Separation of aerosol and solid contaminants from air is primarily dictated by gravity. Contaminants greater than 10 mm in size settle out of the air stream fairly quickly. However, extremely small aerosol particles remain suspended, particularly in flowing – as opposed to still – air. Most coalescing filters are designed to cause combining of smaller aerosols into larger droplets. The enlarged droplets are now susceptible to the effects of gravity. “Coalescence” is the term given to this combining process. The coalescence process can be visualized as the atmospheric conditions at work in a thunderstorm – many small water vapor molecules present in turbulent moisture-laden air condense into aerosols which then collide or come together to form increasingly larger droplet masses until they gain enough weight to react to gravity and fall to earth as raindrops. Coalescing filters eliminate sub-micronic contamination through three concurrent processes, depending on aerosol size: Diffusion: Aerosols .001 to .2 mm., Interception: Aerosols .2 to 2 mm., Direct Impact: Aerosols over 2 mm.
Figure 1. THREE COALESCENCE PROCESSES.
Diffusion
Aerosols and solids in the 0.001 to 0.2 mm range are subject to rapid random Brownian motion, moving completely independently of the bulk air stream as gaseous molecules in flowing air. This motion causes them to migrate from the air stream and to collide with exposed filter surfaces. Solid contaminants adhere permanently to these surfaces via intermolecular forces. Liquid droplets, however, migrate gravitationally down the filter fibers, joining other droplets to form larger masses of liquid which can be drained from the system. While the rate of diffusion activity increases with heightened temperature and pressure, contaminants of this size exhibit random motion – and are subject to diffusion coalescence – even under non-turbulent, low velocity flow conditions.
Interception
For contaminants 0.2 to 2 mm in size, interception is the predominant coalescing mechanism. These contaminants conform to the stream line of the air flow and are the most difficult to remove because they can pass around filter fibers and escape from the filter uncollected. Interception occurs most effectively when the filter fiber diameter is smaller than the aerosol or solid particle diameter. Fibers 0.5 mm in diameter are required for optimum direct interception performance. When contaminants approach to within 1/2 their diameters to these filter fibers, the inertial forces holding them in the air stream are extremely small. This permits interception of the contaminants by the filter fibers.
Direct Impact
Contaminants 2 mm and larger are removed by the direct impact method because they have sufficient mass and develop enough momentum to leave the air flow stream line. These contaminants collide with the filter media, a coalescing process termed inertial or direct impaction.