A Plate Heat Exchanger is an indirect type heat transfer device, where the hot and cold media are separated by a metallic plate (wall) through which heat is transferred. Temperature losses through radiation can be typically be disregarded when considering plate heat exchangers, and thus conduction and convection are the main mechanism of heat transfer.
There are five main types of plate heat exchangers offered by CESCO. Click on the name or picture of each type to learn more:
Typical flow diagram for the most common type of gasketed plate heat exchanger:
Of the three main types of indirect heat exchangers (Plate, Shell & Tube, and Spiral), the most notable advantages of a plate heat exchanger are:
- Thin material for the metallic heat transfer surface -this gives optimum heat transfer, since the heat only has to penetrate thin metal material. In plate heat exchangers, we have the advantage of small plate thickness, between 0.3 and 0.8 mm.
- High turbulence in the medium -this gives a higher convection, which results in efficient heat transfer between the media. The consequence of this higher heat transfer coefficient per unit area is not only a smaller surface area requirement but also a more efficient plant. The high turbulence also gives a self-cleaning effect. Therefore, when compared to the traditional shell and tube heat exchanger, the fouling of the heat transfer surfaces is considerably reduced. This means that the plate heat exchanger can remain in service far longer between cleaning intervals. At the design stage, this means that the designed can use much smaller fouling factors for plate heat exchangers relative to shell and tube type exchangers.
- Flexibility - the plate heat exchanger consists of a framework containing several heat transfer plates. It can easily be extended to increase capacity. Furthermore, it is easy to open for the purpose of cleaning. This only applies to gasketed plate heat exchangers, and not to brazed or fusion welded units. It is typically a one person job to add or remove plates:
- Variable thermal length - most of the plate heat exchangers offered by CESCO are available with two different pressing patterns. When the plate has a narrow pattern, the pressure drop is higher and the heat exchanger is more effective. This type of heat exchanger has a long thermal channel. When the plate has a wide pattern, the pressure drop is smaller and the heat transfer coefficient is accordingly somewhat smaller. This type of heat exchanger has a short thermal channel. When two plates of different pressing patterns are placed next to each other, the result is a compromise between long and short channels as well as between pressure drop and effectiveness.
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To solve a thermal problem, CESCO must know several parameters. Further data can then be determined. The six most important parameters are as follows:
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The amount of heat to be transferred (heat load)
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The inlet and outlet temperatures on the primary and secondary sides
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The maximum allowable pressure drop on the primary and secondary sides
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The maximum operating temperature
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The maximum operating pressure
If the flow rate, specific heat, and temperature difference on one side are known, the heat load can be calculated. The overall heat transfer coefficient of plate heat exchangers under favorable circumstances can be as high as 8,000 W/m2ºC. With traditional shell and tube heat exchangers, the k-value will be below 2,500 W/m2ºC.
Temperature Program
This means the inlet and outlet temperatures of both media in the heat exchanger.
T1 = Inlet temperature – hot side
T2 = Outlet temperature – hot side
T3 = Inlet temperature – cold side
T4 = Outlet temperature – cold side
The temperature program is shown in the diagram below:
Minimizing the Heat Exchanger Size
An important parameter that can be influenced to reduce the size, and therefore the price, of the heat exchanger is to use the highest possible allowable pressure drop, as well as the LMTD. A higher pressure drop will usually result in a smaller heat exchanger. A higher Logarithmic Mean Temperature Difference (LMTD) will also give a smaller unit. With heat recovery, the price of the heat exchanger in relationship to the amount of heat recovered is of great significance, since a profit must be realized to make the project worthwhile. Here is a typical size comparison for the same duty, comparing plate technology to shell and tube technology:
