Single stage coalescer units may be comprised of multiple installation of coalescer cartridges or the entire may be packed with bulk type media (see repack coalescers). This design is used when the differential in specific gravity of the of the two liquid phases is sufficient to provide separation by gravitational force. In the design of single stage units the size of the vessel is of a prime consideration since separation is entirely dependent upon the difference in the density of the two liquids. If the discontinuous phase is lighter than the continuous phase the accumulator chamber or dome is placed on top of the vessel. If the density reverse the accumulator chamber or dome is placed underneath of the vessel.
Coalescer cartridges are mechanical devices to filter solids and separate two immiscible liquids. The most common uses are the filtration and separation of water from various types of hydrocarbons. The free water droplets and solid contaminants in hydrocarbons are, in the vast majority of cases, no more than a haze of microscopic particles suspended in the fuel and invisible to the naked eye.
The measurement of these particles is usually quoted in microns or, more correctly, micrometers. As an indication of these measurements in comparative terms the thickness of a human hair is approximately 50 microns whilst the smallest particle visible to the naked eye is about 40 microns. The majority of water and solid matter particles in a contaminated hydrocarbons are less than 30 microns. It is the function of the coalescer cartridge of the filter separator unit firstly to remove the solids and then to merge the particles of water into larger droplets of five to ten millimeters in diameter before they are ejected to fall, under gravity, to the sump from where the water accumulation can be drained.
Typical coalescer cartridge construction
The construction and design of a typical filter coalescer cartridge is shown right. The fuel/water/solids mixture first flows through a pleated assembly of fine-grade filter media, the pleated configuration is necessary to obtain the optimum area consistent with maximum dirt-holding capacity and efficiency. Following the almost total removal of solid contaminant by this first filtration stage the fuel water emulsion then passes through the coalescing media, graduating from a very fine grade material to a coarse grade material to effect the gradual coalescence of water particles from their original microscopic size to visible droplet size.
The coalescer core, which is an unique construction, provides adequate rigidity to the whole assembly after it is finally sealed within the reinforced nylon end caps using a high-quality fuel-resistant adhesive. Water droplets are finally entering the outer, specially treated, cotton sock. The weave pattern of the outer sock determines the ultimate size of water droplet that emerges from the coalescer stage.
For more information see the section on Filter water separators.