Direct contact heat transfer is of interest in many fields. Direct contactors are used in absorption refrigeration, distillation, electric power generation (as feed water heaters and in conjunction with dry cooling towers as in the so-called Heller Cycle), food processing, drying, emergency cooling of nuclear reactors, quenching of hot gases, etc. Further, the combustion process using liquid fuels is intimately connected to die direct contact evaporation of liquid aerosol sprays. Thus, its use is ubiquitous. Despite this fact, research into the direct contact processes lags well behind that of heat transfer through solid surfaces.

Direct contact heat transfer is generally defined as heat transfer between two of more mass streams without the presence of an intervening wall. The mass streams can be co-current or counter current or even cross-flow. The streams may be composed of, for example, systems that are: liquid-liquid, liquid-vapor, liquid-gas, liquid-solid, gas-solid or even solid-solid. The streams can be composed of the same chemicals as in the case of steam-water, or can be partially miscible, or completely so. One or both of the streams can be disperse (made up of defined masses, panicles, drops, bubbles) or continuous. Quite often, direct contact heat transfer is accompanied by mass transfer such as in the case of dryers or distillation columns, or in condensers. The possibilities are legion. Thus, in this chapter, we must limit ourselves to a few fundamental processes. We refer the reader to the literature or specific review articles such as Sideman (1966) on Direct Contact Heat Transfer Between Immiscible Liquids. Sideman and Moalem-Maron (1967) on Direct Contact Condensation, or Jacobs (n.d.) on Direct Contact Heat Transfer for Process Technologies, for example.

The fundamental processes to be included here are sensible heat transfer between parallel liquid streams in co- or counter flow; sensible heat transfer to a disperse flow of particles, drops, or bubbles, flow with condensation, and flow with evaporation with similar geometry to that for sensible heat transfer.