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 7 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 6.

  1. Essential airflow formulas: The ability to accurately calculate basic geometric areas is the foundational step for any airflow commissioning technician. Without this, using commissioning equipment effectively is impossible.
  • Area of a Rectangle/Square: Length × Width
  • Area of a Circle: π×radius 2
  • or π×(diameter/2) 2

The most critical formula in airflow commissioning, universally applied (though units may vary regionally), is:

  • Q=A×V
  • Where:
  • Q = Flow Rate (in cubic meters per second, m3 /s, for standard metric calculations)
  • A = Area (in square meters, m2)
  • V = Velocity (in meters per second, m/s)

This formula is incredibly versatile. If any two variables are known, the third can always be calculated. For example, if you know the desired flow rate (Q) and the duct area (A), you can calculate the required velocity (V=Q/A). Conversely, if you measure the velocity (V) and know the duct area (A), you can calculate the actual flow rate (Q=A×V).

  1. Unit conversions for flow rate: Flow rates are typically expressed in different units by various suppliers and design documents, often leading to confusion. Common units include:
  • Cubic meters per second (m3/s)
  • Cubic meters per hour (m3/h)
  • Litres per second (L/s)
  • In South Africa and across many global designs, L/s and m3/s are prevalent, with m3/h being less common in current designs. Key conversions are:
  • m3/s to L/s: Multiply by 1 000 (1 m3 =1000L)
  • L/s to m3/s: Divide by 1 000m3/h to m3/s: Divide by 3 600 (since there are 3 600 seconds in an hour)

Practical Calculation Example:

When calculating area from drawing dimensions, always remember to convert millimeters (mm) to meters (m). For example, a 400mm x 250mm rectangular duct has an area of (0.400m×0.250m)=0.1m2. If the measured velocity in this duct is 5 m/s, the flow rate would be (0.1m2 ×5m/s)=0.5m3/s, which converts to 500 L/s. These calculations can and should be performed in the office before site visits, significantly streamlining the on-site commissioning process. Knowing the expected velocity allows for quick verification rather than extensive troubleshooting.

Continued in Part 8… 

 

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