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 9 of a 10-part article.
Thorough documentation and effective training are critical for the long-term success and maintainability of any HVAC system. Freepik
Ultimately, what goes into a system must come out. If the measured airflow at the outlets does not sum up to the airflow being supplied by the fan (after accounting for return air or exhaust), it indicates either leakage within the duct system, a compromised fan performance, or a fundamental issue with the design. Addressing these issues systematically, starting with a thorough understanding of the design and working through potential bottlenecks, is key to successful troubleshooting and achieving optimal airflow.
Critical considerations for accurate airflow measurement
Achieving precise airflow measurements, particularly in complex or constrained environments, demands careful attention to where and how readings are taken.
- Strategic measurement locations: Turbulence is the nemesis of accurate airflow measurement. Avoid taking readings directly within flexible ducts or in short transition pieces, as airflow in these sections is inherently turbulent and inconsistent. Instead, seek out straight duct sections, ideally at least five times the equivalent diameter or resistance length away from any flow-disrupting elements like elbows, dampers, or fan inlets/outlets. While ideal conditions are not always possible on site, one must improvise, focusing on the most stable sections available.
- Fan discharge configuration: Measuring airflow directly at a fan’s discharge without any downstream ducting or a grille is highly unreliable. Without a resistive element to straighten the airflow, an axial fan, for instance, will discharge air in a wide, divergent pattern (e.g., 45-50 degrees radial spread) rather than a unidirectional flow. This makes accurate measurement impossible. It is always best practice to ensure a piece of ducting or a discharge grille is installed immediately after the fan, as this helps to straighten the airflow, keeps the fan operating closer to its design curve, and ensures more accurate airflow readings.
- The principle of conservation: what goes in must come out: A fundamental principle in airflow commissioning is that the total airflow entering a system (e.g., from an air handling unit) must equal the sum of the airflow exiting through all terminal units (minus any intended exhaust or leakage). If the total measured airflow at the diffusers and grilles does not match the fan’s discharge flow, it strongly indicates either significant duct leakage, an undetected blockage, or an issue with the fan’s actual performance. This basic conservation law serves as a crucial sanity check for all measurements.
