Integration of gravity-fed evaporators in CO2 based heat-pump chillers

Abstract

This study is carried out to model a gravity-fed evaporator for CO2 based heat-pump chillers. The gravity-fed evaporator loop consists of a separator, a downcomer, a heat exchanger, and a riser. The working principle is that the evaporation of fluid in the heat exchanger gives rise to a density gradient in the loop. This density gradient generates the buoyancy force which drives the fluid by overcoming all the pressure drops in the loop. Appropriate equations are derived to estimate these pressure resistances. Finally, the balance between the buoyancy forces and pressure resistances dictates the flowrate in the loop. This procedure is implemented in Modelica to develop the simulation model for the gravity-fed evaporator. The loop dimensions are critical to the performance of the system. Hence it is essential to estimate the loop dimensions accurately. In this study, the simulations are carried out to predict the optimum dimensions to achieve the optimum circulation rate in the loop. It is expected that this study will be helpful in designing optimized gravity-fed evaporators for CO2 based heat-pump chillers.