Heat exchangers are any devices designed to perform heat transfers, and include refrigerators, swamp coolers, or radiators. Heat exchangers
work by transferring thermal energy from one liquid to another through a separator or by direct contact. These devices can either be used to direct heat transfers towards an object, as in refrigeration, or away, as in radiators. Because of the various applications of heat exchangers, various designs exist.
work by transferring thermal energy from one liquid to another through a separator or by direct contact. These devices can either be used to direct heat transfers towards an object, as in refrigeration, or away, as in radiators. Because of the various applications of heat exchangers, various designs exist.
Basic Heat Exchanger Principles
The basics of heat transfer involve the second law of thermodynamics, which basically states that two thermodynamic systems, when allowed to interact, will move towards a thermodynamic equilibrium. In other words, when hot and cold substances are brought into contact with one another, the hot substance will cool and the cool substance will heat up, resulting in a median temperature.
A more complex explanation of this law may be found here:http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node30.html
A more complex explanation of this law may be found here:http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node30.html
A heat exchanger works by allowing two liquids of differing thermodynamic equilibria to interact, bringing about thermal transfer. In order to engender this interaction, a heat exchanger is designed according to one of two principle classifications regarding flow direction. Parallel-flow heat exchangers involve fluid flowing in the same direction, in a parallel configuration. Counter-flow heat exchangers are constructed to allow liquid to flow in opposite directions. In both configurations, the design is such that the surface area of the wall separating the two liquids allows maximum surface area between the liquids, which contributes to the most efficient transfer of heat between them.
Heat Exchanger Design Types
A heat exchanger must have a design that allows two different liquids to interact. As a result, most heat exchangers are designed with two adjacent containers for a liquid. The variation in containers leads to many of the basic differences in heat exchanger design.
A shell and tube heat exchanger pumps fluid through adjacent tubes. These tubes can vary in tube diameter, thickness, length, corrugation, layout, and other physical characteristics. For instance, the smaller the tube diameter, the faster the heat transfer can take place and the more compact the unit can be; however, the thin diameter can also lead to malfunction, such as flow interruption. Tube heat exchangers can be designed to hold high pressure, high temperature thermal exchanges.
pumps fluid through adjacent tubes. These tubes can vary in tube diameter, thickness, length, corrugation, layout, and other physical characteristics. For instance, the smaller the tube diameter, the faster the heat transfer can take place and the more compact the unit can be; however, the thin diameter can also lead to malfunction, such as flow interruption. Tube heat exchangers can be designed to hold high pressure, high temperature thermal exchanges.
In order to maximize surface area contact, plates are used in a plate heatexchanger. Multiple flat plates can be stacked one on top of the other in order to form a hermetically sealed volume into which fluid may be pumped. Because the large surface area allows for maximum contact between the liquids; this provides for effective and efficient heat transfer.
There are two variations from standard plate design:
- Plate fin: These exchangers use finned chambers to allow multiple liquids to interact more broadly across the plate.
- Pillow plate: This type of plate incurs pressure after construction to allow the exchanger to billow out like a pillow. The shape allows for efficient heat transfer across the entire surface of the plate. These types of exchangers are mostly used in the dairy industry.
Waste heat recovery units (WHRU) are heat exchangers that employ waste heat, such as the exhaust from an engine or steam, to transfer heat to a liquid. One of the commonly cited benefits of a WHRU is that the use of waste gases reduces pollution.
(WHRU) are heat exchangers that employ waste heat, such as the exhaust from an engine or steam, to transfer heat to a liquid. One of the commonly cited benefits of a WHRU is that the use of waste gases reduces pollution.
In many types of refineries or chemical plants, phase-change heat exchangers are used to convert one medium to another, such as heating a liquid into vapor or cooling vapor into a liquid. The design for phase-change heat exchangers resembles an industrial kettle, typically with a heat source used on a liquid. This method is commonly used to allow for distillation and refinement of an oil or chemical. One major benefit of phase-change exchangers is that they can be used for multiple phase changes. A heat source can heat a liquid into a vapor, and then that heated vapor can be used to heat another medium further on in the cycle.
Source: www.thomasnet.com
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