Reverse relief airflow prevention and building pressurization with a decoupled relief air damper in air-handling units

Outdoor airflow rate, building static pressure, supply air duct static pressure, and relief air plenum static pressure, as controlled variables, are maintained by modulating the speed of the supply and return fans and the position of the outdoor, recirculating, and relief air dampers in an air-handling unit (AHU). The three dampers are often interlinked completely or partially to match independent control inputs with the controlled variables. The traditional damper control has all three dampers interlinked with no control over the relief air plenum static pressure. Reverse relief airflow might occur as the outdoor air damper approaches the closed position. To prevent reverse airflow, one solution is to decouple the relief air damper and maintain positive static pressure at the relief air plenum. Two control methods are available based on the control loop design. The first control methoduses the return fan speed to control the relief air plenum static pressure and the relief air damper to control the building static pressure, and the second control method switches two control inputs. The purpose of this paper is to evaluate the controllability of the reverse relief airflow and building static pressure of these two control methods. The system performance is investigated through steady-state simulations on an AHU at different outdoor airflows using a nonlinear network solution. The simulation results show that both the control methods can well prevent reverserelief airflow. However, the first controlmethod with a constant pressure setpoint may result in negative building static pressure at lower outdoor airflow ratio and either excessively positive building static pressure or excessive return fan power at higher outdoor airflow ratio. The second control method shows better control over the building static pressure control. A reset pressure scheme should be associated with the first control method when the relief damper is fully open.