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Forum topic by JReed3 posted 10-03-2013 12:59 PM 975 views 0 times favorited 13 replies Add to Favorites Watch
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JReed3

81 posts in 1832 days


10-03-2013 12:59 PM

I have reviewed a lot of home made shop filteration systems on this site and across the net. I have come across some very good plans which I will modify for my use. The majority of the systems are using the cage air blowers rescued from old A/C units. I came across an Air Fox 1 HP air blower. My neighbor was moving and I picked this up from him so the price was right. It has 3 speeds that produce 2,500 CFM, 3,000 CFM and 4,000 CFM.

Is this too much CFM even at the lowest setting? Looking at the Jett & Powermatic units they move approximately 1050 CFM’s. So it appears that a low volumn of CFM’s may be in order.


13 replies so far

View mnguy's profile

mnguy

183 posts in 2859 days


#1 posted 10-03-2013 01:14 PM

It’s not a question of too much CFM, but the challenge of getting the face velocity across your filter down to a similar level as the commercial units. Higher face velocities will tend to increase pressure drop across the filter, which could cause particles to push through the media, or in the worst case, the filter to collapse. If you look at a commercial unit, take the size of the intake filter and the number and depth of the pleats, you can get an estimate of the media area, and with the CFM’s, an estimate of the face velocity. I would target a velocity in a similar range. To get the face velocity down at your high flow rates, you either have to make a lot bigger box or use deeper pleated filters. I would go with deeper pleats; many newer HVAC systems use 2” and 4” deep filters, so there are lots of readily available options.

I assume you are using just a pleated filter and not adding a felt bag/sock filter behind that.

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JReed3

81 posts in 1832 days


#2 posted 10-03-2013 01:22 PM

”To get the face velocity down at your high flow rates, you either have to make a lot bigger box or use deeper pleated filters. I would go with deeper pleats; many newer HVAC systems use 2” and 4” deep filters, so there are lots of readily available options.
I assume you are using just a pleated filter and not adding a felt bag/sock filter behind that.

I was looking at making the box about 20”on the sides to accept two 1”x20” pleated filters and then one 2”x20” pleated filter.
Maybe making the unit about 36” long.

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crank49

3980 posts in 2432 days


#3 posted 10-03-2013 01:32 PM

You can make it that size but the velocity through your filters will prevent the filter from capturing the dust.
You have two choices.
Make your filter 2 to 3 times bigger than the commercial units for 1050 CFM, or restrict the inlet of your fan to reduce the CFM you will be pulling through the filter.
I haven’t seen your fan so I don’t know if this is possible. Some fans have the motor inside the air stream for cooling the motor. I would not restrict this kind of fan. Otherwise, restricting the air flow has no negative effect on the fan or its energy usage. In fact, it would reduce the energy usage.

The math for calculating the air flow/velocity.
Figure the area of the filter in square feet.
20 inches by 20 inches is 20×20 = 400 sq. inches
400 sq. in. / 144 sq. in. = 2.77 sq. ft.
The maximum air speed you want across that area is 400 ft per minute. 200 FPM would be much better.
So 2.77 sq. ft. x 400 = 1108 CFM is the max for this size filter.

You could make the inlet a 24” cube and put 20×20 filters on 3 sides. That would give you 300 FPM with a 2500 CFM fan. Should work great.

-- Michael: Hillary has a long list of accomplishments, though most DAs would refer to them as felonies.

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JReed3

81 posts in 1832 days


#4 posted 10-03-2013 02:03 PM

You have two choices.
Make your filter 2 to 3 times bigger than the commercial units for 1050 CFM, or restrict the inlet of your fan to reduce the CFM you will be pulling through the filter.
You could make the inlet a 24” cube and put 20×20 filters on 3 sides. That would give you 300 FPM with a 2500 CFM fan.

That’s a very good idea and easy to do. This is a photo of the Foxx air blower the neighbor gave me. So I should have 3 intake sides or just bring it through 3 filters at one intake point?

