Simulation of the behaviour of a centrifugal chiller during quick start-up

During quick start-ups (0–80,000 rpm in a few seconds) of centrifugal chillers, there is substantial risk for lack of superheating out of the evaporator. Moreover, the compressor passes through the surge zone which can lead to damaging flow reversal. These problems cannot be addressed with experiments and need computing help. The work presented here takes into account the mechanical inertia of the fluid, which can no longer be neglected in the momentum equation written in local form in each component of the chiller. All properties are derived from the internal specific energy and density, which are determined locally by the integration of mass and energy equations applied to finite volumes of the components: evaporator, condenser and pipes. The simulations show that without proper control strategies of the electronic expansion valve, liquid can enter the compressor. New strategies are therefore designed which also help to avoid compressor stall.

► Finite volume model of a centrifugal chiller including fluid mechanical inertia.
► All fluid properties derive from chosen state variables: density and internal energy.
► Quick start up simulations including compressor stall phenomenon.
► Appropriate control strategy to avoid compressor stall and liquid feed.