Heat and mass transfer characteristic of vertical falling film generator with annular structure for ammonia-water system

Abstract

Ammonia–water solution is widely used in absorption cycle systems, but the generation of ammonia from solution requires a great amount of heat consumption. In this work, a vertical-tube heat exchanger was adopted as the generator of an ammonia–water absorption cycle. The generation of ammonia from an ammonia–water solution in the generator was experimentally analyzed. The generator has an annular structure: a falling film of ammonia–water solution flows inside the inner tube, and an upflowing water stream flows in the annular space between the outside surface of the inner tube and the inside surface of the outer tube. First, the heat transfer enhancement effect of the annular structure was confirmed using water as the working fluid; then, the effect was tested for an ammonia–water solution. It was found that the heat transfer coefficient of Annulus I (0.5 mm annulus gap) was 316.2% higher than that of Annulus III (2.5 mm annulus gap) at the water flow rate of 50 g⋅s−1 through the annulus space. When the flow rate of the upflowing fluid exceeds a certain value, the effect of heat transfer for these annuli deviates obviously. The solution in the falling film with an initial ammonia concentration of 18.7 wt. % was used in the generation experiment. The outlet ammonia concentration of Annulus I was 12.7% lower than that of Annulus III at the water flow rate of 33.3 g⋅s−1. Based on the Sieder–Tate formula, the Nusselt number correlation equations were developed with ±10% error.