A 20" planer needs to have around 800CFM to clean it properly.

The question stands, what size pipe should I run? Lets assume this is a single operator shop so the planer will not run with any other tool. All we want to do is deliver 800 CFM to the planer, that's the worst case our system will have to support. Lets also assume that the system will be run with a 2Hp fan.

Here are two examples. A right and a wrong way to design this ductwork. The first (right) way uses 6" diameter pipe run all the way to the planer from the cyclone. This design will work and be able to deliver the desired 800CFM. The second (wrong) way uses all 4" pipe run from the cyclone to the planer. This design will not get the desired 800CFM to the planer. In fact, it will deliver about half that amount.

Lets look at the right way...