Calculating CFM for Blowers

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Forum topic by Wingstress posted 04-26-2009 01:02 AM 3600 views 0 times favorited 4 replies Add to Favorites Watch
View Wingstress's profile


337 posts in 3480 days

04-26-2009 01:02 AM

Topic tags/keywords: dust collection question

I just purchased a home made dust collection system and I am now trying to size out the ducting. The only problem is that every reference to ducting is related to CFM of the dust collector. I have the model number and all the specs of the blower and motor, but I don’t have a CFM rating. Does anyone know how I can approximate the CFM of the system so I can size my ducts? I am an Engineer so if you want to geek out and lay down a bunch of formulas, I’d happily dive into them, otherwise a simple “rule of thumb” or approximation will do…
The dust collector in question was posted a few days ago.

Click for details

Thanks in advance for the help…

-- Tom, Simsbury, CT

4 replies so far

View Woodchuck1957's profile


944 posts in 3729 days

#1 posted 04-26-2009 01:08 AM

A 1 hp blower isn’t enough for a stationary ducting system. Probably the easiest way to find out the CFM is to get the information off the blower and do a search, or call the manufacturer of the blower.

View BlankMan's profile


1490 posts in 3318 days

#2 posted 04-26-2009 01:30 AM

Like getting the manufacturers rating is of any value.

With a Dywer Mark II Manometer and Pitot tube. You can connect a length of pipe to the blower, say 10 feet, install the pitot tube so that it’s centered, close to the blower but I think there is a minimum to keep it away, then you can measure Static Pressure and Velocity Pressure.

You can then calculate Air Velocity using the measured Velocity Pressure and then calculate CFM using the Air Velocity and pipe area.

This will tell you what the blower really can do and not some exaggerated manufacturer supplied number.

While you’re at it you might want to close off the pipe opening a little at a time and take Static Pressure and Velocity Pressure readings then you can make a Static Pressure vs CFM graph so once you have the ductwork designed you can estimate its Static Pressure and using the graph you’ll know your CFM operating point.

Grainger carries the Dywer Manometer and Pitot tubes. I have both and have done this.

One thing I forgot to mention, you need to know barometric pressure and temperature to do the Air Velocity calculation because you need to know, thus calculate, Air Density, so a good small desktop type weather station would be of value.

I’m pretty sure all the instructions and formulas come with the Dywer Manometer, if not, I probably got them somewhere.

Oh and Dywer Mark II Model 26, it goes to 7” H2O.

-- -Curt, Milwaukee, WI

View TopamaxSurvivor's profile


18246 posts in 3641 days

#3 posted 04-26-2009 05:19 AM

Most fractional HP exhaust fans are a few hundred cfm, so my wild guess would be 1000 to 1200. Like WC says, check with the fan manufacturer if there isn’t any info on the label.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Durnik150's profile


647 posts in 3287 days

#4 posted 04-27-2009 12:15 PM

The next time you are in Woodcraft or Rockler, sneak over to the bookshelf and grab a book by Sandor Nagszalanczy (I mauled the spelling but it’s close) called “Woodshop Dust Control.” Most of the book avoids formulas since most people run screaming from them. However, he did put a few in there for the folks who wanted them or were putting together an elaborate system. It’s worth a look!

-- Behind the Bark is a lot of Heartwood----Charles, Centennial, CO

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