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Heat exchangers are equipments used for exchanging or transferring of heat from one fluid to other fluid which is at different temperatures through a solid barrier separating these fluids without mixing the two fluids. Heat exchangers are commonly used in a wide variety of applications such as for heating and air conditioning systems in a household, for chemical processing and power production in large plants. Car radiator is an example for heat exchanger where the heat is transferred from the radiator tubes through the hot water is flowing to the air flowing through thin plates attached to the radiator tubes.
Various thermal resistances are involved in the transfer of heat from one fluid to another. For a heat exchanger the heat transfer equation is given by,
Q = U × A × δT
Where Q is the heat flow rate in W, U is the overall heat transfer coefficient in (W/m2 K), A is the heat flow area in m2 and δT is the temperature difference between the fluids.
Based on the heat transfer applications, there are heat exchangers. Some of them are explained below:
It is the simplest type of heat exchanger. As the name indicates this heat exchanger consists of 2 concentric pipes of different diameters, as shown in below figure. Here one fluid either hot or cold flows through the smaller pipe while the other will flow through the space between the pipes.In a double-pipe heat exchanger, two types of flow arrangement are possible such as parallel flow and counter flow. In case of parallel flow, both the hot and cold fluids enter through the same side to the heat exchanger and move in the same direction. Whereas in case of counter flow, on the other hand, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions. Both the parallel flow and counter flow is shown in the figure below.
This is another type of heat exchanger, which is specifically designed for a large heat transfer surface area per unit volume. In a heat exchanger the ratio of the heat transfer surface area to its volume is called the area density ‘β’. A heat exchanger with ‘β’ = 700 m2/m3 is called compact heat exchanger. Car radiators (‘β’ = 1000 m2/m3), the regenerator of a Stirling engine (‘β’ = 15,000 m2/m3), glass ceramic gas turbine heat exchangers (‘β’ = 6000 m2/m3) and the human lung (‘β’ = 20,000 m2/m3) are some of the examples of compact heat exchangers. In car radiators, which are a water to air compact heat exchanger, the fins are attached to the air side of the tube surface. Compact heat exchangers help to achieve high heat transfer rates between two fluids in a small volume and they are commonly used in applications with limitations on the weight and volume of heat exchangers. In compact heat exchangers, by attaching closely spaced thin plate or corrugated fins to the walls separating the two fluids, large surface area can be obtained. Different from that of double pipe heat exchanger, in compact heat exchangers, the two fluids (hot and cold) usually move perpendicular to each other, and this type of flow configuration is called cross-flow. The cross-flow can be further classified in to two as unmixed and mixed flow, depending on the flow configuration. In the cross-flow, the fluids are said to be unmixed if the plate fins force the fluid to flow through a particular interfin spacing and prevent it from moving in the transverse direction. The cross-flow is said to be mixed if the fluid is free to move in the transverse direction (parallel to the tubes). In case of a car radiator, both fluids are unmixed.
This is the most common type of heat exchanger mainly used in industrial applications. These heat exchangers contain a large number of tubes packed in a shell with their axes parallel to that of the shell. As the name indicates here the fluids are flowing through shell and tube. Heat transfer takes place as one fluid flows inside the tubes while the other flows outside the tube that is through the shell. In order to force the shell-side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing between the tubes, baffles are commonly placed in the shell. Due to their relatively large size and weight, shell and tube heat exchangers are usually not suitable for use in automotive and aircraft applications. The tubes in a shell and tube heat exchanger open to some large flow areas called headers at both ends of the shell, where the tube side fluid accumulates before entering the tubes and after leaving them. The schematic of a shell and tube heat exchanger is shown in figure below.
Shell and tube heat exchangers can be further classified,
Fixed tube heat exchanger: In this type of heat exchangers, tube sheets are welded to the shell. The shells can be cleaned by removing channel cover or bonet. The main advantage of this heat exchanger is they of low cost, simple in construction and requires no expansion joints. As the name indicates, since the tubes are fixed to shell, it is not possible to clean the outside of the tubes and therefore its application is limited to clean services. For a large temperature differential, expansion joints are needed.
