Heat Exchanger<br /><br />A heat exchanger is a heat transfer device that exchanges heat between two or more process fluids. Heat exchangers have widespread industrial and domestic applications.<br />Many types of heat exchangers have been developed for use in steam power plants, chemical processing plants, building heat and air conditioning systems, transportation power systems, and refrigeration units.<br />The actual design of heat exchangers is a complicated problem. It involves more than heat-transfer analysis alone. Cost of fabrication and installation, weight, and size play important roles in the selection of the final design from a total cost of ownership point of view. In many cases, although cost is an important consideration, size and footprint often tend to be the dominant factors in choosing a design.<br /><br />Heat Exchanger Types<br />A heat exchanger is a heat transfer device that exchanges heat between two or more process fluids. Heat exchangers have widespread industrial and domestic applications.<br />Many types of heat exchangers have been developed for use in steam power plants, chemical processing plants, building heat and air conditioning systems, transportation power systems, and refrigeration units.<br />The actual design of heat exchangers is a complicated problem. It involves more than heat-transfer analysis alone. Cost of fabrication and installation, weight, and size play important roles in the selection of the final design from a total cost of ownership point of view. In many cases, although cost is an important consideration, size and footprint often tend to be the dominant factors in choosing a design.<br />Most heat exchangers may be classified as one of several basic types. The four most common types, based on flow path configuration, are illustrated .<br /><br /><br /><br />1. In concurrent, or parallel-flow, units the two fluid streams enter together at one end, flow through in the same direction, and leave together at the other end.<br />2. In countercurrent, or counter-flow, units the two streams move in opposite directions.<br />3. Insingle-pass crossflow units one fluid moves through the heat transfer matrix at right angles to the flow path of the other fluid.<br />4. Inmultipass crossflow units one fluid stream shuttles back and forth across the<br />flow path of the other fluid stream, usually giving a crossflow approximation to counterflow.<br />The most important difference between these four basic types lies in the relative amounts of heat transfer surface area required to transfer the desired amount of heat between the two fluids.<br />In the region in which the fluid temperature change across the heat exchanger is a small percentage of the difference intemperature between the two entering fluid streams, all the units require roughlythe same area. The parallel-flow heat exchanger is of interest primarily for applicationsin this region. Cross-flow units have a somewhat broader range of application,and are peculiarly suited to some types of heat exchanger construction thathave special advantages. The counter-flow heat exchanger requires the least area. Furthermore, it is the only type that can be employed in the region in which the temperature change in one or both of the fluid streams closely approaches the temperature difference between the entering fluids streams.<br />In addition, heat exchangers may be classified as direct contact or indirect contact. In the direct-contact type, heat transfer takes place between two immiscible fluids, such as a gas and a liquid, coming into direct contact. For example, cooling towers, jet condensers for water vapor, and other vapors utilizing water spray are typical examples of direct-contact exchangers.<br />Immiscible Fluids are incapable of being mixed or blended together. Immiscible liquids that are shaken together eventually separate into layers. Oil and Water are typical immiscible fluids.<br />In the indirect-contact type of heat exchangers, such as automobile radiators, the hot and cold fluids are separated by an impervious surface, and they are referred to as surface heat exchangers. There is no mixing of the two fluids. Classification According to Transfer Processes Heat exchangers are classified according to transfer processes into indirect and direct contact types<br />a. Indirect Contact Type Heat Exchangers<br />In an indirect-contact heat exchanger, the fluid streams remain separate and the heat transfers continuously through an impervious dividing wall or into and out of a wall in a transient manner. Thus, ideally, there is no direct contact between thermally interacting fluids. This type of heat exchanger, also referred to as a surface heat exchanger, can be further classified into direct-transfer type, storage type, and fluidized-bed exchangers.<br /><br />b. Direct Contact Type Heat Exchangers<br />In this type, heat transfers continuously from the hot fluid to the cold fluid through a dividing wall. Although a simultaneous flow of two (or more) fluids is required in the exchanger, there is no direct mixing of the two (or more) fluids because each fluid flows in separate fluid passages. In general, there are no moving parts in most such heat exchangers. This type of exchanger is designated as a recuperative heat exchanger or simply as a recuperator. (Some examples of direct transfer type heat exchangers are tubular, plate-type, and extended surface exchangers).<br />Note that the term recuperator is not commonly used in the process industry for shell-and-tube and plate heat exchangers, although they are also considered recuperators. Recuperators are further sub-classified as prime surface exchangers and extended-surface exchangers. Prime surface exchangers do not employ fins or extended surfaces on any fluid side. Plain tubular exchangers, shell-and-tube exchangers with plain tubes, and plate exchangers are good examples of prime surface exchangers. Recuperators constitute a vast majority of all heat exchangers<br />