Using the air-water system, transient experimental measurements of liquid temperature, bubbling height, evaporation rate, gas hold-up and bubble size distributions in a direct-contact evaporator were performed for four gas superficial velocities including operation in both homogeneous and heterogeneous bubbling regimes. Perforated and porous plates were used as spargers. Significant effects of both the sparger and the gas flow rate upon the equipment performance were verified and systematically analysed, revealing some rather interesting features of transient non-isothermal bubbling, such as temporal evolution of bubble size distributions and gas hold-up values. (C) 2004 Elsevier Ltd. All rights reserved.