A new theory is presented, termed the vapor surfactant theory, which explains the heat and mass transfer enhancement mechanism of additives in absorption chillers. The vapor surfactant theory holds that the enhancement additives are surfactants delivered to the surface of the solution film from the vapor and that they produce enhancement by generating intense secondary flows on the film. The secondary flows, driven by surface tension gradients (Marangoni convection), augment the transfer process by providing mixing of the film. The primary variable in the enhancement is the surface concentration distribution of the surfactant additive since the surface tension gradients arise due to non-uniformities in the surface concentration. The vapor surfactant theory is found to explain all observations from several different types of experiments. The new theory is a significant departure from past theories in its emphasis on the primary role of the vapor in the process.

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