Double wall heat exchangers are security type exchangers, having a peculiar construction with a double plate, hence their name ‘double wall’. This ensures that even in case of breaking or cracking of a plate, due to corrosion or other causes, the mixing between the primary and secondary fluid is avoided.
This is a kind of construction that can be applied both with brazed plate exchangers and gasketed plate exchangers, but also in shell and tube exchangers. This is employed wherever the mixing between the fluids is absolutely to be prevented. That’s the case of food industry for example, where products that have to be cooled before bottling, such as mineral water, beverages, milk or wine, don’t have to mix with cooling water, maybe not even potable.
Further applications are with hydraulic oil and diathermic oil, where the presence of water within the oil could lead to damages or also be extremely dangerous. The cooling of oil in power converters is another application field, where the mixing of oil and water can cause very expensive damages.
In gasketed heat exchangers the breakage of a gasket doesn’t lead to fluids’ mixing, because leakages go outside of the exchanger. Fluids can mix only in case of a crack in a plate. With double wall heat exchangers, even in case of cracking of a plate the leakage flows outside of the exchanger, making it immediately visible. In case of double wall shell and tube exchangers the leakage is not so immediately visible. In this case, there is a collecting chamber where leakages flow, and it can be equipped with special sensors alerting the maintenance team or the plant manager that a leakage happened, and thus that there is a cracked tube.
Clearly, double wall exchangers involve much higher costs, because at least the construction materials are doubled. In addition, the air space between the two walls decrease the thermal exchange coefficient, thus requiring an increase of the thermal transfer surface. This is even more impacting on shell and tube exchangers, yet usually offering lower thermal transfer rates, which further decrease with the addition of a tube in tube and an additional air space.
Anyway, double wall exchangers are a mandatory choice in all of those applications where it is necessary to avoid the mixing between the fluids.
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