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 5 of a 10-part article.
Thorough documentation and effective training are critical for the long-term success and maintainability of any HVAC system. Freepik
- Pressure drop across filters: Understanding typical pressure drops across various components, especially filters, is vital.
- Washable pleated filters: Typically exhibit a pressure drop of 60-75 Pascals (Pa).
- New, unloaded filters: Around 50 Pa.
- Loaded filters: Can see significantly higher pressure drops. Pressure drop is inversely proportional to the filter’s area; a larger filter area generally means less pressure drop for the same airflow. This highlights a crucial check during commissioning: if a fan is part of a retrofit but the existing ductwork and filters are retained, their condition must be assessed. A washable pleated filter with a pressure drop exceeding 150 Pa, for instance, strongly indicates a blockage, a severely loaded filter, or a design miscalculation, requiring immediate investigation.
- Fan curves and duty points: Before arriving on site, it is good practice to obtain and understand the fan curve for each fan in the system. The fan curve plots the fan’s airflow rate against the static pressure it can deliver at different speeds (low, medium, high). Knowing the design duty point (e.g., 900 m3/h at 220 Pa static pressure) allows the commissioner to:
- Verify fan operation: Confirm if the fan is operating at the correct speed setting (e.g., medium speed for the example given) to achieve the design airflow and pressure.
- Identify system issues: If a fan is delivering the required airflow at a significantly higher static pressure than designed (e.g., 900 m3/h on high speed at 380 Pa), it indicates an issue within the system, such as excessive duct resistance or a blockage. This proactive check can save hours of troubleshooting on site.
While general office environments might not require extremely high static pressures, specialised applications like clean rooms or hospital operating theaters often operate at much higher static pressures (700-800 Pa) due to stringent filtration and air change requirements. Being aware of these design specificities from the outset is crucial.
