The Mass Airflow Triangle:  What Is the Difference Between ACFM, SCFM, and MCFM? Errors That We All Make.

University of Virginia to present a webinar on mass airflow triangle with AABC

The Mass Airflow Triangle:  What Is the Difference Between ACFM, SCFM, and MCFM? Errors That We All Make.


There can be a lot of confusion associated with a simple term such as airflow. Airflow can mean several different things though. Airflow can be used to refer to velocity, volume, or even mass. Mass flow is typically a foreign concept to the TAB industry and the HVAC industry. Let’s face it, when have you ever seen a mass flow design on a schedule or submittal?  When have you ever balanced to a mass flow rate?

Mass flow is much more important than volumetric flow. ASHRAE 62.1 ventilation rates are based on SCFM values, not ACFM. Room and building cfm differentials for pressurization are based on SCFM values, not ACFM, and yet we conflate the difference.

Density is often understood by many in the industry to have a minor impact on airflow measurements and calibrations, and it is often overlooked due to the thought of these minor changes. However, this is not the case, and this session will show examples of how far off we can get things wrong. Errors of over 10%-15% can be shown.

We will also be discussing the triangle relationship of mass flow, volumetric flow, and density.  ACFM and SCFM always coexist correctly in a proper relationship, but measured cfm (MCFM) often does not.

Learning objectives: 

  1. Define the terminology.
  2. Understand the mass flow triangle relationship.
  3. Examine cases where measurements and calibration can go wrong, and how to properly test in the future.
  4. Learn about new control strategies that will result in proper airflow control.

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Nov 15 2023


2:00 pm


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