motor inside of the box I am building?

I don't know the velocity or size of the motor and there are no markings on it. I checked last night. However it would seem to me that in such a small space I can change the air pretty quickly with that fan. 

Can I just take it apart from the housing and mount both the blades and the motor inside a box with some HEPA filters before the fan?

I am having some trouble finding a squirrel cage fan and this occured to me last night as I was working in the shp.


----------



## woodnthings (Jan 24, 2009)

*just off the top of my head*

The squirrel cage motors/blowers are meant to operate with a restriction. The motor in a box fan isn't. If you restrict it somewhat probably not an issue...but possibly it may overheat inside that box, and it will be difficult to know. FIRE? 
Personally i would leave the fan intact, just rotate it so the switches can be reached easily and make a 1/4" or so open box on the sides. Then you can add several filters across the draw end maybe with strips to retain them. It's worth a try but keep an eye on it for heat.  bill


----------



## Gene Howe (Feb 28, 2009)

I'd hold out for a squirrel cage! 
Surely, there are HVAC folks around who usually have a pile of them "out back". Mine came from my own furnace after a retrofit AC unit was installed. The installer said they sell used ones for $10.00 and that $ goes towards Friday's beer for the crew.:thumbsup:


----------



## wmodavis (Dec 26, 2010)

How do you fit an eighteen foot box fan in your 12 foot shop? That's a big fan!


----------



## WWWorker (Jan 12, 2011)

*har har*

That fan is huge! I have it installed by the Jolly Green Giant!


----------



## Nic (Jul 11, 2011)

http://www.surpluscenter.com/electric.asp?catname=electric - check here for squirrel cage fans.


----------



## WWWorker (Jan 12, 2011)

Thanks Nic, That is a handy place to go. My next question is how big of a blower do I need? I have calculated my cubic feet around 2400. How big should I buy?


----------



## woodnthings (Jan 24, 2009)

*I checked that site*

And none of those squirrel cage blowers are large enough for your space.
Based on these specs at the bottom (1044 CFM max) you might want to get something like this: Amazon.com: JET 708620B AFS-1000B 550/702/1044 CFM 3-Speed Air Filtration System with Remote and Electrostatic Pre-Filter: Home Improvement

If you search shop air filtration units you'll find that www.Grizzly.com www.pennstateindustries .com and other make similar units.

If you want to make one I suggest a visit to your local and hopefully friendly furnace repair or heating and cooling store where they will have a take off blower from a furnace. I got one for $40.00 with 3 speeds... and don't forget to ask which color wires go where. You can get a 4 position rotary switch to select each speed. A box of plywood to surrond the blower and some furnace filters will give you a good start. Buy the filters first and make the box to suit that size...slightly bigger than the blower unit...:laughing:.
Who knows, their sheet metal guy might just bend up a cover for you at a reasonable cost and a few donuts thrown in. It's surprising what happens when you show up in person and have a good attitude. :yes:
Search home or shop built air filtration units for more info.  bill


----------



## dbhost (Jan 28, 2008)

FWIW, if you are going to go the buy it route instead of build it, I can certainly feel good about recommending the Grizzly G0572. I hear great things about the Jet as well. Haven't seen any posts about the PSI units, but they are so simple it is kind of hard to screw them up.


----------



## WWWorker (Jan 12, 2011)

No way am I buying it. I can't afford it. I have to build it myself..... so my question still is this:

What size blower do I need?

I saw a fan on that website that would move 500 cfm. By my rudimentary math skills if I bought and installed that one wouldn't I change the air like every five minutes?

I know it isn't a squirrel cage but would that work?


