Direct contact heat exchangers

Alongside conventional heat exchangers, which transfer heat through a material wall between two fluids at different temperatures, there's a whole range of industrial devices where the exchange wall is eliminated: cooling towers, solid-gas recuperative exchangers, gas-liquid contactors for heating water from gaseous effluents, air humidifiers, bubble and barometric condensers.... However, direct-contact heat exchangers remain relatively uncommon due to intrinsic constraints and a lack of understanding of the hydrodynamic and thermal behavior of these systems, which involve complex physical processes (multiphase media in flow with or without change of state). Clearly, direct contact heat exchangers are not a panacea, but in certain specific cases they are an interesting alternative, as they offer the possibility of reducing investment costs and increasing exchange performance compared with a conventional exchanger. This article presents the basics needed to understand and design heat exchangers operating on the principle of direct contact, a heat transfer method that is commonplace in nature, since it is involved, in particular, in the water cycle between the earth and the atmosphere through evaporation and condensation, in the formation of fog and snow in the atmosphere, in the gelling of river water and in the solidification of lava, etc....

For notations and symbols, see end of article (p. 29).