Chart relating air speed with CFM?

[Wade Lippman]

I just bought an anemometer on ebay for $12. Outside it seems to work reasonably, but I guess I will have to wait for a hurricane to be sure.

I tested a 4" duct that I use to collect lathe sanding dust. It shows 70mph. (well, between 65 and 73, but centered on 70) As nearly as I can figure that is 500cfm, but I could be wrong. Is there a chart anywhere that relates air speed with CFM?

When I bought it, Grizzly CS told me the 3hp didn't work effectively with only one 4" port open, as it needed more air flow to run at full speed. I should open another port, and the one I was using would actually pull more air because the impeller could get up to speed.
I tried that, and with another port open on the other side of the main Y, the air speed went down to 55mph. Does that disprove Grizzly's advice?

[John Stankus]

If I did my math correct. For a 4 inch duct. Multiply mph by 7.68 to get to cfm

This does not account for velocity gradient across duct, but should be pretty good
The process is convert miles per hour to feet per minute first by multiplying by 5280 ft per mile. And then by 1hr per 60 minutes. Then find the cross sectional area of the duct by pi x radius squared. Convert the radius to feet first and you will get the area in square feet. Multiply cross sectional area by velocity in feet per minute to get cfm.

John

[Gerry Grzadzinski]

If I did my math correct. For a 4 inch duct. Multiply mph by 7.68 to get to cfm

That's the number I got. 537 CFM

[John Stankus]

For other duct sizes

Duct diameter in inches	1	2	3	4	5	6	7	8	9	10
Multply mph by	0.48	1.92	4.32	7.68	12.00	17.28	23.52	30.72	38.88	48.00

to get cfm

John

[Paul Stoops]

Somehow the information you quoted from the Grizzly CS either got mixed up or you were misinformed. The DC fan runs at a fixed speed, regardless of the amount of air it is moving. The number of runs open does not affect the fan speed. However, the volume of air that can be moved will vary with the number of ports open at the same time. It is true that the DC will probably not move the specified or maximum amount of air with only one 4" port open, due to the restriction of the small port -- and opening additional ports will increase the total air volume moved. However, as you noted by your testing, the velocity of the air in the ducts will reduced with multiple ports open. Keep in mind that there are a lot of variables in play at one time, such as the diameter and length of the ducting, the number of runs, the number and type of fittings, size and configuration of your machine ports, etc. For these reasons, most of us hobbyist woodworkers operate our DC's with only one port open at a time to maximize the airflow thru the port being used.

Paul -- Auburn, WA

[Alan Bienlein]

I just bought an anemometer on ebay for $12. Outside it seems to work reasonably, but I guess I will have to wait for a hurricane to be sure.

I tested a 4" duct that I use to collect lathe sanding dust. It shows 70mph. (well, between 65 and 73, but centered on 70) As nearly as I can figure that is 500cfm, but I could be wrong. Is there a chart anywhere that relates air speed with CFM?

When I bought it, Grizzly CS told me the 3hp didn't work effectively with only one 4" port open, as it needed more air flow to run at full speed. I should open another port, and the one I was using would actually pull more air because the impeller could get up to speed.
I tried that, and with another port open on the other side of the main Y, the air speed went down to 55mph. Does that disprove Grizzly's advice?

Not sure if you have a cyclone or a bag unit but if you have a cyclone then the advice you got from Grizzly customer service is correct as the cyclone needs a certain amount of airflow for maximum separation efficiency. If it's a bag unit then it doesn't really matter.

[David V Carpenter]

This chart might be easier and it contains other info that you might use.Duct DP 2s.pdfDuct DP 1s.pdf