AC-2344 -- Visual Observations in an Ammonia Evaporator

It is generally agreed that visual observations are of much value in understanding the physical phenomena that occur in evaporators. While several such studies on R-ll, R-12, and R-22 evaporators have been reported, no such efforts seem to have been devoted to ammonia evaporators. Ammonia differs markedly from other common refrigerants because it is practically insoluble in oil, while R-ll, R-22, and R-12 are soluble. For this reason, results of visual studies on these refrigerants are not directly applicable to ammonia evaporators.

Extensive studies on a recirculation type ammonia evaporator were made by the author at the Institute of Refrigeration, Technical University of Norway at Trondheim, Norway. In these experiments, local heat transfer coefficients and local pressure drop rates were measured, and simultaneous observations through sight glasses were made. Correlations developed from these data for predicting heat transfer and pressure drop rates are presented in another paper. In that paper, visual observations are discussed only briefly.

In this study, flow pattern data from several test runs are presented along with the measured heat transfer and pressure drop rates to examine any possible interrelation. The development of flow patterns along the length of the evaporator for various mass flow rates and heat fluxes is described with other phenomena like tube wetting, film climbing, etc. Flow pattern data are compared with two correlations, resulting in maps that can be used for predicting flow patterns. The behavior of oil observed in the sight glasses and its apparent effect on heat transfer and pressure drop are discussed.