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Dust Collection Terms

Dust Collection Terms

CFM (Cubic Feet per Minute) provides the amount of air your blower puts out. That amount is decided by the capacity of your blower and how much you choke down its inlet and outlet. Hobbyist dust collector makers rate their blower CFM as the maximum flow with no inlet or outlet blockages. In the real world we have ducting, fittings, separators, cyclones and filters that add resistance. Resistance is called static pressure that is measured as inches of water. To know how a blower will really perform, we must know its fan curve showing the CFM at different static pressure levels. As a rule of thumb, you can expect working CFM levels to be about half the maximum rated.

FPM (Feet per Minute) FPM says how fast the air is moving. FPM can be computed by multiplying CFM by 144 (1 square foot in inches) then dividing that result by the square inches of area for the ducting. For instance a 1.5 HP blower rated at 1100 CFM will give about 550 CFM at a working level. If you use this 1.5 HP blower with a 6″ duct this computes to 550*144=79,200 divided by the duct area = pi*3*3=28.2743 giving 2801 FPM, which is not enough to prevent dust build up in 6″ ducting.

Static Pressure (SP) measures pressure in terms of inches of water. That pressure can be how powerful your blower is, or how much resistance is created by each component in your dust collection system. For any given CFM you can measure the resistance of each component or entire ducting runs. Each collection hood, piece of flex hose, duct pipe, duct fitting, separator, and filter has its own resistance at any given CFM. In addition to measuring, you can use a table that gives the typical resistance for each component then total the resistance for each component to get an estimate of the overall static pressure for the run to any particular tool. The run in a shop with the highest overall resistance can then be used to size the blower needed to overcome that resistance and to size the ducting needed to move ample air without clogging.

 

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