Duct Design

PVC and Metal Pipe

When considering ducting for your shop, there are two popular material choices: PVC and metal.

When Clear Vue Cyclones started making cyclones, we exclusively recommended the use of PVC pipe for the following reasons:

  •     It's usually less expensive than metal pipe
  •     It's readily available from plumbing supply houses
  •     It is easy to work with.

The "Sewer and Drain" PVC pipe we recommend for ducting may be found at plumbing supply companies such as HD Fowler, Ferguson, or at a local vendor in your area.  We recommend purchasing non-gasketed pipe.  You will find the pipe referenced as ASTM D-2729, ASTM D-3034 and/or SDR 35.  Compatible pipe will have the following outside diameters (OD):

  •     4" PVC: 4.215" OD
  •     6" PVC: 6.275" OD
  •     8" PVC: 8.4" OD

While PVC is a viable option, there are those who prefer metal.  Clear Vue Cyclones is a distributor for Nordfab and can help you with both designing and ordering your ductwork. All of the ducting included in our designs and quotes are Nordfab Quick-Fit (QF) metal ducting products. The Nordfab Laser “Quick-Fit” Piping System has been used in many different industrial applications, and under various negative static pressures. Pipes come in 5’ lengths with a rolled lip on each end, thus providing reinforcement every 5 feet, which presents a sound structural design that should be stronger than any pipe in its class. This quality ducting is easy and fast to install, easily disassembled, modified and re-useable. Most customers cut their installation time by more than 45% using Nordfab Quick-Fit ducting.

Duct Design Consideration

Proper ducting design requires an understanding of the interplay between the air volume and air velocity required to move dust/debris. An air speed of at least 3800 feet per minute in vertical runs (known as down drops) is required to avoid clogs that block our airflow. An air speed of about 2800 feet per minute is required to keep horizontal runs clear. If adequate air speed is not maintained, these runs build dust piles inside the duct. These piles constrict air flow within the pipe, which in turn increases air velocity. This combination causes the dust piles to grow longer, increasing the possibility of fire.

To ensure safe margin, most air engineers design their systems to move at least 4000 feet per minute in vertical runs and 3000 feet per minute in horizontal runs. Because air will not compress at typical dust collection pressures, it is important to run duct of the proper size. At these air speeds, reducing to a 4" duct cuts the airflow to 349 CFM, a 5" duct cuts the flow to 545 CFM, a 6" duct to 785 CFM and a 7" duct to 1069 CFM. Most small shop tools get great "chip collection" with 4" ducting and ports, but good fine dust collection requires moving 1000 CFM at each tool.

Clear Vue Cyclone blowers are specially engineered with higher pressure impellers to move at least 1000 CFM with 6" diameter duct. This was done so we could take advantage of readily available, less expensive and easier to work 6" diameter duct and PVC pieces. If you have a two-car garage sized shop, there is no need to use anything larger than 6" ducting mains with a Clear Vue Cyclone. If you have a larger shop or very long duct runs, you can reduce your ducting resistance significantly by using either 7" or 8" diameter horizontal main ducts. Because air runs slower in these larger mains, it is recommended that you do not go larger than 8" diameter in order to keep the run clear. For vertical runs, it is recommended that you do not go larger than 7" as the airspeed will slow enough to cause plugging.

If you run PVC pipe, we recommend that you do not glue it together. Instead, fasten each fitting with a short #6 or #8 sheet metal screw through the fitting and into the pipe. This will keep the fittings from working loose and allows you the flexibility to change your layout later, if necessary. We recommend sealing the joints on all PVC slip fittings with either aluminum duct tape or a tiny bead of silicone caulk at the seam.

Tips to remember:

  • Use as little flex hose as possible (3-4 feet), as it will reduce air flow.
  • Avoid hard 90 degree turns. Instead, use two 45 degree angles or three 30 degree angles.
  • Keep your duct runs as short as possible.
  • Take advantage of blast gates in strategic areas to help control and maximize air flow.
  • Use wye fittings and avoid tees.
  • Use tapered reducers, when necessary.
  • Ensure your system is airtight.
  • Use hoods to help collect dust and debris.

Static Electricity and Grounding your System

Grounding your system is never overkill, especially if you live in a dry climate. There is no evidence of static causing fires or explosions in small shops, but getting a good shock when you are working with fast moving blades, bits and cutters can be dangerous. To get a good ground with a metal ducting system, make sure you use aluminum tape on the joints. When using flexible hose, use hose with a grounding wire that can be connected to both the duct and either your cyclone or machine. You cannot ground PVC as it is an insulator. You can, however, bleed off the static well enough to avoid dangerous shocks. To prepare your PVC duct, run a strip of aluminum tape inside the duct and a matching strip outside. Each run must be connected with a wire between ducts. Typically, a wire is placed on the sheet metal screws used to secure the fittings. Make sure you are using wire reinforced flexible hose and ground that to your duct and your cyclone or tool. Grounding your PVC will help dissipate any static charges created by the air and wood debris flowing through your ducts. A very light spray of WD-40 will stop the static buildup inside your Clear Vue Cyclone.

If you are uncertain as to whether your system is properly grounded, Clear Vue Cyclones recommends that you hire a certified electrician to ground or verify the grounding of your system.

For more information on grounding, please visit Rod Cole's in depth article "Grounding PVC and Other Dust Collection Myths": http://www.woodcentral.com/articles/shop/articles_221.shtml