Variable-air-volume terminal units I: Steady-state models

Variable-air-volume (vAv) systems have become one of the most popular forms of air conditioning in recent years. They represent approximately 51% by value of the UK market for terminal units. Although VAV applications in the UK have recently declined with increasing interest in natural ventilation and low-energy buildings, terminal units are still used in many existing VAV air-conditioned buildings as well as some installations with displacement ventilation. VAV systems can be difficult to control for both maximum efficiency and stable operation, especially under part-load conditions. Steady-state terminal unit models are required for the network analysis of such systems to detect possible terminal unit 'starvation' and to determine part-load efficiency. This paper deals with the steady-state models of common vAv boxes. A future paper will describe the dynamic analysis. The measurements and modelling of three commercial VAV terminal units, two pressure-independent and one pressure-dependent, are described as well as the rig on which the measurements were made. These models are an improvement over the simple opposed-blade damper model that is currently in use as a simplification of the VAV terminal unit in simulation and network analysis programs. Depending on terminal unit design, the use of opposed-blade damper models could under-estimate the volume flow rate by as much as 65% where the damper is fully open under the same authority and percentage damper angle.