A Moving-Boundary Model of a Centrifugal Chiller System

Dynamic models of liquid chillers are gaining importance in recent years as tools for evaluating feedback controllers and to study transient system performance in general. Such system models consist of component models for the heat-exchangers, the compressor and the expansion device. The modeling of the heat-exchangers is particularly significant as they constitute the largest thermal capacitances in the system and thus determine the system response. Existing models of liquid chillers use either highly simplified lumped parameter approximations for the heatexchangers or highly detailed finite-volume formulations. A moving boundary formulation applied to both shelland-tube construction heat-exchangers within a system model with a centrifugal chiller has not been found in the literature. This paper presents a complete system dynamic model of a centrifugal liquid chiller using this approach for both the evaporator and the condenser. The heat-exchangers are flooded types with refrigerant on the shell-side and water as the secondary fluid. The dynamics of the compressor and expansion valve are also included. Simulation results are presented for the system model during different load-change transients. The model is validated using data from a 90-ton single-stage, centrifugal chiller test stand with R134a as the refrigerant. The model is shown to predict system behavior accurately while running faster than real time on a contemporary desktop computer.