rrbrown said:

It does benefit you to run 6" duct and then reduce to 4" at the machine. The longer the run of 4" duct the more the air slows and starves the DC. Where as by running larger duct and reducing down it increases the velocity of the air to try and keep up with the larger duct size.

Ok this seems a bit confused. From physics and fluid dynamics I know that Q=v*a, where Q is the volume, v is the mean flow velocity, and a is the cross sectional area of the duct. This means the velocity is roughly twice as high in a 4-inch duct as it is in a 6-inch.

Friction loss is given by the duct equation, which I'm not going to try and do on the iPhone, but there are several online calculators.

Assuming an air volume of 2800-cfm, the 4-inch will have almost 8 times as much head loss as a 6-inch, and 33 times as much as an 8-inch inch.

The 4-inch connection to the machines will restrict flow somewhat, but since the run length is relatively short the increased head loss will not be a major loss, and the increased velocity will help lift large particles out of the machine and into the larger main run.

The carrying velocity for chips and large sawdust is about 4000-feet per minute. Which means for a dc pulling 2800-cfm, that a 10-inch duct will give the least head loss, while maintaining sufficient velocity to carry any sawdust or chips you would ever produce, in a horizontal run.

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