Design Considerations for 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 and greater control of the ventilation system.

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 passive 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. However a common concern for building owners is – how do I maintain acceptable indoor quality when there is no accurate airflow measurement in the first place?

Unfortunately due to poor regulation and practise this is a common scenario for most owners today.

COVID-19 is a good reason to fix our bad indoor air quality. As a first step engineers need to limit their liability by providing airflow measurement in the minimum outdoor air intake of each recirculating air handling unit (AHU) to document design compliance when the building is handed over to the owner.

Owner Project Requirement

Building Management System (BMS) sub contractor provides a direct outdoor airflow measurement station (AFMS) capable of measuring the outdoor air intake flow of each air handler unit.

Installed accuracy of the airflow measurement station +/- 10% of signal.

An alarm indicates when the airflow value varies by 15% or more from the outdoor airflow setpoint. Measuring display of AFMS is positioned so that O&M staff can have safe access without aids. The measuring display unit is located 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 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. Direct airflow measurement can also provide better tracking of ventilation rates and assurance that minimum ventilation is achieved for occupants.

Additional monitoring of room population and CO2 concentrations will further optimise the control strategy to help mitigate the risk depending on the room function.

RETROFIT AIRFLOW MEASUREMENT STATIONS

Best Practice Recommendations
  • 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