BT-17 Estimation of Nucleate Boiling Film Breakdown in Falling Film Evaporation

J. W. Palen

This report presents a new method for determining the nucleate boiling film breakdown heat flux in falling film evaporators. Existing data from HTRI falling film evaporation studies at Lehigh University were used to calibrate a theoretical expression relating heat flux to entrainment. An entrainment ratio of 0.0055 mass units liquid to mass units vapor gave the best criterion for film breakdown. Based on this value, a maximum heat flux limit for design of falling film evaporators is provided. We recommend that this expression replace the presently used rule-of-thumb maximum temperature difference of 25 °C (45 °F).