Mechanisms of Drying of Skin-Forming Materials. I. Droplets of Materials Which Gelatinised at High Temperature

Heat and mass transfer rates related to single droplets suspended from a nozzle were studied in a 42mm 1D. horizontal wind tunnel designed to supply hot dry air, to simulate conditions in a spray dryer. A novel rotating glass nozzle facilitated direct measurements of droplet weight and core temperature. Revised correlations were obtained for heat and mass transfer coefficients, for evaporation from pure water drop1ets. The drying of droplets of suspensions was then investigated. Dried crusts were recovered and examined by Scanning Electron Microscopy. Skin-forming materials were classified into three types according to the mechanisms of skin formation. A description is given of studies on single-droplets of the first type i.e. materials which gelatinised at high temperatures, namely custard and starch. An increase in the drying temperature resulted in increased resistance to vapour diffusion due to increased granule swelling and the crust resistance was completely transfered to a skin resistance at drying temperatures > 150 °C. A mathematical model was proposed for the drying of this type of material. The experimentally-observed times at which a skin appeared on the droplet's surfaces were in excellent agreement with those predicted from the model.