Experimental and Analytical Investigation of Ammonia-Water Desorption in Microchannel Geometries
Determan, Matthew D.
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An experimental and analytical study of a microchannel ammonia-water desorber was conducted in this study. The desorber consists of 5 passes of 16 tube rows each with 27, 1.575 mm outside diameter x 140 mm long tubes per row for a total of 2160 tubes. The desorber is an extremely compact 178 mm x 178 mm x 0.508 m tall component, and is capable of transferring the required heat load (~17.5 kW) for a representative residential heat pump system. Experimental results indicate that the heat duty ranged from 5.37 kW to 17.46 kW and the overall heat transfer coefficient ranges from 388 to 617 W/m2-K. The analytical model predicts temperature, concentration and mass flow rate profiles through the desorber, as well as the effective wetted area of the heat transfer surface. Heat and mass transfer correlations as well as locally measured variations in the heating fluid temperature are used to predict the effective wetted area. The average wetted area of the heat and mass exchanger ranged from 0.25 to 0.69 over the range of conditions tested in this study. Local mass transfer results indicate that water vapor is absorbed into the solution in the upper stages of the desorber leading to higher concentration ammonia vapor and therefore reducing the rectifier cooling capacity required. These experimentally validated results indicate that the microchannel geometry is well suited for use as a desorber. Previous experimental and analytical research has demonstrated the performance of this microchannel geometry as an absorber. Together, these studies show that this compact geometry is suitable for all components in an absorption heat pump, which would enable the increased use of absorption technology in the small capacity heat pump market.