May 2, 2018

Clogged ducting

1. What information do we need to start designing a system?

  • First, you need to know about your dust. Important details: how it’s being generated, how toxic or hazardous it is, what exposure levels OSHA considers too high, whether it is combustible and/or explosive, particulate size, physical characteristics.
  • Combustible dust will require special precautions to be worked into the system design to protect the facility and workers from deflagration or explosion. Dust testing can determine how explosive dust is and what precautions are needed.
  • A general layout of the facility and each location where dust is being generated.
  • A sketch of the duct layout, including the location of the dust collector, fans, and duct branches.
  • A plan for the type of hood that will be used at each of the dust capture locations and how much CFM each one will require.

Large Duct run with flex hose drops

2. What are the basic components of a dust collection system?

  • Hoods: must be correctly designed for each type of machine, and must be efficient at removing the dust produced by that machine.
  • Ducts: must be correctly sized to allow proper airflow, keep air moving, and not have too many bends or elbows to slow down airflow; this can allow particulate to drop out of the airflow or can create a point for wear and tear on the ductwork.
  • Fan: the fan must be powerful enough to keep air moving through all the ductwork at a high enough velocity. Drops in velocity or not enough velocity can allow dust to fall out of the air stream.
  • Collector: must be correctly sized for the system, with an air to cloth ratio that makes it able to filter all the air coming into the collector. Must have correct filter material to handle the size and type of particulate (DeltaMAXX nanofiber for most applications, or spunbond, PTFE, or other specialized filters for particular applications). Filters must be fire-retardant if the dust is flammable.
  • Fire Prevention: devices such as spark arrestors can help keep sparks from entering the dust collector. A water or chemical fire suppression system can extinguish sparks or flame when a sensor detects them.
  • Venting/Exhaust: If air is being returned to the building, it must be clean enough to meet all health and safety standards. If dust is toxic, an extra layer of safety in the form of HEPA filters might be needed. A backdraft damper can prevent backdrafts from allowing dust back into the system. An abort gate with spark detector can sense a spark or flame and can divert the flame in a safe direction.


3. What is the process for designing a system?

  • System design should start with identifying each place that a hood or other source capture point needs to go (anywhere that dust is generated).
  • Use appropriate calculations to figure out how much CFM you need at each of these points
  • Determine the minimum duct velocity. This is based on the transport velocity (the air flow needed to keep your particular dust moving in the air stream).
  • Calculate the size of duct for each branch. This is based on the CFM and the transport velocity and will be different for different spots along the ductwork.
  • Increase duct velocity at each branch to maintain transport velocity until all branches are connected to the main duct.

4. Will this be an ambient system or source capture system?

  • Ambient system: removes air from the entire work area, filters it, and recirculates it back into the area, diluting contaminated air with clean air.
  • Air changes per hour: the number of times per hour that the total amount of air in the area is changed from old/contaminated to new/clean. This is calculated by the cubic volume of the work area and the number of air changes required to maintain air quality.
  • Source capture/close capture: captures dust at each point where it is generated throughout the facility.
  • This type of system must have properly designed hoods at every capture point and properly calculated airflow. Specialty hoods can be designed for almost any application.
  • Static pressure in these systems is an algebraic formula that includes loss at hoods, flex ducts, transitions, and straight runs. Minimal use of elbows and flex ducts will greatly improve system efficiency.
  • For dangerous or toxic dust where exposure must be kept to an absolute minimum, a close capture system will keep the material from entering the air of the general workplace.


We hope this set of articles about ductwork help to answer some of your questions. Please thank the knowledgeable and experienced Charlie Miller for providing so much valuable information and sharing his wisdom.