They consist of a perforated cylindrical container housing the filtering septum with small pleats at constant pitch. The air to be filtered comes in from the inlet opening and expands by investing the filtering septum, crosses it and comes out from the holes the outer aluminium metal sheet is provided with. They apply in all those cases requiring very high filtering efficiency, but with small air flow. They can work in laminar or turbulent flow.

For more information: see the data sheet