Measuring dust collector cfm?

[DaveBozeman]

I have a small Handheld Anemometer where I can measure wind speed at the blast gate.. is there anyway to know how many cfm it would equate too?

I just need a ballpark, to see if I have anywhere near what I need for a new drum sander.

thank you

[TDKPE]

That's easy. 

Run the DC with an open duct or blast gate. Be aware that air flow is not uniform across the end of an open duct, with highest air speed being at the center (assuming round), down to zero at the surface (theoretical).  Take an average across the radius, from center outward to the wall.  

Multiply the diameter in inches by itself and by 3.14, then divide by 4.  That's the area in square inches.  Divide again by 144 for square feet of cross-sectional area.  

Depending on what your anemometer reads in, like knots or mph or ft/s or ft/min, you'll have to multiply air speed in ft/min by cross sectional area in square ft to get cubic feet per minute (ft/min*ft^2 = ft^3/min).  That means converting air speed in whatever units it reads to ft/min.

Easiest to just use an online conversion web page if you don't have ft/min, like the one below, which is already drilled down to "speed" unit conversions.  

Online Conversion - All Speed and Velocity Conversions

Be aware also that the open end of a thin wall duct is a poor entrance shape, and even a small bit of bell mouth flare will improve the air flow.  On the other hand, more flow restriction than an open duct will reduce air flow, so maybe it's enough of a wash that you don't even have to consider it.  

[brianwelch]

That's easy. 

Run the DC with an open duct or blast gate.  Be aware that air flow is not uniform across the end of an open duct, with highest air speed being at the center (assuming round), down to zero at the surface (theoretical).  Take an average across the radius, from center outward to the wall.  

Multiply the diameter in inches by itself and by 3.14, then divide by 4.  That's the area in square inches.  Divide again by 144 for square feet of cross-sectional area.  

Depending on what your anemometer reads in, like knots or mph or ft/s or ft/min, you'll have to multiply air speed in ft/min by cross sectional area in square ft to get cubic feet per minute (ft/min*ft^2 = ft^3/min).  That means converting air speed in whatever units it reads to ft/min.

Easiest to just use an online conversion web page if you don't have ft/min, like the one below, which is already drilled down to "speed" unit conversions.  

Online Conversion - All Speed and Velocity Conversions

Be aware also that the open end of a thin wall duct is a poor entrance shape, and even a small bit of bell mouth flare will improve the air flow.  On the other hand, more flow restriction than an open duct will reduce air flow, so maybe it's enough of a wash that you don't even have to consider it.  

Good to know...now to find me an anemometer...
Thanks

[Juss1]

Good to know...now to find me an anemometer...
Thanks

Amazon carries them.  Haven't used this model so no experience but the price is right.

https://www.amazon.com/HOLDPEAK-Anemomet...Y2s9dHJ1ZQ==

[jteneyck]

You can determine CFM without spending money.  Just make yourself a water manometer Link with a piece of plastic tubing and measure the static pressure in the duct of interest.  Convert that to CFM:  Link

John

[DaveBozeman]

Does this site seem accurate ?

https://www.engineering.com/calculators/airflow.htm

[TDKPE]

Does this site seem accurate ?

https://www.engineering.com/calculators/airflow.htm

Working it out by hand for 50 mph through a 6" round duct, and plugging that into the spreadsheet (radius of 3" as an input), gives the same 864 cfm.  

Much easier than working it out by hand.  I'm just old-school I'm afraid and my go-to move is pencil and paper and my trusty antique HP10C, with a bit of CAD or solid modeling thrown in when necessary.  

[jteneyck]

Working it out by hand for 50 mph through a 6" round duct, and plugging that into the spreadsheet (radius of 3" as an input), gives the same 864 cfm.  

Much easier than working it out by hand.  I'm just old-school I'm afraid and my go-to move is pencil and paper and my trusty antique HP10C, with a bit of CAD or solid modeling thrown in when necessary.  

Ha. I have a HP 15C.  Must be nearly 30 years old and I still use it almost daily.  I often think things through and do the first calculations manually but I'm happy to use on-line calculators once I understand the underlying fundamentals.  The stuff offered at Engineering.com is almost always well done.  

John

[TDKPE]

Ha. I have a HP 15C.  Must be nearly 30 years old and I still use it almost daily. 

I bought mine new in the early 80's, and went through engineering school and did the EIT and PE exams with it.  It's pretty beat up, but still runs strong.  I also have a 42S, though I'm much faster with the 10C, probably since I'm still more used to the horizontal layout.  

I also use DW's 12C (financial) just for simple math, and it lives in a drawer in the kitchen.  Math functions are the same, of course (RPN), but I don't remember or never knew what most of the functionality is on that thing.

Oh, and nobody asks to borrow my calculator after the first try. RPN is an acquired taste.

[DaveBozeman]

I reread your post.. I missed the radius, and used diameter.. the numbers are more what I expected

250cfm at the blast gate, 150cfm to the tool.. system needs an upgrade