If you want to maximize the efficency (airflow) of your shop-vac or dust collection system you must consider the airflow or CFM of the fan (shop-vac, etc ). The idea is to move the air and dust as slow as you can, but never so slow that the large pieces begin to fall out and collect in the duct. This "optimum" speed is around 2700 CFM, usually 3000 CFM to be safe.
If you have multiple machines or "drops" to connect to the duct system, it becomes necessary to size the duct allowing for each opening in the system and the diameter, or required airflow for that opening. This results in a duct system that is largest at the collector, and gradually decreases in size, usually at each drop. With a properely designed duct system you will have equal airflow at every drop on the system, instead of tremendous pull at the first drop, and diminishing pull at each drop further away
The other VERY important factor to consider when planning a duct system is "Static Pressure". ANYTHING that restricts airflow will produce a degree of "static" pressure. Every time the air stream changes direction, flows over an obstruction (seam, ridge), passes through a filter, even general length will add more static pressure, in varying degrees. The pressure at all points of the duct must be considered when building a system.
NOTE*** If you are going to use PVC pipe, you should use the thinnest available pipe and not "schedule 40". The schedule 40 pipe has a minimal inner diameter due to the thickness which results in large amounts of static pressure at the joints and elbows.
The good news is that a shop vac is designed for static pressure so if you have a simple system with one drop then all of the above isnt nearly as important. If you have an actual dust collecter with a blower, then you are wasting your money without a properely designed duct system.
My company designs and installs large industrial systems and we have much experience with wood dust and various ways of collecting it. We also design and sell duct systems (big and small) that are fully do-it-yourself with no drilling or screwing to do. And if you need to move the duct to a new location, it can be taken down and reassembled in any configuration you need, with NO LOSS in structural integrity. See www.nordfab.com for details
If you have any questions concerning dust collection or our duct systems, please send me an email and I will do my best to fully answer any questions you may have.


Best Regards,

Rusty Chappell
Air Pollution Systems
AirPollutionSystems@mediaone.net

P>S> We are in the process of developing a small-medium size woodshop, duct-system, line and would be interested in designing some duct systems for free, and possibly passing through the duct materials at cost. Email me if interested