----------



## woodnthings (Jan 24, 2009)

*I don't think the CFM specs are on furnace blowers*

If you want to build one the cheapest way, is get a used furnace blower. Make sure it will work on 120 V as some are 220v motors! 
You probably have less than $100.00 if you get a used furnace blower, a rotary switch and make a plywood box...1/2" will do. Cut the end off an extension cord for a power cord. 
Make a top and bottom identical. 
Make a left and right side with a space for the filters.
Make a frame for the filters.....wire it up, turn it on. 
Take pix, post here! :yes: bill


----------



## WWWorker (Jan 12, 2011)

OKay but do I need a certain size blower? Or will anyone of them do?


----------



## firemedic (Dec 26, 2010)

wilbwworker said:


> OKay but do I need a certain size blower? Or will anyone of them do?


I picked up a 1/3 hp blower for $30 from my AC... Man does it move some air. I haven't put it in a box yet, but the plan is similar to what your doing. You probably don't need one that big, but you definitely don't need one bigger than that...

~tom ...it's better to remain silent and be thought a fool than to open one's mouth and remove all doubt...


----------



## firemedic (Dec 26, 2010)

Or you could always go this route:

http://www.chiefsupply.com/3092-Tempest-Variable-Speed-Electric-Blowers.aspx



~tom ...it's better to remain silent and be thought a fool than to open one's mouth and remove all doubt...


----------



## woodnthings (Jan 24, 2009)

*This one will do*



wilbwworker said:


> OKay but do I need a certain size blower? Or will anyone of them do?


http://cgi.ebay.com/FURNACE-SQUIRRE...614?pt=LH_DefaultDomain_0&hash=item2a0d2fcfbe
Perfect! size and voltage is correct. :thumbsup: bill
Others are listed below...


----------



## darrollr (Aug 22, 2011)

*Lets learn from each other!*

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”, 1/10th 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 5th 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.


----------



## darrollr (Aug 22, 2011)

These are some great links for info & examples of DIY ambient air cleaner/purifiers:
http://www.kevinsbrady.net/DustCollection.html#9
http://www.woodcentral.com/shots/shot522.shtml
http://www.bt3central.com/showthread.php?t=24144
http://lumberjocks.com/projects/392
http://www.twistedknotwoodshop.com/airfilter.htm
http://webcache.googleusercontent.com/search?q=cache:AAnoq2Fa6MwJ:wood.bigelowsite.com/air_filter/+wood.bigelowsite&cd=1&hl=en&ct=clnk&gl=us&source=www.google.com
http://www.ncwoodworker.net/forums/showthread.php?t=23055
http://www.dustcollectorexperts.com/ambient/


----------



## darrollr (Aug 22, 2011)

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. 




 
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.


----------



## GeorgeC (Jul 30, 2008)

I would just install an exhaust fan up near the peak of the roof. That was it can be both a cooling fan and exhaust the dust to the outside.

Of course in the dead of winter you might not want that fan running.

George


----------



## darrollr (Aug 22, 2011)

My shop is a conditioned space so I dont want to push my heated or cooled outside. Plus I live in Oklahoma and it is 100+ and high humidity, so I dont want to pull that hot humid air inside. I also dont have any windows so the air would have to come in from an open door which would also let in uninvited pests.


----------



## BWSmith (Aug 24, 2010)

darrollr

Are you intending this air cleaner as a "do-all"?Meaning general shop air/dust management,AND a welding fume cleaner?

JMO,I'd have two(or more)separate units.Have a fume extractor as close as possible to welding area.These have special pre-filters in them because of some rather nasty contaminates.And depending on the welding processes,again JMO.....some need to be exhausted outside,"make-up" air be damned.

24G is a little lite for an enclosure.....It's gonna flex unless you're makin mighty small panels.My gut feeling is you're making alot of assumptions.....based on engineering.....about area's where the filter quality has more to do with it than anything else.This is one of those times when engineering and testing(building mock ups)is the de riguer.Then the results get programed into specialized computer design systems.That along with YEARS of experience is what you get(hopefully)when you buy a commercial unit.Not trying to dissuade your DIY.......but when you ask for exact dimmensions or spacing of things like filters....this is where IMO,you're makin a mistake.

