Open absorption dehumidification systems bear the potential to drastically reduce the electrical energy demand for air conditioning, making use of low-grade heat streams such as solar or waste heat. Newly developed absorbents, such as ionic liquids, which are less corrosive against standard heat-exchanger materials, allow for designing new heat and mass exchangers, which reject the heat of absorption during the actual absorption of moisture in the absorber. Nevertheless, reliable correlations for the coupled heat and mass transfer processes in open absorption systems are scarce, not only because of the corrosivity of conventional absorbents. The current study is concerned with establishing design rules for those heat and mass exchangers using an NTU model for both the heat and the mass transfer. The obtained equations are linked through the thermodynamic vapor pressure equilibrium of the specific absorbent at the interfacial area between the air flow and the absorbent solution flow. For given parameters, such as volume flows of air and solution, as well as the cooling and hot water temperatures, this NTU−Model is solved for a given experimental setup using falling film horizontal tube heat and mass exchangers for two different ionic liquids. A comparative analysis between both the two different working fluids and the simulation and the experiments is conducted. The validated simulation is then used to identify improved operational conditions for the existing set−up and further investigations on better heat exchanger designs for future applications will follow.