Coalescer is a technological device performing coalescence. Coalescence is the process by which two or more droplets or particles merge during contact to form a single daughter droplet. The primary use of coalescers is to separate emulsions in various processes. Basically coalescer works as reversed emulsifier. In the area of compressed air purification, coalescing filters are used to separate liquid water and oil from compressed air using a coalescing effect. These filters additionally remove particles.

Coalescing is the separation of liquid aerosols and droplets from a gas. Using a coalescing filter element installed in a housing with three ports the wet gas sample passes though element inside to outside. The inner capture layer is a high efficiency coalescing layer and the outer is a coarser drainage layer. The fine fibers of the inner layer capture the fine liquid aerosols and droplets and they run together along the fibers to form large drops within the depth of the element. These large drops are then forced to the outside of the filter element and then drain to the bowl of the housing by gravity. Coalescing filter elements will also remove particulates at the same efficiency as particulate type elements of the same grade.