Design considerations for accurate airflow measurement

Ventilation is the process of bringing air from outside a building inside, distributing it, and later expelling the air outside. One of the easiest and most effective ways to improve indoor air quality is have accurate measurement for greater control of ventilation.

The quality of the air we breathe in the workplace plays an important factor in our well-being. Ever felt tired or drowsy in a meeting room? Could not focus and head felt heavy? Your air quality might be to blame.

Ventilation can assist to reduce moisture, odours, smoke, heat, or dust from the room and replenish it with air from outside. In addition, ventilation can also help with managing exposure risk resulting from the handling of hazardous substances in research labs.

Mechanical ventilation offers a controllable, responsive way to provide adequate indoor air quality when natural ventilation e.g., operable windows or louvres, is not available for occupants. Australia’s standard for AS 1668:2 specifies minimum ventilation rates for design compliance.

COVID-19 is a good reason to fix our bad indoor air quality. As a first step engineers need to improve compliance and verification of ventilation is a good start.

Owner project requirement example

Provide an airflow measurement station (AFMS) to measure the outdoor air intake flow (L/s) of each air handler unit (AHU).

Installed accuracy of AFMS: +/- 10% of signal.

Indicate a warning alarm when airflow value varies by 15% or more from the outdoor airflow setpoint.

Position operator display of AFMS so that personnel have safe access without aids. Locate the measuring display unit at least 1100 mm above the floor and no more than 1900 mm above the floor.

You can’t improve what you don’t measure

External variations in wind, stack effect and inaccurate damper positioning on the intakes of mechanical equipment, will often result in widely fluctuating outdoor air intake flow rates (± 50% is not uncommon) if not directly measured and controlled.

Do it right the first time. Whenever the outdoor air ventilation rates are modulated, use of airflow measurement needs to be considered as dampers are not linear flow devices. Airflow measurement can also be used to set limits, such as the building component or the maximum capacity of the mechanical equipment.

Measurement provides better tracking of ventilation rates and validates that minimum ventilation is achieved.

Additional monitoring of population and CO₂ concentrations in dense locations will further optimise the control strategy to help mitigate the risk.

Best practice

• Replace differential pressure (DP) devices on BMS and ventilation systems including pitot tubes, pitot arrays, piezo rings and devices that measure the pressure drop across a mixed air chamber, louver, cooling or heating coil or obstruction
• Each sensor node must use two (2) bead‐in‐glass thermistors and be individually factory calibrated and marked to NIST traceable standards

References

1. AS1668 Part 2 § 2.3.1 Intakes for outdoor air are located and arranged so that all conditions of operation including effects from wind and other factors do not cause the flow rate of outdoor air to be reduced below the minimum requirements.
2. AIRAH Technical Handbook Edition 5 2013