Energy efficient duct design and control of VAV systems: where is the fan energy savings?

Energy efficient operation of variable speed fans in Variable Air Volume (VAV) systems ishighly dependent on both the type of duct design as well as the type of control strategy thathas been implemented in association with the volume flow demand profiles of each individualzone in the building. The quantification of energy savings due to duct design and the effects offan control have generally been poorly understood, even with very simplistic types of controlstrategies, e.g. static pressure P + I control which have often been employed. The introductionof networked Building Energy Management Systems (BEMS) has offered the possibility toimplement more advanced control methods using terminal unit feedback (commonly knownas box polling), which can be employed to improve the optimal usage of fan power in VAVsystems. The distribution pattern of volume airflow in a multi-zone VAV system with a ductloop is also analysed under various volume flow demand conditions. This paper quantifiedsome of the key issues pertaining to radial and duct loop designs as well as the contribution ofvarious fan control methods to the overall fan energy savings in a VAV system.