These devices are used to separate in the most efficient manner the very fine drops carried away by a gas or steam, either at a boiling stage, vacuum evaporation or bubbling of a liquid. In fact there are two phenomena, one mechanical and one physical, but no filtration at all since the size of the held drops has no realtion with the infinitely greater size of the pore or open space. The head loss or gas passage resistance is practically imperceptible, however demisting can reach up to 99% efficiency.