Variable Speed Drive (VSD) Applications in Dual-Duct Constant Volume Systems

Models have been developed for static pressure and potential supply fan energy savings by using variable speed drive (VSD) in dual-duct constant volume systems. Experiments have been performed using a full size dual-duct constant volume system installed in a 68,000 ft (6,317 m) office and classroom building. The measured static pressure variations and the energy savings agree with the model projected values. The VSD saves the fan power by as much as 35%, reduces the total airflow by 15%, and decreases the excessive static pressure on the terminal box dampers. This paper presents the systems models, the experimental methods and the results. INTRODUCTION Single-fan, dual-duct (SFDD) constant volume air-handling units (AHU) have been installed in many medical facilities, office buildings and library facilities since 1940’s. They are especially popular in hot and humid climates because they offer good temperature and humidity control. When a constant speed fan is used, the static pressures in both the hot and cold ducts are higher than the design specifications under partial load conditions. Terminal box dampers are often over-pressurized, which creates noise and vibration problems and causes excessive airflow in parts of the building where the single actuator terminal boxes are used [Liu et al., 1997]. The performance of the SFDD systems can be improved by converting them to dual-fan, dual-duct (DFDD) systems [Joo and Liu, 2002]. In such a conversion a dedicated hot air fan is added to the system, and variable speed drives are added to both the hot and cold air fans. The fan speeds are controlled to maintain the required static pressures at the selected duct locations. The dual-fan conversion requires major mechanical retrofits. This paper investigates the potential supply air fan energy savings by using VSD in single-fan, dualduct constant volume systems. The VSD modulates the fan speed to maintain the lower of the hot duct and cold duct static pressures at the set point. The theoretical models have been developed. Experiments have also been performed using a full size system in a 68,000 ft building. Both the theoretical models and experiments are presented in this paper. MODELS The single-fan, dual-duct constant volume unit provides both hot air and cold air to rooms where terminal boxes mix cold and hot air to accommodate the room load variation. When the cooling load decreases, the hot air flow increases and the cold airflow decreases. When the cooling load increases, the hot air flow decreases and the cold airflow increases. The total airflow of the AHU remains constant. Figure 1 presents a single-fan, dual-duct constant volume system with static pressure control. The static pressure control system consists of a controller, two static pressure sensors and a variable speed drive.