Coalescing is the separation of liquid aerosols and droplets from a compressed air or gas stream. It is also the separation of 2 liquids, usually a hydrocarbon based liquid and water.
In a compressed air/gas application, the airflow is normally inside to outside. The fine glass media captures the liquid aerosols and droplets as they pass through the filter wall. The aerosols and droplets run together along the fibers to form larger drops within the depth of the element. These large drops are then forced to the outside of the filter element. The oil that collects on the outside layer of the filter drains to the bottom of the filter housing by gravity. The oil is periodically drained off.
In a Liquid/Liquid application, the Coalescing filter element separates the Oil/Water mixture. Since water is heavier than oil, the water collects in the bottom of the filter housing and is periodically drained off.