Air lift pumps are finding increasing use where pump reliability and low maintenance are required, and where corrosive, abrasive, or radioactive fluids must be handled. Although air lifts are used in nuclear fuel reprocessing plants, no general, theoretically sound equation has been proposed in the literature for tall air lift design. Such an equation is developed from two-phase flow theory to predict the height to which an air lift pump operating in the slug flow regime can lift a given volumetric flow rate of liquid, given the air flow rate and pressure at the point of gas intduction. The widely used drift-flux model for the prediction of holdup is combined with an appmximate relationship to predict pmsure loss, and is substituted into the total pressure differential. ing a momentum balance. However, this analysis was accurate only in the design of short pumps, since there was no provision for variation in gas volumetric flow rate over the tube length. For taller pumps, the method has to be applied incrementally. In the analysis below, a differential momentum balance is integrated over the whole pump length to provide a closed-form equation that is valid for air lifts of any height operating in bubble or slug two-phase flow. This new equation compares favorably with data in the literature, and with experimental data from a 38 mm dia. testinstallation. CONCLUSIONS AND SIGNIFICANCEResults from a 38 mm air lift test installation support a new design equation, based on a two-phase flow momentum balance.The test apparatus was constructed to provide for the operation of the air lift in either vacuum or overpressure conditions, and four different liquids were used in the tests. Operating curves plotted from the results over a range of air flow rates agree with curves given by the new design equation. In addition, the new designCorrespondence concerning this paper should be addressed to N. N. Clark. R. J. Dabolt is presently with Chem Nuclear Systems, Columbia, SC.equation is able to predict the performance of very tall air lifts used by Shaw (1920) in mine dewatering, and so appears valid over a wide range of operating conditions and configurations. In using this equation, one need know only the air flow rate, essential measurements of the pump, and friction factor for the flow in order to predict the liquid flow rate. Since all of these terms are readily available to the design engineer, this new equation will prove easier to use than energy balances, which require a knowledge of the pump efficiency, or empirical correlations, which are often cumbersome and not universally applicable.