Please either have your duct supplier design the duct sizing or get familiar with the Bill Pentz spreadsheet and design yourself.? Big mains will enable higher CFM only if the port at the machine (the smallest opening the air flows through) is about the same cross sectional area.

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Simple mental experiment.? You have a garden hose bib with say 60 psi of city water pressure.? The 60 psi is like the pressure measurement of a dust collector stated in inches water gauge.? There is also a metric equivalent.? You open the hose bib a little (like a small port) and you get water flowing at some gallon per minute rate. This would be like CFM in dust collection.? Now if you put a bigger water pipe behind the hose bib but you didn’t open the bib any further you would get exactly the same gallons per minute.? You could put a 12” water line behind the hose bib and not get any more water at that setting.? If you want more gallons per minute you have to open the hose bib up.? Or you could increase the pressure in the water line.? The engineering field is called fluid dynamics and that science tells you that to double the gallons per minute you need to increase the pressure by 4X.? So to double the gallons per minute you would need 240 psi of water pressure, or you could open the hose bib a little further.

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My Felder KF700SP has about a 5” port on the side of the machine but inside it’s connected to a 4” flex hose and they connects to a port under the blade that’s equivalent to about a 3” diameter duct.? That 3” diameter is like the hose bib.? When does a large main help, it’s necessary when you are using multiple machines at one time or when you have a machine that has really big ports.? On your table saw with a typical collector you will be lucky to pull 400 cfm.? Doesn’t matter if you have a 5” duct, 6” duct, 8” duct or 20” duct.? The suction pressure will determine how much air moves through that port.? But if the main is really big and you have small effective ports, the velocity of the air in the main will be low and if too low, dust will settle in the main.

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Big mains for a one person shop are usually a mistake.

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I use an Oneida Smart collector that varies the pressure to maximize CFM.? More pressure is the only way to get more cfm through a small port.? My collector has a 7” inlet and I ram 7” main to the planer and drum sander and then 6” everywhere else.? If you have a professional design the layout (free from must duct sellers) or like $100 from Oneida if you don’t buy duct from them, they will use software tools to optimize all the parameters.

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The think for people on maximizing duct diameter is to minimize friction from the air flowing. Friction is real but because the smallest passage for air is the main factor in overall resistance, the big duct doesn’t help much.

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With Bill Pentz’s spreadsheet you can model sample duct runs and play with changing duct sizes to see the impact.? It also flags when velocity is too low.

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