Development of techniques for in-situ measurement of heat and mass transfer in ammonia-water absorption systems

Abstract

An experimental investigation of heat and mass transfer in a horizontal tube falling-film ammonia-water absorber was conducted. A tube bank consisting of four columns of six 9.5 mm (3/8 ) nominal OD, 0.292 m (11.5 ) long tubes was installed in an absorber shell that allowed heat and mass transfer measurements and optical access. A test facility consisting of all the components of a functional absorption chiller was fabricated specifically for this investigation. Several variations of the basic system set up were fabricated to enable testing over the wide range of conditions (nominally, desorber solution outlet concentrations of 5 - 40% for three nominal absorber pressures of 150, 345 and 500 kPa, over solution flow rates of 0.019 0.034 kg/s.) Measurements at the absorber were used to determine heat transfer rates, overall thermal conductances, solution-side heat and vapor-side mass transfer coefficients for each test condition. For the range of experiments conducted, the solution heat transfer coefficient varied from 923 to 2857 W/m2-K while the vapor mass transfer coefficient varied from 0.0026 to 0.25 m/s and the liquid mass transfer coefficient varied from 5.51×10-6 to 3.31×10-5 m/s depending on the test condition. The solution heat transfer coefficient increased with increasing solution flow rate; however, the vapor and liquid mass transfer coefficients seem to remain unaffected with the variations in solution flow rate and were found to be primarily determined by the vapor and solution properties. The experimental heat and mass transfer coefficients were compared with the relevant studies from the literature. Based on the observed trends, heat transfer correlations and mass transfer correlations in the vapor and liquid phases were developed to predict heat and mass transfer coefficients for the range of experimental conditions tested. These correlations can be used to design horizontal tube falling-film absorbers for ammonia-water absorption systems.