| A study was conducted to measure heat transfer coefficients and pressure drops for evaporation of both pure ammonia and ammonia, mixed with a miscible oil in a plain and an enhanced tube over the range of conditions that would occur in a direct expansion air coil. Heat transfer coefficients were determined using both the log-mean temperature difference and a locally determined heat flux.;It was found that the average heat transfer coefficients in the microfin tube were 20% to 70% larger than those in the smooth tube, for the same operating conditions. The addition of oil was found to increase heat transfer for a circulating concentration of 1% and decrease performance at a concentration of 5%.;The average two-phase pressure drop measured in the microfin tube was found to be from 10% lower to 20% higher than those measured in the smooth tube. The presence of oil was found to have a larger effect in the smooth tube than in the microfin tube as evidenced by the fact that the tube oil penalty factors, which show the relative performance of the microfin tube to the smooth tube at the same oil concentration, tended to be less than 1.0.;During the course of the study flow patterns in both the smooth and the microfin tube were observed and it was found that the predominant flow pattern in both tubes was stratified-wavy. Based on these observations a stratified flow heat transfer model was developed for both the smooth and the microfin tube, and these models were found to accurately predict the majority of the heat transfer coefficients measured in the study. |
