Comparison of different fan control strategies on a variable air volume systems through simulations and experiments

Through a variable frequency drive (VFD), a variable air volume air handling system can significantly reduce supply fan power under partial load conditions. Typically, the VFD on a supply fan motor is modulated to maintain a supply air duct static pressure set point. The static pressure set point can be either constant or dynamically reset in response to different load conditions. In this paper, comprehensive mathematical models are established to describe the performance of a VFD-motor-fan system under three different static pressure control strategies: constant static pressure set point, static pressure reset by total airflow rate, and static pressure reset by highest zone demand. The total input power to the system, including the energy imparted into the air as well as the energy losses from the VFD, motor, and fan, are simulated and compared among the different static pressure reset strategies with different minimum airflow ratios. The simulation results show that more than 50% of electrical power savings can be realized by static pressure reset when the diversity of the zone thermal loads is moderate. In addition, although a lower minimum airflow ratio can result in more power savings, there were no significant power saving by reducing the minimum airflow ratio from 30% to 10% due to relatively high energy losses of the fan, motor and VFD at low load and speed conditions. Finally, experiments were carried out to demonstrate the performance comparison of three different control strategies.