Pressure Loss Coefficients for VAV Terminal Units

The goal of duct design is to deliver the proper quantity of conditioned air to various zones in a building as efficiently as possible. Duct design methods utilize predictions of total pressure loss through the duct system, instead of static pressure, since that is what dictates the fan energy needed to deliver the required air flow rate. Variable air volume (VAV) systems regulate the conditioned air flow rate supplied to the served space through the use of a control damper. Likewise, VAV terminal units often incorporate a hot water reheat coil (or electrical heating element) to maintain the space temperature at a desired condition. This study utilizes static pressure drop data taken from one representative manufacturer’s catalog to develop total pressure loss coefficient correlations for single duct VAV terminal units having a circular inlet duct and a rectangular outlet. The inlet damper position changes depending on the heating or cooling requirements for the space. However, in this study the control dampers for each box were assumed to be in their wide open position, since this represents the design condition that must be achieved at the maximum heating or cooling load. The additional pressure loss across a hot water reheat coil or electrical heating element (if present) must also be accounted for when determining the total pressure loss through the box. Separate correlations are presented for cases including a plain box, as well as terminal units that have either hot water reheat coils consisting of one to four tube rows, or an electrical heating element. In every instance the correlations account for static pressure and velocity pressure changes between the box inlet and outlet, based on cross section changes. The use of the correlations developed in this study expedite those calculations, and therefore facilitate the utilization of automated numerical duct design programs that may require extensive iterative calculations, e.g., the static regain method. It is acknowledged that the total pressure loss coefficients derived in this study may not exactly predict the performance of every terminal box supplied by each VAV box manufacturer. However, most VAV terminal unit manufacturers employ a single round or oval damper, which contributes minimal pressure loss in the wide open position. Therefore, it is anticipated that the correlations generated by this investigation should be widely applicable to all single duct VAV boxes having various numbers of hot water reheat coils or reheat electric coils, provided that the reheat coil geometries are geometrically similar to those considered in this investigation and the same area ratios are compared. The correlations are useful for duct designers, particularly when the analysis requires extensive iterative calculations (e.g. the static regain method), and are proposed for inclusion in the pertinent databases to catalog duct fitting loss coefficients.