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mnguy

183 posts in 2859 days


#5 posted 10-03-2013 04:21 PM

To build on and somewhat correct my initial post, the actual key performance criteria for the filters is media velocity, not face velocity. For a given filter face area (20” x 20”, for example), you can decrease the media velocity by increasing pleat depth; more media per unit of air flow. Face velocity is easy to calculate (thank you crank). I am not sure that calculating effective media area is as simple as dividing the face velocity by the face area:media area ratio, but you get the idea. Crank’s concept of a cube with (3) 20” x 20” filters, one on each of three sides of the cube, is a great idea. Adding filters in depth will not increase your media area / drop your media velocity. The only reason to add filters in front of each other is to ‘protect’ the second filter in line, say using a cheaper, lower efficiency filter in front of a more expensive, higher efficiency filter. IMO, high quality (not highest) residential pleated filters with MERV of 8 – 10 will do a great job of capturing wood dust, are easy to tap off and somewhat renew, and cheap enough to throw away.

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Fred Hargis

3928 posts in 1954 days


#6 posted 10-03-2013 04:42 PM

I think I’d pass on using that blower. There’s a reason those squirrel cage blowers are used, they’re made for the continuous running and the volume/speed you need.

-- Our village hasn't lost it's idiot, he was elected to congress.

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JReed3

81 posts in 1832 days


#7 posted 10-03-2013 05:04 PM

”I think I’d pass on using that blower. There’s a reason those squirrel cage blowers are used, they’re made for the continuous running and the volume/speed you need”

I don’t think that statement is really true. The squirrel cage style are made to run at short intervals. The A/C or heat reaches it’s selected temperature range and it automatically shuts off. These type of blowers are made to run for long periods of time, drying out wet floors, carpet, etc.

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JReed3

81 posts in 1832 days


#8 posted 10-03-2013 05:07 PM

”To build on and somewhat correct my initial post, the actual key performance criteria for the filters is media velocity, not face velocity. For a given filter face area (20” x 20”, for example), you can decrease the media velocity by increasing pleat depth; more media per unit of air flow. Face velocity is easy to calculate (thank you crank). I am not sure that calculating effective media area is as simple as dividing the face velocity by the face area:media area ratio, but you get the idea. Crank’s concept of a cube with (3) 20” x 20” filters, one on each of three sides of the cube, is a great idea. Adding filters in depth will not increase your media area / drop your media velocity. The only reason to add filters in front of each other is to ‘protect’ the second filter in line, say using a cheaper, lower efficiency filter in front of a more expensive, higher efficiency filter. IMO, high quality (not highest) residential pleated filters with MERV of 8 – 10 will do a great job of capturing wood dust, are easy to tap off and somewhat renew, and cheap enough to throw away.”

I think I’ll give this a try.

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higtron

207 posts in 2138 days


#9 posted 10-03-2013 05:46 PM

I’ve been toying with the idea of an air filtration/down draft table using a squirrel cage fan on a rolling cart that could be used as work table, at the right height it could be a outfeed table

-- A friend will help you move, a good friend will help you move a body

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JReed3

81 posts in 1832 days


#10 posted 10-03-2013 06:00 PM

”I’ve been toying with the idea of an air filtration/down draft table using a squirrel cage fan on a rolling cart that could be used as work table, at the right height it could be a outfeed table”

I have seen some of these people use as a sanding table and it looks like this would work out very well. With the rollers you could probably place it in the shop near table saw, planer etc. In my shop, I will need something wall or ceiling mounted.

View exelectrician's profile

exelectrician

2327 posts in 1888 days


#11 posted 10-03-2013 06:05 PM

Suck up the dust then push it outside your shop, that way you know no dust remains inside.

-- Love thy neighbour as thyself

View crank49's profile

crank49

3980 posts in 2432 days


#12 posted 10-03-2013 07:12 PM

Speaking in generalities here.
A squirrel cage blower usually has a cylindrical cage with a very large number of short forward curved blades. The blades look like louvers arranged in a cylindrical shape.
This type of fan/blower is used where large volumes of air and very little static pressure is required.
A good choice for an air filter, but would be a bad choice for a dust collector.
Again, there can be exceptions, but that is generally the engineer’s reason to select this type of fan.

-- Michael: Hillary has a long list of accomplishments, though most DAs would refer to them as felonies.

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bigblockyeti

3666 posts in 1181 days


#13 posted 10-03-2013 09:10 PM

I think you’d be okay with the blower on it’s lowest setting, but you’d need substantially more filter surface area than the existing application you have referenced, as they do move less air.

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