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Forum topic by Kevin_WestCO posted 06-20-2011 07:43 PM 2089 views 0 times favorited 6 replies Add to Favorites Watch
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62 posts in 2746 days

06-20-2011 07:43 PM

Topic tags/keywords: question trick tip

I’m beginning to build a cyclone system for my shop. After researching for a couple weeks I’ve nailed down the design for the separator but I’m having a problem with the required air flow. I’m fairly knowledgeable in the HVAC field but I cannot figure out the required or desired velocity/CFM for a dual 4” system. I’ve been looking at pre-built systems trying to find some kind of idea of what they are pulling CFM wise but everything is stated in the horsepower of motor with very little airflow information. I’m not requiring a large amount of dust collection for my shop but would like it to be adequate for an upgrade later down the road (Over sizing will be better in this case.) I’m planning on using a hard mounted duct blower on this system.

Does anyone know the actual CFM their dust collection system is pulling. If I can get an idea of a couple systems I can get a reasonable benchmark for my design. Has anyone built a complete system from scratch rather then using a dust collection system and then adding a cyclone?

Thanks for looking!

6 replies so far

View brtech's profile


1052 posts in 3121 days

#1 posted 06-20-2011 08:26 PM

These and many other questions you may have will be answered at:

View crank49's profile


4032 posts in 3169 days

#2 posted 06-20-2011 09:55 PM

I would not use a squirrel cage blower. Will not develope enough static pressure. Will load up and get clogged with dust. Will not pass chips and shavings. Squirrel cages are ok for air filters, where they only have to pass air that has already been through a filter.

You need to maintain 3500 feet per minute velocity through the ducts; minimum.
Calculate the square feet of the duct. Divide this into the CFM.

4” dia = 2×2 x pi = 12.56 square inches.
12.56 square inches / 144 = .0873 square feet.

350 CFM / .0873 square feet = 4009 feet per minute.

So, a 4” duct connected to a 350 CFM collector will have a velocity of just over 4000 FPM which is good.

All this assumes the collector has enough static pressure to do this. That is where we get messed up by the marketing hype around all the collectors on the market. No one wants to claim exactly what CFM their system generates at what specific static pressure. You can find 3 different fans with 12” impellers and 2hp motors and one might claim it generates 12” static pressure, another might claim it delivers 1400 CFM, another might say it delivers 900 CFM. These guys claim whatever they think will sell their machine. A fan like this might be able to deliver 1400 CFM without any ducts and filters attached to it, and it might only be able to do this with the motor running at a 125% overload, which will burn it up in a few minutes, but they can claim it can deliver 1400 CFM. And their competitor who said they deliver 900 CFM, with the same fan, will lose sales.

View Kevin_WestCO's profile


62 posts in 2746 days

#3 posted 06-21-2011 12:40 AM

I was looking at a Radial Blade Blower used at a habit store here in town. I think it might be a little small but I’m hoping for the money it might work out. Its a Dayton, here the link:

Considering most of the units for sale are 1hp minimum and this one is 1/2hp.

View bigike's profile


4055 posts in 3487 days

#4 posted 06-21-2011 01:13 AM

I’m just interested to see how this comes out! Please keep us posted?

-- Ike, Big Daddies Woodshop,

View crank49's profile


4032 posts in 3169 days

#5 posted 06-21-2011 03:12 AM

The fan in your link is NO horsepower. It’s a fan without a motor. Why spend $201 dollars for a motorless fan when you can buy the whole fan, 2 hp motor, on/off switch, bag filter and dropout bag, on a base with casters from Harbor Freight for $139? (With a coupon from Wood magazine)

Or, if you don’t like Harbor Freight, you can buy the exact same filter made in the same Chinese factory, but with a red, green or white coat of paint and sold by American sounding company names for $200 to $400 .

If you want to do it yourself, buy the $139 dust collector and throw all the parts away but the motor and fan and you still are $60 ahead and will have a 2 hp blower able to really generate about 800 honest CFM at about 9” static pressure. This will really handle a 5” duct alone, or a 4” and a 2 1/2” at the same time. It will even handle two 4” ducts but with reduced velocity; but enough for table saws, band saws, sanders, etc. Things that make dust mostly. For a lathe, jointer or planer you really want to jack up the velocity to keep those chips and shavings in suspension so I would pull all the 2 hp worth of air through a single 4” duct for those machines and just run one at a time.

Many LJs have bought the HF 2 hp collector and added a pre-filter in the form of what is referred to as a trash can separator to catch the heavy chips and big dust before they go through the fan.. Then they often replace the 5 micron bag with a cartridge filter for increased surface area which reduces the back pressure on the blower. The beauty of these LJ modified collectors is the cartridge filter captures the really fine dust down to 0.5 microns which is he biggest health hazzard..

View boardmaker's profile


35 posts in 2835 days

#6 posted 06-22-2011 03:30 AM

Crank49 nailed it. One thing he didn’t mention is that many also just exhaust outside. You can’t do it in a heated or cooled shop though. Whatever floats your boat.

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