Three-dimensional evaporation process in aqua-ammonia absorption refrigerators using helium as inert gas

Abstract Helium is proposed as a hydrogen substitute in inert gas absorption refrigerators, in order to eliminate the danger of a possible explosion. Attention is focused on the evaporator of such units and a solution is obtained for the three-dimensional evaporation process of a liquid ammonia stream of circular meniscus cross-section flowing parallel to an ammonia-helium gas mixture. The analysis is based on a finite- difference solution of the three-dimensional momentum, heat and mass transfer differential equations within a nearly horizontal evaporation tube. Calculations are also performed using hydrogen instead of helium in order to compare their behaviour as inert gases in aqua-ammonia absorption refrigerators. Under corresponding conditions, higher entropy increase and cooling capacity are observed in the case of helium. Generally, the behaviour of the latter is deemed satisfactory and therefore helium may replace hydrogen.