Extension of the Effectiveness Method to Volatile Absorbents in Absorption Chillers: application to an NH 3 –H 2 O Combined Generator Integrated into a Single-Stage Cycle

This study investigates the use of an enlarged effectiveness method to evaluate the characteristics of multifunctional exchangers and their impact on absorption machines performances. The enlarged effectiveness method consists in calculating the physical fluids characteristics (mass flow rates, temperatures and ammonia concentration) at the outlet of a component, using the fluids characteristics at the inlet and the mass, thermal and species effectiveness. The evolution of the effectiveness as a function of the operating conditions are described using a set of three non-dimensional numbers: the equilibrium factor, the number of transfer units and a modified Jakob number. The method is applied to a compact combined generator, where the desorber and rectifier sections combined in a same exchanger replace a flooded generator–separators–rectifier assembly, in an NH3–H2O absorption machine dedicated to air conditioning. The model predicts an improvement of the coefficient of performance by up to 10%, and a raise of the vapor ammonia concentration by +0.008. Varying the solution mass flow rate or the heat source temperature at the combined generator inlet controls the machine cooling capacity while keeping the performance coefficient nearly constant. The solution temperature entering the combined generator controls the ammonia concentration exiting the generator.