Investigations on a virtual airflow meter using projected motor and fan efficiencies

Abstract

Airflow measurements are not as accurate as needed in air-handling units due to space and cost limitations of physical meters. Generally, the airflow in an air-handling unit is propelled by a fan driven by a motor and, hence, the airflow rate is related to fan-motor system performance, which can be affected by other measurable variables, such as fan head and motor power. Theoretically, a virtual airflow meter can be developed to virtually obtain the airflow rate from measured fan head and motor power along with projected motor and fan efficiency models. Because variable frequency drives have been widely installed in HVAC systems, a comprehensive motor efficiency model is needed to project motor efficiency under variable frequency and voltage. At the same time, an in situ fan efficiency curve needs to be projected through a calibration process corresponding to actual fan head measurement. This article explores a theoretical model of virtual airflow meters in order to identify the relationship of fan airflow rate with measurable fan head, motor power, and power frequency and voltage; then, demonstrates a procedure to implement a virtual airflow meter and validate the virtual fan airflow meter through an experiment. The results show that airflow measurements from the virtual airflow meter match very well with a conventional duct mounted airflow meter with an standard deviation of 0.0177 m3/s (37.5 cubic feet per minute [CFM]) for instant measurement and 0.0142 m3/s (30.1 CFM) for a 5-min moving average with the measured airflow range between 0.45 m3/s (950 CFM) and 0.70 m3/s (1,480 CFM).