Air Speed vs Air Flow: their impacts on leaf blower

How about CFM?

CFM is the abbreviation of Cubic Feet per Minute. When it is about the leaf blower use, this basically means the volume of the air that the machine is able to blow every minute. Concretly, it is the flow of air leaving the machine. You must consider a high CFM when picking a leaf blower.

For example, a 1000 CFM leaf blower means that each minute, when set at its maximum power, 1000 Cubic Feet (approximately 9.3 m²) will be blown out of the tube.

In concrete terms, consider an empty box of 1000 cubic feet: 10 feet wide, 10 feet long and 10 feet high. Then a 1000 CFM leaf blower will be able to fill in with air within 1 minute.

Air speed (MPH) and Air flow (CFM) of a leaf blower are linked to each other

As we mentioned earlier, Cubic Feet per Minute (CFM) is the air flow (the volume of the air per minute) and air speed stands forthe speed at which the air leaves the tube.

When buying a leaf blower, those two characteristics must match. Indeed, a tool that blows very strongly (high MPH) with a small amount of air (CFM) is useless. Consider a straw, when you blow inside a cup of water the MPH is high and the water waves a lot. However you can’t clean a whole garden with a straw as you need more air per minute.

Mathematical explanation of Air speed and Air flow

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Now let's take a closer look at how those two importants characteristics are linked to each other. Be aware, a little bit mathematics and physics will be done here. But relax, it is quite simple! Let's note Q the air flow in CFM meaning every minute Q Cubic Feet leaves the tube of the machine. Let's note v the air speed in MPH meaning every hour, the air travels v miles. Finally, let's note S the surface of exit of the tube in square foot (kinda door but in our case it's a circle door).

Through the surface S, every minutes there is Q Cubic Foot air that is blown. We can deduce easily the speed of the air. Q divided by S equals the speed of the air in feet per minute. Q / S = v airspeed (foot per minute). So we multiple by 60 to convert the hours in minutes : 60 x Q / S = airspeed (foot per hour). We now divide the result by 5280 to get the result in miles per hour: (60 X Q) / (S x 5280) = v (miles per hour).

Concrete example of calculation

Let's apply this new formula! Assume we have a leaf blower with an Air Speed of 180 MPH and an Air Flow of 650 CFM at its turbo setting. By manipulating the previous formula, we get can get the width of the tube!

Surface = (Q x 60) / (v x 5280) = (650 x 60) / (180 x 5280) = 0,041 foot square.

As you know since secondary school, Surface = π x radius² hence radius = √(surface / π) = √(0.041 / π) = 0.114 feet = 1.368 inches = 3.47 cm. Multiply this result by 2 and you get the diameter!