Dust Collection System Design

Dust collection design can be complicated, here are a few simply steps to designing an effective and efficient dust collection system. We have simplified the dust collection system design process for smaller shops, it is important to know federal, state and localities have codes and regulations enforced by AHJ (Authorities Having Jurisdiction) governing sales, construction, installation and or use of dust collection systems.

Air Handling Systems by Manufacturers Service Co., Inc. cannot guarantee compliance, and cannot be responsible for how the Product is installed or used. Before purchase and use of a Product, please review the Product application, national & local codes, regulations, and be sure that the Product, dust collection design, installation, and use will be in compliance.
  1. Download our Air Handling Design Guide. We can also offer you Tips on Installing your Air Handling System.
  2. Follow these written directions:

    Dust collection system diagram

    Before you start designing your Air Handling System you need a floor plan of your shop area including the following:
    1. Location of dust producing machines: indicate size & location of dust pick ups on each machine.
    2. Desired location of dust collector unit.
    3. Floor to joist measurement.
    4. Any obstructions that would interfere with the run of the duct.

    Shop floor plan diagram
  3. Familiarize yourself with these terms:
    • CFM - Air Volume in cubic feet per minute.
    • FPM - Velocity of Air in feet per minute.
    • SP - Static Pressure. This is expressed in inches water gauge. It is resistance to air at rest in a duct, and is also commonly called "resistance", "friction", "friction loss", or "pressure loss."
    • VP - Velocity Pressure: expressed in inches water gauge. It is kinetic pressure in the direction of flow necessary to cause air at rest to flow at a given velocity.

Calculations

It is best to do the following calculations before you purchase your Dust Collector or the necessary ductwork.
  1. Duct Velocity (FPM)
  2. Proper Size of Duct
  3. Total CFM Required
  4. System Resistance (SP)
  1. Duct Velocity

    Use the chart below to determine the velocity of your system.

    Type of Dust Velocity in Branches Velocity in Main
    Metalworking Dust 4500 FPM 4000 FPM
    Woodworking Dust 4000 FPM 3500 FPM
    Other Light Dust 4000 FPM 3500 FPM
  2. Determine the size of each branch

    There are several ways to determine the diameter of the branches.
    1. If the machine has a factory installed collar, the manufacturer has determined that the machine needs that size branch under normal circumstances.
    2. If the machine has a metric diameter outlet, convert it into inches, and round off to the nearest inch. When writing up your parts list you may need to order a custom reducer.
    3. If the outlet is rectangular you need to determine the equivalent round diameter. When you write up your list use a transition.
    4. If the branch is smaller than 3" diameter plan using a reducer near the machinery to increase the branch to 3". Figure the CFM for 3 "(195 CFM).
    Determine CFM requirements for each branch. Under the proper velocity note the CFM of each branch. If working with wood dust, use 4000 FPM in branches ( see above - A) Duct Velocity ).
  3. Determine Diameter of Main Duct
    1. Determine which machines are your primary machines. A primary machine is the machine(s) that will operate at the same time under the worst conditions. (If you normally operate two machines, but once a week need to operate a third machine at the same time, then you must size your system for all three machines.) We generally highlight the primary machines on the drawing.
    2. Sizing the Main Trunk Line. When sizing the main truck line start with the primary machine farthest from the duct collector. Run that size duct until the next primary branch enters the main. Increase the main size at that junction to accommodate the CFM total of the two primaries. You will follow this practice all the way to the collector, sizing all primary junctions, to accommodate total CFM of all primaries at that point. Do not increase main duct size when a branch other than a primary enters. Your total CFM requirement is the total of all primary branches. When not using a primary machine you will close blastgate and divert suction to a secondary machine.

      Example:
      You have 3 primary machines. You have already assigned the branch diameter and CFM requirements.
      Table Saw, Lathe 4" Diameter 350 CFM
      Radial Saw 5" Diameter 550 CFM

      A 4" branch will be run from the Table Saw until it joins with the 4" branch from the Lathe. At this point your main starts and you need to increase the pipe to handle the combined CFM (350+350 = 700). Using the CFM Chart 1 look up 700 CFM under the appropriate velocity (3500 FPM in the main for wood dust), then look at the corresponding diameter (6"). Run 6" pipe in the main from the Lathe until the branch of the Radial Saw joins the main.