Design the thing as a test mule from the get-go.So instead of responding with,filters need to be spaced,X % of their thickness(which would require a loooong list of exact parameters).....wouldn't it be easier to have them on a sliding rack?Now you can space them and measure the results.Just sayin,BW


----------



## darrollr (Aug 22, 2011)

BW,

Thanks for the input. 
I do have a fume extractor directly at the welding work station. I also am getting a clearvue cyclone dust collector for point of origin dust collection. I already have one of these DIY air cleaners currently running in my shop and has worked fine so far. However, it was my prototype and I just moved into a bigger shop and I need to build another one to make up the difference in shop volume. I just want to make this new one better. My question about the filter spacing was pertaining to backpressure buildup or circulation problems between filters. I know now that they can be as close together or as far apart as you want, it will make no difference.

I think youll see from my design that the 24 ga. should suffice. I will test that theory during fabrication. I can make it heavier if need be. I have dealt with the emgineers of several manufacturers and the commercial systems really arent that great. 

I spoke yesterday with Bill Pentz, the designer/inventor of the clear vue cyclones dust collectors. He is a very brilliant man who was very helpful. We went over some formulas and discussed what it would take to produce a quality system. For anyone who hasnt been to his site, http://billpentz.com/, I highly recomend it.


----------



## darrollr (Aug 22, 2011)

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.


----------



## woodnthings (Jan 24, 2009)

*Interesting concept*

My only concern would be immediate recirulation of the previously filtered air, which would reduce efficiency somewhat. A front to back directional flow would "seem" to counter that, but air is invisible unless you run a "smoke" test on it and see how it flows in the room. There are smoke bombs used to determine air flow commercially in wind tunnels if you really are concerned. We used small lengths of yarn at the GM wind tunnel when testing car aerodynamics. on rear spoilers and such. They would sometimes remain static and sometimes return on themselves. Usually they just blew in the wind.....  bill


----------



## darrollr (Aug 22, 2011)

http://www.wynnenv.com/ambient_filters.htm
This link will better explain the design reasoning.


----------



## woodnthings (Jan 24, 2009)

*So that's a pusher fan?*

And it draws the air through the filters? From an air conditioning unit possibly? I like the concept better now that you have posted the link. I wonder about a hex shaped unit, 6 sided. 4 sides keep it simple though. It gives me some ideas I have for a couple of 220v high volume axial blowers that were on a bag air filtration unit. 
Thanx for posting! :thumbsup: bill


----------



## darrollr (Aug 22, 2011)

It is an attic fan. Used to mount on the gable end of a houses attic to push the hot attic air out and suck in new cooler air from the vents in soffit and opposite gable end of attic. It is 1320 cfm.


----------



## BWSmith (Aug 24, 2010)

Coanda or racetrack.........first is center of the rm and second type is perimeter style.Its shouldn't be a case of one vs the other....its a case of which type suits the individual shop in question.

We're racetrack here,and gotta say...really don't want a motor anywhere near dust.We use belt pwr,with motor on top of enclosed squirrel cage,completely separated from air/dust stream.But thats what works here.

I'm really looking into "wet scrubbers" (water)......it greatly reduces filter costs.Just an observation.......when doing research on this or that project/build whatever.....be prepared for a cpl things.One is marketing,in a painted bullseye on your back sort of way.And another is fabrication.....and this is moreso than engineering.This shows up most if the company involved weighs or leans heavily/completely on a specific material or trade.And for whatever reason dosn't want to go out of their area of expertise.Maybe its management,liability?Maybe stupidity?Too set in their ways?

I know its very apparent when looking at water scrubbers.Traditional air to air companies,"we don't don't do plumbing"..."you'll have to go down the street to XYZ".And vis-versa.Sheet metal weldors/fabricators who don't know what a threaded fitting looks like?Not getting hung up on it,just interesting.BW


----------

