Heat exchanger

A heat exchanger is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. They are widely used in refrigeration, air conditioning, space heating, power production, and chemical processing. One common example of a heat exchanger is the radiator in a car, in which the hot radiator fluid is cooled by the flow of air over the radiator surface.

Flow arrangement

Heat exchangers may be classified according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is most efficient, in that it can transfer the most heat. See countercurrent exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.

For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence. Image:Heat_exc_1-1.png|Fig. 1: Shell and tube heat exchanger, single pass (1-1 parallel flow) Image:Heat_exc_2-1.png|Fig. 2: Shell and tube heat exchanger, 2-pass tube side (1-2 crossflow) Image:Heat_exc_2-2.png|Fig. 3: Shell and tube heat exchanger, 2-pass shell side, 2-pass tube side (2-2 countercurrent)

Types of heat exchangers

A typical heat exchanger is the shell and tube heat exchanger which consists of a series of finned tubes, through which one of the fluids runs. The second fluid runs over the finned tubes to be heated or cooled.

Another type of heat exchanger is the plate heat exchanger. It directs flow through baffles so that the fluids are separated by plates with very large surface area. This plate type arrangement can be more efficient than the shell and tube. Advances in gasket technology have made the plate type increasingly practical.

A third type of heat exchanger is the regenerative heat exchanger. In this, the heat from a process is used to warm the fluids to be used in the process, and the same type of fluid is used either side of the heat exchanger. (These heat exchangers can be either plate and frame or shell and tube construction.)

A fourth type of heat exchanger uses an intermediate fluid or solid store to hold heat, which is then moved to the other side of the heat exchanger to be released. Two examples of this are adiabatic wheels, which consist of a large wheel with fine threads rotating through the hot and cold fluids, and heat exchangers with a gas passing upwards through a shower of fluid (often water) and the water then taken elsewhere before being cooled. This is commonly used for cooling gases whilst also removing certain impurities, solving two problems at once.

Phase-change heat exchangers

In addition to heating up or cooling down fluids in just a single phase, heat exchangers can be used either to heat a liquid to evaporate (or boil) it or used as condensers to cool a vapor to condense it back to a liquid. In chemical plants and refineries, reboilers used to heat incoming feed for distillation towers are often heat exchangers. Distillation set-ups typically use condensers to condense distillate vapors back into liquid.

Power plants which have steam-driven turbines commonly use heat exchangers to boil water into steam. Heat exchangers or similar units for producing steam from water are often called boilers. In the nuclear power plants called pressurized water reactors, special large heat exchangers which pass heat from the primary (reactor plant) system to the secondary (steam plant) system, producing steam from water in the process, are called steam generators. All power plants, fossil-fueled and nuclear, using large quantities of steam have large condensers to recycle the water back to liquid form for re-use.

Air conditioning systems and refrigerators use heat exchange in their condensers to cool refrigerant vapor back to liquid in their refrigerant cycles. Air conditioning systems also have cooling coils which act as heat exchangers, not only cooling down air, but also condensing excess humidity from the air into liquid condensate. In order to conserve energy and cooling capacity in chemical and other plants, regenerative heat exchangers can be used to transfer heat from one stream that needs to be cooled to another stream that needs to be heated, such as distillate cooling and reboiler feed pre-heating.

Monitoring and maintenance

Condition monitoring of heat exchanger tubes may be conducted through eddy current inspection. This is often simulated through the use of computational fluid dynamics or CFD.

A serious problem of some heat exchangers is fouling. As very often cooling water from rivers or sea is used, a lot of biological debris enters the heat exchanger and build layers, decreasing the heat transfer coefficient. Another possible problem is scale, which is chemical deposit layers such as calcium carbonate or magnesium carbonate.

In case of plate heat exchangers they need to be cleaned periodically. To do this they will be de-assembled and re-assembled. In case of tube heat exchangers there are possibilities like acid cleaning, bullet cleaning etc. (off-line cleaning).

In large-scale cooling water systems for heat exchangers, water treatment such as purification, addition of chemicals, and testing, is used to minimize fouling of the heat exchange equipment. Other water treatment is also used in steam systems for power plants, etc. to minimize fouling and corrosion of the heat exchange and other equipment.

Heat exchangers in nature

Heat exchangers occur naturally in the circulation system of whales. Arteries to the skin carrying warm blood are intertwined with veins from the skin carrying cold blood causing the warm arterial blood to exchange heat with the cold venous blood. This reduces overall heat loss by the whale when diving in cold waters. Wading birds use a similar system to limit heat losses from their body through their legs into the water.

In species that have external testes (such as humans), the artery to the testis is surrounded by a mesh of veins called the pampiniform plexus. This cools the blood heading to the testis, while reheating the returning blood.

External links

HVAC | Mechanical engineering

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