Variable-air-volume terminal units II: Dynamic model

Abstract

Variable-air-volume (VAV) air conditioning systems are used extensively in air-conditioned buildings. This paper extends the steady-state measurements described in a previously paper to obtain a dynamic model of one of the pressure-independent terminal units. These have sometimes been difficult to tune due to interaction with other VAV equipment, e.g. the supply fan or other terminal units. Hence validated, dynamic pressure-independent terminal unit models are essential for future investigations of plant interaction. This paper describes the derivation of a representative dynamic model based on a commercially manufactured European terminal unit including the integral flow controller and actuator gear common in Europe. The dynamic terminal unit model contains detailed sub-modules for the flow controller, variable-speed actuator, damper linkages with hysteresis, damper, fan and velocity sensor. The sub-modules are assembled on a readily available simulation platform. Using them, accurate validated results have been obtained, showing that instability can occur. The modules will be useful to simulators wishing to model real VAV systems.