How to Size Ductwork for Your Dust Collection System

Having an efficient dust collection system is essential for a variety of reasons; they protect the health and safety of employees, keep businesses regulation compliant, and help save money by preventing the need for manual dust removal. Whether your shop is large or small, investing in a dust collection system is a must. Read on to learn about how to size ductwork for your dust collection system to fit your shop’s specific requirements. 

Terms to Learn

Before you can begin your calculations, you will need to familiarize yourself with the following terms: 

  • CFM (Cubic Feet per Minute): volume of air moved per minute
  • FPM (Feet per Minute): velocity of the airstream

Sizing Your Ductwork

Preparations: Before you begin calculating for ductwork size, take measurements of your shop floor plan and create a diagram. Use your diagram to plot the most efficient route (with the fewest number of turns or bends) for your duct lines. 

Step 1: The first thing you need to do is determine the FPM needed for your system based on the type of dust that will be produced. Use this helpful guide: 

  • For metalworking dust: 5000 FPM in branches, 4500 FPM in main trunk line
  • For woodworking dust: 4500 FPM in branches, 4000 FPM in main trunk line
  • For plastic/other light dust: 4500 FPM in branches, 4000 FPM in main trunk line

Step 2: Next, you will need to determine the diameter of each branch line. To do this, you can consult the manufacturer, or use the diameter of a factory installed port. Convert rectangular ports to the equivalent round diameter and metric ports to the nearest inch. You will need reducers for ports with diameters of less than 3”. 

Step 3: Now, use your FPM and diameter measurements to determine the CFM requirement for each branch. To find the required CFM, multiply FPM by the cross-sectional area (ft2). 

CFM = FPM x (𝝅r2)

Step 4: After determining CFM for each branch, you will need to define your heaviest use scenario so that you can make sure your system will meet it. Do this by identifying your high-use machines (those that frequently operate simultaneously). Do not include infrequently used machines in your calculations or you will end up with an over-designed dust collection system that is more expensive up front and costs more to operate. 

Step 5: With the data gathered in the last 4 steps, you should be able to size all of your branch lines. Now, you need to size the main trunk line. For in depth instructions on how to complete this step (and previous steps), including examples and helpful tables for determining CFM and FPM, click here to view Spiral Manufacturing’s dust collection design pamphlet. 

Contact leading duct manufacturer, Spiral Mfg. at 763-755-7677 or for more information on dust collection system design.