By Eamonn Ryan

The following is derived from a Cape Town TechTalk on 10 June by Theo van der Linde, Pr. Tech, Head of Project at SFI Group, on the topic ‘Basics of Airflow Commissioning’. This is Part 6 of a 10-part article.

Thorough documentation and effective training are critical for the long-term success and maintainability of any HVAC system.

Thorough documentation and effective training are critical for the long-term success and maintainability of any HVAC system. Freepik

…continued from Part 5.

  1. Multiple measurement points: To obtain a truly accurate average airflow velocity within a duct, especially in larger ducts (for instance, 1×1 meter), it’s essential to take readings at multiple points. Standard guidelines suggest specific grids for rectangular and circular ducts. For example, inserting a hot wire anemometer or Pitot tube and slowly traversing it across the duct, or taking readings at pre-determined points, allows the instrument to calculate an average velocity. While this level of detail might be critical in highly sensitive environments like biosafety labs or hospital theatres, a simpler approach might suffice for less critical applications, such as a general office with 150 diffusers, focusing on key points to confirm reasonable averages. The aim is to ensure “beyond a reasonable doubt” that the average velocity is accurate.
  2. Recommended velocities and on-site efficiency: Knowing the recommended air velocities for different duct sizes and applications is a powerful tool for efficient commissioning. For instance, if a 200mm diameter duct is expected to maintain an average air speed of 7 meters per second (m/s) based on design calculations, a quick reading with a hot wire anemometer that shows a velocity between 6.8 and 7.5 m/s can immediately confirm satisfactory performance. This drastically reduces the time spent on site, as opposed to lengthy diagnostic sessions. While airflow characteristics are generally better in round ducts compared to square ones, understanding the expected velocities for both allows for rapid assessment and targeted adjustments, especially for counter-cooling applications. Being prepared with these velocity benchmarks in mind allows for swift verification and more effective commissioning.

 

Air velocity guidelines and fundamental calculations

Understanding recommended air velocities and mastering basic calculations are cornerstones of effective airflow commissioning. These principles allow for rapid assessment and precise adjustments during the commissioning process.

  1. Air velocity guidelines by application: The optimal air velocity within a duct system varies significantly based on the application of the ventilation system.
  • Industrial/specialised systems: Often feature higher velocities due to the need for greater air changes or contaminant removal
  • Office environments: Typically operate at lower velocities to minimise noise and ensure occupant comfort
  • Laminar airflow systems (e.g., clean rooms): Require very low, uniform velocities to maintain a stable, unidirectional airflow critical for contamination control

A common rule of thumb across the industry for air velocity over filters and coils is approximately 2.5 m/s face velocity. This metric is calculated by dividing the total airflow rate by the effective cross-sectional area of the filter or coil. Emphasising unit consistency is paramount here; mixing units (e.g., airflow in m3/h with area in m2) will lead to incorrect velocity calculations.

Continued in Part 7…

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