Dynamic performance of an electronic zone air temperature control loop in a typical variable-air-volume air conditioning system

Abstract

Variable-Air-Volume (VAV) air conditioning systems are highly dynamic and nonlinear. Few detailed stability studies have been conducted on a practical or typical VAV zone air temperature control loop. In this study, numerical nonlinear dynamic models were built from validated component models of a commercially available thermostat, damper controller-actuators, terminal unit in a typical branch duct, and a simplified fully-mixed zone model from literature. The simulation models were employed to study the dynamic response of a typical VAV electronic zone air temperature control loop to a step disturbance in the zone cooling load. Both pres-sure-dependent and pressure-independent flow control strategies were evaluated. Closed-loop simulation on a medium-size zone, employing the typical thermostat with a time constant of 10 minutes, showed that the thermal storage effect associated with the total zone interior surface area including its contents (e.g. furniture) stabilizes the zone air temperature dynamics. The presence of furniture in a zone also enhances stability in zone air temperature control due to an increase in the total zone surface area. The performance of typical pressure-dependent and pressure-independent flow controllers were compared. With steady branch-duct inlet static pressure, both pressure-dependent and pressure-independent flow controls provided stable zone air temperature control, with slightly superior performance given by the latter. With large fluctuating branch-duct inlet static pressure such as found in a poorly designed or large complex VAV air conditioning system, the dynamic performance of zone air temperature response by pressure-independent flow control is excellent while that by pressure-dependent flow control results in zone air temperature swings. The pressure-independent flow controller is thus recommended for use in large complex VAV air conditioning systems. The simulation results obtained agree with the field results and provide a foundation for further research.