Heat Exchanger..docx (Size: 237.99 KB / Downloads: 61)
A device designed to transfer heat between two physically separated fluids; generally consists of a cylindrical shell with longitudinal tubes; one fluid flows on the inside, the other on the outside.
Any of several devices that transfer heat from a hot to a cold fluid. In many engineering applications, one fluid needs to be heated and another cooled, a requirement economically accomplished by a heat exchanger. In double-pipe exchangers, one fluid flows inside the inner pipe, and the other in the annular space between the two pipes. In shell-and-tube exchangers, many tubes are mounted inside a shell; one fluid flows in the tubes and the other flows in the shell, outside the tubes. Special-purpose devices such as boilers, evaporators, superheaters, condensers, and coolers are all heat exchangers. Heat exchangers are used extensively in fossil-fuel and nuclear power plants, gas turbines, heating and air conditioning, refrigeration, and the chemical industry. See also cooling system.
A device used to transfer heat from a fluid flowing on one side of a barrier to another fluid (or fluids) flowing on the other side of the barrier.
When used to accomplish simultaneous heat transfer and mass transfer, heat exchangers become special equipment types, often known by other names. When fired directly by a combustion process, they become furnaces, boilers, heaters, tube-still heaters, and engines. If there is a change in phase in one of the flowing fluids—condensation of steam to water, for example—the equipment may be called a chiller, evaporator, sublimator, distillation-columnreboiler, still, condenser, or cooler-condenser.
Heat exchangers may be so designed that chemical reactions or energy-generation processes can be carried out within them. The exchanger then becomes an integral part of the reaction system and may be known, for example, as a nuclear reactor, catalytic reactor, or polymerize.
Heat exchangers are normally used only for the transfer and useful elimination or recovery of heat without an accompanying phase change. The fluids on either side of the barrier are usually liquids, but they may also be gases such as steam, air, or hydrocarbon vapors; or they may be liquid metals such as sodium or mercury. Fused salts are also used as heat-exchanger fluids in some applications.
Most often the barrier between the fluids is a metal wall such as that of a tube or pipe. However, it can be fabricated from flat metal plate or from graphite, plastic, or other corrosion-resistant materials of construction.
Types of heat exchangers
Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and Tube heat exchangers are typically used for high pressure applications (with pressures greater than 30 bar and temperatures greater than 260°C). This is because the shell and tube heat exchangers are robust due to their shape.
There are several thermal design features that are to be taken into account when designing the tubes in the shell and tube heat exchangers. These include:
Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered.
Plate heat exchanger
Another type of heat exchanger is the plate heat exchanger. One is composed of multiple, thin, slightly-separated plates that have very large surface areas and fluid flow passages for heat transfer. This stacked-plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger. Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. In HVAC applications, large heat exchangers of this type are called plate-and-frame; when used in open loops, these heat exchangers are normally of the gasketed type to allow periodic disassembly, cleaning, and inspection. There are many types of permanently-bonded plate heat exchangers, such as dip-brazed and vacuum-brazed plate varieties, and they are often specified for closed-loop applications such as refrigeration. Plate heat exchangers also differ in the types of plates that are used, and in the configurations of those plates. Some plates may be stamped with "chevron" or other patterns, where others may have machined fins and/or grooves
Moving bed heat exchangers essentially exist of a huge number of square tubes which are arranged in heat exchanger packages one above the other. The ends of the tubes are closed with end plates. Behind the plates are reversing chambers for the cooling or heating medium. The sides of the external tubes are equipped with steel plate strips which hold the product in the shaft. To protect the environments or the product quality, doors that close the side walls can be fitted. Above and under the heat exchanger are feed respectively discharge hoppers. Different conveyor facilities for bulk materials, as for example conveying screws, bucket conveyors or similar are downstream systems.
The cooling or warming of the bulk materials in the Moving bed Cooler happens indirectly; via water, thermal oil or steam. The heating or cooling medium flows through the square tubes. Medium and product flow in cross countercurrent to each other. The coolers work according to the Moving Bed Principle. I.e. the product forms a product column which flows continuously down between the cooling pipes. A discharge bottom with variable openings regulates dwell time and flow rate.
Moving bed heat exchangers can be used for cooling or warming of all free-flowing bulk materials which correspond to the requirements of the apparatus, concerning grain size and angle of repose. The heat exchangers often can be found after rotary kilns and dryers to cool e.g. mineral (quartz sand, Ilmentit etc.) or chemical products (fertilizer, soda etc.). The entry temperatures of the products can reach up to 1200 °C.
Moving bed heat exchangers have a relatively compact construction. Because of the working principle they need only a small base. However, depending on their application they can build relatively high. Because of having only few moved parts they have low electrical requirements and are low-maintenance. Problems with noise or dust contamination of the environments do not occur.
Monitoring and maintenance
Integrity inspection of plate and tubular heat exchanger can be tested in situ by the conductivity or helium gas methods. These methods confirm the integrity of the plates or tubes to prevent any cross contamination and the condition of the gaskets.
Condition monitoring of heat exchanger tubes may be conducted through Nondestructive methods such as eddy current testing.
The mechanics of water flow and deposits are often simulated by computational fluid dynamics or CFD. Fouling is a serious problem in some heat exchangers. River water is often used as cooling water, which results in biological debris entering the heat exchanger and building layers, decreasing the heat transfer coefficient. Another common problem is scale, which is made up of deposited layers of chemicals such ascalcium carbonate or magnesium carbonate.