Reply by crank49

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Posted on My first dust collector

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4032 posts in 3859 days

#1 posted 08-30-2016 03:15 AM

I’ll try to explain with two examples.
1. Propeller fans move a large volume of air with very low pressure.If you hooked a propeller fan to the end of a vertical tube of the same diameter as the fan it would move a large volume of air. Probably about 10,000 CFM with a 16” fan. Reduce the diameter of that tube to 2” and the flow of air will almost be non existant. Thats because the propeller fan only generates a fraction of an inch of pressure. The smaller the pipe gets, the faster the air tries to move but velocity requires pressure.
2. A shop vacuum fan generates a lot of pressure with low volume of air being moved. Hook the vac fan to the same 2” tube and it’ll generate enough pressure to pick up a bowling ball. But, enlarge the tube back to 16” diameter and the air flow will be so tiny it wouldnt blow out a candle.

See, all dust collecting efforts require the right balance of flow and pressure. Google “velocity pressure chart”. Velocity is how fast air moves. The corresponding velocity pressure is the pressure required to make air move at a given speed. If you have a 4” duct and you want to force 400 CFM of air to pass through it, figure the cross sectional area of the duct in square feet. you should come up with about .087 sq.ft. Now take your 400 CFM and divide it by that area. 400/.087= 4597. That is 4597 FPM (feet per minute) the air will have to flow to get 400 CFM through that 4” duct. Find the velocity pressure in the chart you Googled for 4597 FPM and that is the pressure required to generate that flow.

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