This paper presents a study on the dynamics of a water-cooled centrifugal chiller from two aspects: dynamic simulation modeling and stability analysis. First, the transient model is developed in Modelica with Dymola and the TLK/IfT Library (TIL). The centrifugal compressor is modeled in detail based on the turbomachinery theory. The chiller capacity control is achieved by the combination of variable inlet guide vane (VIGV) and variable speed drive (VSD). The shell-and-tube heat exchangers are discretized based on the finite volume method with convenient and numerically efficient two-phase property evaluations. A thermal expansion valve (TXV) is used to regulate the mass flow rate. These models are interconnected to form a centrifugal chiller system. Second, the stability of the chiller is studied based on Lyapunov analysis. The centrifugal chiller is effectively a “closed-loop” concatenation of the centrifugal compressor, condenser, expansion device and evaporator because the refrigerant flow stays within the closed path of the chiller system. The inherent unstable aspect of the compressor dynamics (e.g. surge) becomes more intrigued because the input and output flow are correlated, as compared to the typical “open-loop” centrifugal compressor applications such as compressed air systems or turbochargers. The Lyapunov stability analysis presented herein constructs partial foundation for future nonlinear control design for the centrifugal chiller systems.

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