There is a wide variety of designs for heat exchangers that are not dealt with exhaustively here. An important distinction between heat exchangers for water and air flows should be explained:
In a direct current heat exchanger, the two media that z. B. separated by a thermally conductive plate, in the same direction. In the best case, a complete equalization of the temperatures is achieved at the end. The final temperature is then between the two inlet temperatures. For example, two equally strong water flows with inlet temperatures of 60 ° C or 20 ° C could lead to a common outlet temperature of approx. 40 ° C. The temperature adjustment is associated with a loss of exergy.
In a countercurrent heat exchanger, the two media flow in opposite directions. This enables a stronger heat exchange. Ideally, the outlet temperature of each medium can approximately correspond to the inlet temperature of the other medium. For example, two equally strong water flows with inlet temperatures of 60 ° C or 20 ° C could lead to the initially warm flow being cooled to 20 ° C and the other being heated to 60 ° C. In this extreme case, practically no exergy would be lost.
A behaviour between these extremes occurs with the cross-flow heat exchanger, in which the two material flows flow approximately at right angles to one another. There are also mixed forms such as the cross- countercurrent heat exchanger.
An ideal heat exchanger would not only not lose any heat (e.g., through heat conduction to the outside air), but would also retain all of the exergies of the material flows. In principle, this is almost possible with the countercurrent heat exchanger (see above) if the flow rates are not too high. It is then z. B. with a ventilation system that the exhaust air (with a room temperature of 20 ° C) largely on the fresh air with z. B. 0 ° C transfers, so that the fresh air is preheated to almost 20 ° C and the exhaust air is discharged at almost 0 ° C. The ventilation heat losses in good systems and lower performance levels are only around 10% of what would be without heat recovery would get lost. The so-called temperature efficiency (also known as the heat recovery rate) is then around 90%.
The better the heat conduction in the heat exchanger works, the higher the material flows can be without reducing the efficiency of the heat transfer. Large surfaces can contribute to this, as can thin separating layers have made of a material that conducts heat well (e.g., stainless steel). An important parameter is the heat transfer coefficient, which can be influenced not only by heat conduction but also by effects at the interfaces.
As a rule, heat exchangers must meet several additional requirements:
• The flow losses (friction losses of the material flows during passage) should be as low as possible in order to keep the energy expenditure for pumps or fans low. A measure of the flow losses is the pressure loss at a certain volume flow.
• A heat exchanger must be able to withstand a certain temperature range and possibly a certain pressure load.
• Blockages (e.g., due to deposits of dust, dirt or lime) must be avoided as far as possible. If necessary, the cleaning should be easy.
• The materials used should be resistant to aggressive substances, e.g., B. in condensing boilers against acidic condensate.
• Often the material flows must remain reliably separated in order to e.g., B. to prevent pollutants from entering the room air.
• A compact design that is inexpensive to manufacture is often desirable.
In some cases, constructive compromises are necessary here. For example, plate heat exchangers with thinner (and more numerous) channels have better heat transfer, but higher flow losses and a greater risk of contamination, and small designs save material but limit the efficiency of heat transfer.
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