www. refrigerationandaircon. co. za RACA Journal I September 2025 9
TECH TALK measure air velocity at diffusers and assess how air is induced and mixed within a space to ensure optimal comfort and effectiveness. Air balancing is an iterative process, demanding repeated adjustments and measurements to perfectly optimise air movement, making it a systematic approach rather than a single-point check.
ESSENTIAL TOOLS FOR AIRFLOW COMMISSIONING Effective airflow commissioning relies on specialised tools, each serving a unique purpose in achieving accurate measurements:
• Capture hoods( balometers) are invaluable for measuring volumetric airflow directly at grilles and diffusers. These devices, often appearing as a cone with a foam sealer, integrate a vane or hot wire anemometer to take multiple readings and calculate average flow rates, ensuring a complete seal for accurate terminal measurements.
• For in-duct measurements or when direct grille access is unavailable, a hot wire anemometer is indispensable. This telescopic probe, featuring a heated wire and airflow sensor, is inserted into ductwork to provide precise velocity readings, crucial for verifying main air supply.
• Pressure differential sensors( manometers) are critical for two primary functions: measuring room differential pressure in controlled environments( hospitals, labs) to prevent cross-contamination, and verifying static pressure across AHUs and fans to confirm system operation at design pressure points.
• Lastly, pitot tubes, typically used with manometers, are essential for accurate air velocity measurements within ducts, particularly in higher velocity applications. They derive velocity from total and static pressure differentials, offering a robust method for in-duct readings.
Regardless of the specific brand or design, the fundamental principles of these instruments remain consistent, enabling the accurate and reliable airflow measurements essential for comprehensive commissioning.
SITE READINESS AND SYSTEM FUNDAMENTALS A major emphasis of effective commissioning lies in ensuring the site is genuinely ready before the commissioning team mobilises. This critical pre-check phase prevents wasted time, resources and project delays. Essential site readiness includes verifying all diffusers and grilles are installed, power is supplied to all terminal units, ducting is complete and intact( especially
flexible ducts), the full HVAC system is installed as per design, and all critical snags are rectified. Adhering to standards like ASHRAE Guideline 1.1-2007 provides comprehensive checklists to ensure this systematic approach.
A fundamental understanding of system design parameters and basic calculations is also crucial. A common pitfall is inconsistent unit conversion; airflow rates from suppliers often vary – including m3 / h, m3 / s or L / s- necessitating careful conversion to consistent units before any calculations. Understanding pressure drop across filters is vital; a washable pleated filter with a pressure drop exceeding 150 Pa, for instance, strongly indicates a blockage or severe loading. Fan curves and duty points are equally important. By knowing the design airflow and static pressure, commissioners can verify fan operation and identify system issues, such as excessive duct resistance or blockages, proactively saving troubleshooting time.
Real-world installations demand adaptive strategies for accurate measurement and diligent troubleshooting.
Accurate airflow measurement requires laminar flow, ideally found in straight duct sections at least five times the equivalent diameter away from disturbances like elbows or transitions. Avoid measuring in flexible ducts due to inherent turbulence. Furthermore, never measure directly at a fan ' s discharge without downstream ducting or a grille, as this leads to highly inaccurate, non-unidirectional airflow. It is always best practice to ensure a piece of ducting or a discharge grille is installed immediately after the fan.
A fundamental principle is conservation: what goes in must come out. If the total measured airflow at outlets doesn ' t match the fan ' s discharge( accounting for exhaust / return), it indicates significant duct leakage, an undetected blockage, or compromised fan performance. Post-measurement, permanent access plugs are crucial to seal holes, prevent leaks and to mark points for future access.
When facing low airflow, systematic troubleshooting is paramount. Start by verifying the source( fan / AHU power, speed, current draw). Inspect for upstream restrictions or blockages, such as clogged filters, obstructed intake louvres, or debris in ductwork. Check for faulty components, including the fan itself( even new ones can be defective), misaligned actuators / dampers, or uncalibrated sensors that might be causing the VSD to operate incorrectly. Also, always review the original design; discrepancies like undersized fans or altered duct routes due to space constraints can severely impact performance.
In complex scenarios, collaboration and expert assistance are key. Provide specialists with a clear overview of checks already performed, and allow them the space to assess the situation. Ultimately, ensuring proper installation quality, regularly calibrating sensors, and confirming correct fan direction are final checkpoints vital for ongoing system performance. RACA
www. refrigerationandaircon. co. za RACA Journal I September 2025 9