The enhancement of heat transfer for ammonia evaporating outside vertical tubes

Experiments were conducted to determine the heat transfer performance of single vertical tubes with ammonia evaporating from a thin film on the outside of the tube. The three aluminum tubes employed were of 2.54-cm (1-in.) nominal diameter and 1.22-m (4-ft) length: the first tube had smooth inside and outside surfaces; the second one had a smooth inside surface, but 48 flutes on the outside; and the third tube had 34 flutes on the inside and 42 flutes on the outside. Three different ammonia feed rates [3.15 x 10^-6 m³/s (0.05 gpm), 6.13 x 10^-6 m³/s (0.10 gpm), and 15.77 x 10^-6 m³/s (0.25 gpm)] were used in the tests for each tube, while the heat load was varied from 690 W (2360 Btu/h) to 3990 W (13,620 Btu/h).

Evaporation heat transfer coefficents, based on the total tube outside surface area, are reported as composite coefficients, which include the resistances of both the evaporating film and the tube wall. Composite evaporation heat transfer coefficients ranged from 2720 W/m²⋅K (480 Btu/h ft²⋅°F) to 12,020 W/m²⋅K (2120 Btu/h ft²⋅°F). In general, the largest heat transfer coefficients were obtained at the highest heat load and ammonia feed rate tested. Corresponding composite temperature differences ranged from 1.3 K (2.3°F) to 5.7 K (10.2°F), while reflux ratios, the ratio of ammonia feed rate to evaporation rate, ranged from 1.1 to 12.6.

A brief photographic study illustrates the presence of boiling during some of the ammonia evaporation test runs. During thin film evaporation of ammonia, no appreciable heat transfer enhancement was obtained when utilizing fluted surfaces in stead of smooth ones.