U tube heat exchanger: In this heat exchanger only one tube sheet is present. Here as one end of tube is free, it can expand or contract in response to stress differentials. As the tube bundles are not fixed the outside of the tubes can be cleaned by removing it. But the inside of the tube cannot be cleaned effectively. Therefore U tube heat exchangers are usually not used for services with dirty fluids inside the tube. Similar to fixed tube heat exchanger, this is also of low cost.
Floating head heat exchanger: Here one tube sheet is fixed with respect to shell whereas one end is free to float within the shell. Floating head heat exchangers are costlier and versatile. As one end is floating, it permits free expansion of the tube bundle. Cleaning of both inside and outside of tubes is possible in this type of heat exchanger. Therefore this can be used in dirty services. The schematic of a floating head heat exchanger is shown below.
Plate and frame heat exchanger consists of a sequence of plates with corrugated flat flow passages. The hot and cold fluid flows through alternate passages and thus each cold fluid stream is surrounded by two hot fluid streams, resulting in very effective heat transfer. Also plate and frame heat exchangers can be expanded with the increasing demand for heat transfer by simply mounting more plates. These types of heat exchangers are well suited for liquid to liquid heat exchanges applications, given that the hot and cold fluid streams are at about the same pressure.
Regenerative heat exchanger involves the interchange passage of the hot and cold fluid streams through the same flow area. Through porous mass, hot and cold fluids flow alternatively. During the flow of the hot fluid, heat is transferred from the fluid to the matrix of the regenerator and from the matrix to the cold fluid during the flow of the cold fluid. The matrix thus serves as a temporary heat storage medium. The dynamic type regenerator also involves a rotating drum to ensure the continuous flow of the hot and cold fluid through different portions of the drum by storing and rejecting heat from hot stream and cold stream respectively. Also the drum serves as the medium to transfer the heat from the hot to the cold fluid stream.
Apart from the above explained types of heat exchangers, there are other types of heat exchangers with names that reflect the specific application for which they are used. For example, a condenser is a heat exchanger that cools and condenses the fluid as it flows through the heat exchanger. A boiler can also be called as a heat exchanger in which one of the fluids absorbs heat and vaporizes. A space radiator is another heat exchanger in which transfer of heat from the hot fluid to the surrounding space takes place by radiation.
The performance of heat exchangers usually decline with time due to the accumulation of deposits on heat transfer surfaces. The layer of deposits adds additional resistance to heat transfer path and this in turn decreases the rate of heat transfer in a heat exchanger. The net effect of these accumulations on heat transfer is usually represented by a fouling factor 'Rf' , which is the measure of the thermal resistance introduced by fouling. One of the most common types of fouling is the precipitation of solid deposits in a fluid on the heat transfer surfaces. This type of fouling occurs not only in heat exchangers, but also in areas where the water is hard. The scales of such deposits come off by rubbing and the surfaces can be cleaned from such deposits by chemical treatment. Apart from the deposits outside certain mineral deposits will formed on the inner surfaces of fine tubes in the heat exchanger and this will seriously affect fluid flow and thereby the heat transfer. In order to avoid this critical problem, water in power and process plants is treated and its solid contents are removed before it is permitted to circulate through the system. Corrosion is another form of fouling, which is common in the heat exchangers used in chemical process industry. In this case, the surfaces of heat exchangers are fouled by the deposit of the products of chemical reactions. This form of fouling can be prevented by coating metal pipes with glass or using plastic pipes instead of metal ones. Algae growth in warm fluids is a serious factor which causes fouling of heat exchangers. This type of fouling is called biological fouling and can also be prevented by chemical treatment. Apart from this fouling factor also depends on the operating temperature, velocity of the fluids and the length of service. For a new heat exchanger, the fouling factor is zero and it increases with time as the solid deposits build up on the heat exchanger surface. Therefore in applications where there is chance for fouling to occur, it should be considered in the time of design of heat exchangers.
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