Evaporation rates in spray drying

Heat and mass transfer coefficients have been determined during the evaporation of water sprays produced by pneumatic atomizing nozzles in a vertical, cocurrent spray dryer, 8‐in. in diameter and 14 ft. high, under various operating conditions. The progressive evaporation of the spray down the chamber was followed by taking samples of the drying air and of the water droplets at various distances from the nozzle, and a new volumetric method for the accurate determination of air humidities was developed. For droplet spray ranging in mean diameter from 11.5 to 38.5 microns, the heat and mass transfer coefficients were found to be essentially the same as for single, stationary droplets evaporating in still air, the similarity being attributed to the almost equal diffusivity of the drop and of the air stream, resulting in negligible relative velocity. Certain important aspects of spray dryer design are discussed, and a method is outlined for the calculation of the residence time in the evaporation zone.