Direct contact heat transfer is a fundamental process by which heat transfer occurs in nature. It is used for the improvement of cooling or heating processes, in terms of characteristic time, compactness and energy costs. Direct contact heat transfer is potentially more efficient than conventional heat exchange because it eliminates performance reductions caused by fouling of the heat exchange surfaces and the associated energy losses. The major benefit lies in the dramatic increase of the specific contact area between the interacting fluids. A major difference between direct contact and conventional heat exchangers is that the former permit mass transfer as well as heat transfer between the two fluids. In chemical and process plants, mass transfer is usually the main objective and the heat transfer is a necessary but secondary feature. Direct contact devices designed specifically for mass transfer are widely used in industry, whereas direct contact heat exchangers, excepting cooling towers and condensers, are relatively rare.
[...] The evaporative heat transfer process takes place mainly within a central region, which contains baffles to stabilize the mixing process and increase the area of gas-liquid interface by breaking up the gas into small bubbles. The general circulation pattern is indicated by arrows in figure A large vessel is required to provide sufficient free liquid surface for gas to escape without entraining too much liquid Figure Submerged combustion concentrating evaporator Submerged combustion evaporators are particularly useful for the concentration of corrosive fluids, such as acids, and for viscous liquids and slurries that would be difficult to handle in a conventional heat exchanger. [...]
[...] APPLICATIONS The direct contact heat transfer is used in high efficiency water heaters for a number of applications. For example, Ocean Thermal Energy Conversion Warm surface seawater if pumped through a heat exchanger where the low boiling point fluid is vaporized. The expanding vapor turns the turbo generator. Then, cold, deep sea water ( pumped through a second heat exchanger(condenses the vapor back into a liquid, which is then recycled through the system. Steam Generation in Nuclear Reactors One method of generating steam in innovative nuclear reactor applications involves water coming into direct contact with a circulating molten metal. [...]
[...] Another form of direct contact condenser that is sometimes included in process plants is the pool condenser in which the vapor is passed into a pool of cold liquid through a pipe submerged in the pool. This a convenient system for handling vapor that is, say, accidentally or deliberately, released from a process plant. Similar vapor suppression systems have been employed in nuclear plants. Figure Direct contact spray condensation using cooled condensate as a coolant Figure Direct contact spray using cold water as coolant and a subsequent separation of immiscible condensate 3.Water Cooling Towers Cooling towers represent the most extensive application of gas-liquid direct contact heat transfer. [...]
[...] The packed bed may be stationary or may form part of a continuous process where by the particles are mechanically driven through the heat exchanger. MATERIALS AND METHODS DIRECT CONTACT HEAT EXCHANGERS In attempting to classify direct contact heat transfer, it has been noted that equipment employing this process serves many functions and comes in many forms. In this section some of the types of equipment used are described Direct Contact Evaporators Injection of a hot gas into a pool of liquid is one of the cheapest and simplest methods of evaporation; direct contact evaporation of this kind utilizes the sensible heat of the gas to provide the required latent heat of evaporation without the need for complex metal heat exchanger surfaces. [...]
[...] Forced gas flow includes Gas-vapor jets Liquid-Liquid Heat Transfer Heat may be transferred from one immiscible liquid to another by direct contact of the two fluids, as illustrated in the figure below. To achieve a useful rate of heat transfer, the area of interface between the two is increased by the formation of droplets, and sometimes enhanced by mechanical agitation. This principle is employed in liquid-liquid contactors used for heating or cooling of process fluids; one of its main disadvantages is the problem of separating the two fluids afterward, particularly if very small droplets are produced by agitation. [...]
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