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 the air quality of a building is to control the indoor environment through ventilation.

The quality of the air we breathe in the workplace plays an important factor in our well-being. Ventilation can help to reduce airborne infectious disease transmission, moisture, odours, smoke, heat, or dust from the indoor environment and replenish it with air from outside. 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., windows, is not an option for occupants.

Australia’s standard for mechanical ventilation AS 1668:2 specifies minimum ventilation rates for design compliance. A concern for owners can be – how do you maintain acceptable indoor quality when there is no airflow measurement? Unfortunately due to poor regulation and practise this is a common scenario for most owners in 20221.

Engineers can limit their IAQ 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.

Principal Project Requirement

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

Performance: airflow measurement station shall have an installed accuracy of +/- 10% of signal.

An alarm must indicate when the airflow value varies by 15% or more from the outdoor airflow setpoint.

Measuring display of AFMS must be positioned so that O&M staff can have access without aids. The measuring display unit must be located at least 110 cm above the floor and no more than 190 cm 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.

Thermal dispersion using two bead‐in‐glass thermistors. Image courtesy of EBTRON.

Whenever the outdoor air ventilation rates are modulated, use of airflow measurement must 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.

Outdoor airflow measuring station for small packaged HVAC units. Courtesy of Ebtron

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

Note 1: Ref: 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.