Answering Your Airflow and Ductwork Questions: Part Two

Answering Your Airflow and Ductwork Questions: Part Two

Clogged ducting

1. What information do we need to start designing a system and what are the considerations for ductwork?

  • 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 to be too high, whether it is combustible and/or explosive, particulate size, physical characteristics.
  • Combustible dust will certainly require special precautions in the system design to protect the facility and workers from deflagration or explosion. Dust testing can determine how explosive dust is and the needed precautions.
  • A general layout of the facility and each location generating dust.
  • A sketch of the ductwork layout, including the location of the dust collector, fans, and ductwork branches.
  • A plan for the type of hood to use at each of the dust capture locations. Also, how much CFM each one will require.

Large ductwork run with flex hose drops

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

  • Hoods: must be the correct design for each type of machine. They must also be efficient at removing the produced dust by that machine.
  • Ducts: must be the correct size to allow proper airflow and keep air moving. They also must not have too many bends or elbows to slow down the airflow. This can allow particulates to drop out of the airflow. It can also 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 the correct size for the system. It must have 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. This could be DeltaMAXX nanofiber for most applications, or spunbond, PTFE, or other specialized filters for particular applications. Further, 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 returning air to the building, it must be clean enough to meet all health and safety standards. If dust is toxic, it may necessitate an extra layer of safety in the form of HEPA filters. 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, diverting the flame in a safe direction.


3. What is the process for designing a ductwork system?

  • System design should start with identifying each place that a hood or other source capture point needs to go. In other words, anywhere that generates dust.
  • 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 airflow 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. It filters 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 changes. This is from old/contaminated air to new/clean. The calculation is made with the cubic volume of the work area and the number of required air changes to maintain air quality.
  • Source capture/close capture: captures generated dust at each point throughout the facility.
  • This type of system must have properly designed hoods at every capture point. Perform and double-check your airflow calculations. Specialty hoods can be designed for almost any application.
  • Static pressure in these systems is an algebraic formula. It includes a loss at hoods, flex ducts, transitions, and straight ductwork runs. Minimal use of elbows and flex ducts in the ductwork runs 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.

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Answering Your Airflow and Ductwork Questions: Part One

Answering Your Airflow and Ductwork Questions: Part One

Diagram illustrating formulas for static pressures in ductwork with no airflow

1.  What do the different types of pressure measurements mean for airflow in ductwork?

  • The basic behavior of airflow: it always travels through the past of least resistance, from higher pressures to lower pressures.
  • STATIC PRESSURE: potential pressure exerted in all directions in the ductwork with no airflow, measured with a manometer. It can also be positive (pressure outward on the duct) or negative (pressure pulling inward on the duct)
  • VELOCITY PRESSURE: pressure necessary to move air through the ductwork at a certain velocity. Further, this is kinetic energy, and the pressure is in the direction of the airflow.
  • TOTAL PRESSURE: a calculation of system static pressure and velocity pressure (the formula is TP=SP+VP). It can be positive or negative. It is the overall energy content of the air stream.


fan and silencer atop CMAXX at Fabtech trade show

2.  When we do our airflow calculations to determine our fan size, what’s the difference between SCFM and ACFM?

  • SCFM is Standard Cubic Feet per Minute. This is a measurement based on a standard temperature and air pressure.
  • Temperature and air pressure can be significantly different in different areas (e.g. higher altitude, cold or hot climate, high humidity). Then can also vary in different applications (high/low temperature or humidity applications).
  • ACFM is Actual Cubic Feet per Minute. This is a measurement of the actual airflow conditions that your system will be operating under.
  • Fan manufacturers are able to make these airflow calculations. However, they need to be provided with accurate data about temperature and humidity conditions in your application.


Duct line along ceiling

3.  What is conveying velocity and why is it important?

  • The unit of measurement for conveying velocity is feet per minute (FPM). It is the required velocity to pick up dust in the air stream and move it through the ductwork.
  • It is calculated by the CFM at the point of capture and the square foot cross-section of the ductwork.
  • This is extremely important because if the conveying velocity drops too low at any point in the dust transport, the dust will drop out of the airflow.
  • There are a variety of tables to help engineers calculate the minimum conveying velocity for different kinds of dust.
  • Combustible materials might need a higher minimum conveying velocity. This is because an accumulation of this material in the ductwork could cause a deflagration or dust explosion.
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