Measurements of Heat and Mass Transfer Coefficients During Absorption of Water Vapor by Lithium Bromide and (Li, K, Na) NO<sub>3</sub> Mixtures

A knowledge of heat and mass transfer coefficients in heat pump fluids, plays an important role in the design of absorption machines. Heat and mass transfer coefficients as well as subcooling are measured for absorption of water vapor in (Li, K, Na)No<sub>3</sub> and Lithium Bromide (LiBr) mixtures.The rate of absorption of water vapor is obtained from the difference in concentration of mixtures between inlet and outlet streams across the absorber. In situ concentrations of aqueous salt mixtures over temperature ranges between 80 to 135 °C were calculated from density measurements. This technique of measurement is a reliable and convenient but not a very accurate (± 0.8 wt% salt) method of measuring the in situ salt concentration. Results show that the subcooling at the absorber exit is not only a property of the fluid, but depends strongly on the process conditions. The subcooling in LiBr mixtures without additive is shown to vary between 2.2 and 24.3 °C and the film heat transfer coefficient between 1365.2 and 801.1 W/m<sup>2</sup>.K respectively, depending upon process conditions. These empirical results will prove to be of value to heat pump manufacturers because they have a strong bearing on costs and performance. Heat and mass transfer coefficients in aqueous salt solutions ate presented as a function of dimensionless numbers.