Live Exchanger – Plate heat exchangers are one of the most common types of heat exchangers in use today; Another common type of heat exchanger is the shell and tube heat exchanger. The spiral heat exchanger is also used for industrial applications, but its use is less compared to the other two types of heat exchangers.
Plate heat exchangers have gained widespread use throughout the engineering world because they are efficient, robust and relatively easy to maintain.
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Plate heat exchangers consist of a relatively small number of parts. Because plate heat exchangers are used to transfer heat, they require inlets and outlets where the flowing medium – or fluids – can enter and leave the heat exchanger. The fluid can be a liquid or a gas. Since liquids are often assumed to be just liquid, we will use the term liquid medium to avoid confusion.
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Gaskets and plates are used to separate the liquid media and prevent them from mixing; gaskets are only glued to one side of each board. The boards hang on the support rod and are pressed together using clamping screws. When the boards are pressed together, they are referred to as a “set of boards”. The guide rail ensures correct alignment of the boards when the stack of boards opens and closes.
The final components of interest are the two covers at opposite ends of the board bundle. One lid is movable, while the other is fixed. The movable cover and the fixed cover are also sometimes referred to as the frame plate and pressure plate. Note that the inputs and outputs are only mounted on the hard cover.
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In this article, we will assume that a hypothetical plate heat exchanger has two flowing media, one cold and one hot. The hot medium must be cooled with a cold medium, and this happens in a plate heat exchanger.
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The hot medium enters the heat exchanger through the hot medium inlet. Seals guide the hot medium as it flows through the heat exchanger. Each plate has a changing sealing pattern. The hot medium flows into the gap between a pair of plates, but does not flow into the gap between the next pair of plates because the seals prevent this. The process continues so that every other set of plates is filled with the hot flowing medium.
At the same time, the cold medium enters the heat exchanger through the cold medium inlet, but this time the seals are positioned so that the cold medium can flow into the space where no hot medium is present. The heat exchanger is now full of both hot and cold flowing media. Each medium flows from its respective outlet and the process is continuous.
Note that the two flowing media are always side by side throughout the heat exchanger. Thus, when flowing through the heat exchanger, the flowing media have a flow pattern of hot, cold, hot, cold. Both flowing media are completely separated from each other by the packing and plates, they do not mix.
Due to the close proximity of the flowing media, heat is exchanged between them. The hot medium heats the plate, and the plate transfers some of this heat to the cold fluid medium; thus the temperature of the hot medium falls, while the temperature of the cold medium rises.
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The plates of a plate heat exchanger may appear to have a simple design, but each plate is full of interesting design features. E.g:
Plates are not the only part of a plate heat exchanger with extensive structural elements, gaskets also have interesting structural elements:
The flow through the plate heat exchanger can be parallel, transverse or opposite. Plate heat exchangers usually use countercurrent flow because it is the most efficient type of flow for heat transfer. A countercurrent is sometimes known as a countercurrent.
Because plate heat exchangers are used in a wide variety of applications, they must be designed to withstand the process conditions in which they operate, which may include corrosive and erosive environments. Plate heat exchangers can be constructed from a variety of materials, including metals, alloys and plastics. Different materials make the plate heat exchanger more suitable for different applications. For example, if a particular flow medium reacts aggressively when in contact with certain metals, polymer-based materials such as Teflon can be used instead. wheel, energy recovery wheel or heat recovery wheel is a type of energy recovery heat exchanger placed in the supply and exhaust air streams of air handling units or rooftop units or in the exhaust gases of an industrial process to recover heat energy. Other variants include thalpy wheels and dryer wheels. A cooling-specific thermo wheel is sometimes referred to as a Kyoto wheel.
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The thermal wheel consists of a circular honeycomb matrix of heat-absorbing material that slowly rotates in the supply and exhaust air flows of the air handling system. As the heater wheel rotates, heat is captured from the exhaust air flow in one half of the rotation and released into the fresh air flow in the other half of the rotation. In this way, waste heat energy is transferred from the waste air flow to the matrix material and from the matrix material to the fresh air flow. This increases the temperature of the supply air stream by a value proportional to the temperature difference between the air streams or “thermal gradient” and depending on the efficiency of the appliance. Heat exchange is most effective when the currents run in opposite directions, as this results in a favorable temperature gradient across the thickness of the disc. The principle works in reverse, and the “cooling” energy can be recovered in the supply air flow as needed and the temperature difference.
The heat exchange matrix can be aluminum, plastic or synthetic fiber. The heat exchanger is rotated by a small electric motor and a belt drive system. The motors are often controlled by an inverter for better control of the outlet air temperature. If heat exchange is not required, the engine can stop completely.
Because heat is transferred from the exhaust air stream to the supply air stream without passing directly through the exchange medium, the overall efficiency is usually higher than any other air side heat recovery system. The smaller depth of the heat exchanger matrix compared to a plate heat exchanger means that the pressure drop across the unit is usually lower in comparison. In principle, the thermal wheel is selected for surface speeds between 1.5 and 3.0 meters per second. second (4.9 and 9.8 ft/s), and a rough “ssible” efficiency of 85% can be expected at the same volume of air flows. Although there is a small current requirement to turn the wheel, the current consumption of the motor is usually low and has little effect on the unit’s seasonal efficiency. The ability to recover “latt” heat can improve gross efficiency by 10-15%.
Usually, the transfer of heat between air currents from the device is called “ssible”, which is an exchange of energy or thalpa that leads to a change in the temperature of the medium (in this case air), but without changing the humidity content. . However, if the humidity or relative humidity in the return air stream is high and allows condensation in the equipment, this will cause a “latt” of heat to be released and the heat transfer material will be covered by a film of water. Despite the correspondingly low heat absorption, because part of the water film evaporates in the opposite air flow, the water will reduce the thermal resistance of the boundary layer of the heat exchanger material, thus improving the heat transfer coefficient of the unit and thereby increasing efficiency. The energy exchange of such devices now includes both possible and slow heat transfer; in addition to the change in temperature, there is also a change in the moisture content of the air currents.
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However, the conductive film will also slightly increase the pressure drop across the device, and depending on the spacing of the matrix material, this can increase the resistance by up to 30%. This will increase the fan’s power consumption and reduce the unit’s seasonal efficiency.
Aluminum matrices are also available with an applied hygroscopic coating, and its use or the use of porous synthetic fiber matrices allows the adsorption and release of water vapor at much lower humidity levels than normally required for condensation and batten heating. to transfer. The advantage of this is still higher heat transfer efficiency, but it also results in drying or humidification of the air streams, which may also be desirable for the particular process served by the supply air.
During the automotive industry’s interest in gas turbines to power vehicles (around 1965), Chrysler invented a unique type of rotary heat exchanger.
Which consisted of a rotating drum made of corrugated metal (similar in appearance to corrugated cardboard).
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