      Here again you need to increase your main to handle the total CFM (700+550=1250 CFM). Using the chart again you will see that 1250 CFM is slightly more than volume for 8" diameter. Drop back to 8" diameter so as not to go below transport velocity. Run the 8" duct in your main from the Radial Saw to your Dust Collector.

      If you are installing an indoor re-circulating dust collector you need not calculate any more duct diameters. If you are attaching ductwork to the exhaust side of your duct collector it is accepted practice to use a duct diameter two diameters larger on the exhaust side than on the inlet side, thus minimizing exhaust and duct resistance.

      Charts
  4. Figure System Resistance (SP)

    The total static pressure is several factors added together. They are entry loss, dirty filter loss, static pressure of the worst branch duct, static pressure of the main duct, and static pressure of the return duct.
    1. There are more complicated ways to figure the entry loss of your system, but we find it usually equals a loss of 1" water gauge. (Use 1" as a constant.)
    2. If your system has filters, add in 2" loss. (If you do not have filters add zero).
    3. The Worst Branch, is the branch with the greatest resistance. The branch with the greatest resistance is usually a smaller diameter with the most lineal footage of pipe elbows. Static pressure of worst branch and main duct can be calculated by using the following Chart 2. Chart 2 is based on 100 feet of pipe; therefore, you have to convert all elbows to an equivalent of pipe.

      To convert 90° and 45° elbows to equivalent feet of pipe use our Elbow to Straight Pipe Conversion chart. When figuring the feet of pipe count lateral type branches as 45° elbows. Flexhose has a lot of resistance depending on the amount of bends included in the installation. For this reason we suggest you keep hose to a minimum. Multiply your length of flexhose on your worst branch by 3 for equivalent length of straight pipe.

      Example: Determine Static Pressure of Worst Branch
      Static Pressure (Inches of Water Gauge) in Worst Branch (4" Table Saw)

      Description - 4" Diameter Equivalent to Straight Pipe
      Straight Pipe 20'
      2 - 90° elbows 12'
      2 - 45° elbows 6'
      5' flexhose (3x) 15'
      Total equivalent straight pipe after conversions 53'


      Example: Static Pressure in MAIN DUCT 6" and 8"
      The static pressure of the Main Duct is done the same way, except you figure it out for each diameter in the Main, starting farthest away and working toward the collector.

      Description - 6" Diameter Equivalent to Straight Pipe
      Straight Pipe 20'
      Total equivalent straight pipe after conversions
          700 CFM in 6" diameter = 3.5" S.P. per 100'
          700 CFM in 6" diameter = .70" S.P. per 20'
      20'
      Description - 8" Diameter Equivalent to Straight Pipe
      Straight Pipe 20'
      2 - 90° elbows 30'
      Total equivalent straight pipe after conversions
          1,250 CFM in 8" diameter = 2.4" S.P. per 100'
          1,250 CFM in 8" diameter = 1.3" S.P. per 55'
          (8" Diameter runs to self contained Dust Collector)
      55'
      Total Static Pressure - 1" + 2" + 3.71" + .70" + 1.3" = 8.71" S.P. Inches Water Gauge!

Wrapping Up and Ordering Parts

If clean air return duct is required, duct resistance should also be calculated. Now you have all the information you need to make an educated decision in purchasing your dust collector. You have determined the Velocity, CFM, Static Pressure and the size of the ductwork. To develop your list of materials required, go through the system; this time starting at the dust collector and list each part you will need. Don't forget the assembly equipment such as: pop rivets, hangers, strapping, caulking, and couplings. To order please call: (800) 367-3828; fax: (800) 438-7135; or order online. If you have any questions while you are designing your system give us a call.

For additional information on designing your Air Handling System, we recommend reading Woodshop Dust Control, by professional woodworker Sandor Nagyszalanczy. Sandor provides practical, shop-tested solutions to total dust control so you can build the right system without complex calculations.