A new improvement to the characteristic equation method applied to different single-effect LiBr-H2O absorption chillers

Abstract

The characteristic equation method was developed to be applied to absorption machines using characteristic equations to control and predict the thermodynamic performance of these machines. Since the conventional characteristic equation method was established, different approaches have been taken to improve the accuracy of the results of the characteristic equations. The Dühring parameter (B) represents a linear relationship between the internal boiling temperatures at a given concentration of solution and has usually been adopted as a constant fixed value in the characteristic equation method. The influence of the Dühring parameter (B) on the accuracy of the main results is important to the characteristic equation method and has not been analyzed further. This study analyzed this parameter in the characteristic equation method and a new approach to this method was proposed. This analysis gives a better insight into the influence of this parameter on the characteristic equation method and this approach provides a new use of this method to achieve better thermodynamic prediction and control of absorption chillers of different capacities under different operating conditions. The deviations between the fixed B value usually adopted for the LiBr/H₂O solution and each value of the B obtained by the model at different concentrations of solution and internal temperatures affected the accuracy of the main results of the characteristic equations. An approach using a linear adjustment between B and the thrust temperature (Δt_thrust) was put forward to reduce these deviations. The results of the characteristic equations were more accurate using this adjustment than using the fixed B value. Then, the characteristic equation method was applied to six LiBr/H₂O single-effect absorption chillers using the proposed new approach. This approach was compared with the conventional method and with other main approaches. The heat removed (${\dot{Q}}_E$) and COP results from the characteristic equations and the thermodynamic modelling were compared. All the average deviations using the proposed approach were less than 2 %, which represents an improvement in the accuracy of the results of the characteristic equations and gives a better prediction and control of absorption chiller performance. Therefore, the proposed approach contributed to improve the accuracy of the characteristic equation method in order to refine its application to absorption chillers.