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When you talk about dust collection, woodnthings is right. It is about air flow but more importantly it is about the velocity of the air to keep the dust in suspension. Corners of a square box decrease the velocity of the air as the air and suspended dust particles abruptly change direction. The cfm ratings tell you how much volume the fan is capable of drawing through the system. The diameter of the duct dictates velocity at that point. Change the diameter changes the velocity. But in order to draw dust particles through the system, the velocity of the air has to be high enough to capture the dust and keep it suspended. We call the critical speed of the air the capture velocity. In order to achieve that flow rate, there has to be a source of air available to be drawn into the system. When you design a system, you have to recognize where the dust will be generated and maximize the velocity of the air at that spot to capture the dust. Once the dust is captured, it has to stay suspended so the air velocity has to remain high enough. The larger diameter of the duct, the slower the air travels risking dust coming out of solution and clogging the system. The more friction in the duct, the slower the air. The more chaotic the air flow, the lower the velocity. This is why spiral ducts have a better track record. But be careful that the spirals all force the air to travel in the same direction otherwise you introduce turbulence and drag reducing velocity.
 
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