It’s easy to understand why we filter compressed air. It keeps end use safe and clean for products and people, removing any harmful contaminants that may enter the air stream. Understanding the terminology is a different story.
Let’s start with the contaminants. They can originate both from ambient intake air and within the system itself, as bits of corroded pipe or droplets of lubrication. There are three main types of contaminants found in compressed air.
Particulates: Particulates are small pieces of solid material, from dust, dirt and pollen from intake air to metal particles from pipes. Different filtration methods are used depending on the size of the particulate, including sieving, inertial impaction, interception and diffusion.
Aerosols: Aerosols are small droplets of liquid dispersed throughout compressed air. In oil-injected compressor systems, aerosols are usually from the lubricant.
Vapors: In compressed air systems, vapors are lubricants or other liquids that have converted to gaseous form. These can be especially difficult to remove and require a specific filter.
Each contaminant requires a different method of filtration.
Dry particulate filters remove the solid particulates in the air. There are three mechanisms filters use to remove solid particulates of all sizes from the compressed airstream.
Sieving: Sieving is a mechanism filters use to remove harmful particulates. The filter media is made up of fiber glass layers that trap large particles. These particulates are then mechanically removed from the system. Sieving works for particles larger than 1mm, but most particles in the air stream are much smaller.
Inertial Impaction: Inertial impaction occurs when particles that are too heavy to flow with the airstream become trapped in the fiber media. A similar process called interception occurs when lighter particulates with large diameters flowing with the airstream are stopped by the filter.
Diffusion: Diffusion only occurs with small particles that move erratically through the air instead of following the stream. Their random path is caused by collision with gas particles, a phenomenon called Brownian motion. Because these particles are moving more freely through the filter, they are more likely to be intercepted by the filter media and removed from the airstream.
Aerosols and Vapors
Two types of filters are used for removing aerosols and vapors. Coalescing filters remove liquids (and some particulates) and vapor removal filters use adsorption to remove vapors.
Coalescing: Coalescing is a process that brings small droplets of liquid together to form larger droplets. As they increase in size, the droplets fall from the filter media into a moisture trap and out of the airstream. Coalescing filters are used to remove aerosols and particulates, but cannot capture vapors.
Adsorption: Adsorption is the chemical process vapor removal filters use to capture gaseous lubricants. In this chemical process, vapors bond with the surface of the adsorption media (or adsorbent.) Adsorbents can be made of a variety of materials, but activated charcoal is commonly used in filters because of its high surface area. If adsorption media comes into contact with aerosols, the media becomes oversaturated and is no longer effective. Because of this, coalescing filters are typically used before vapor removal filters.
Filters are small components that make a big difference in the purity of compressed air. Discover more about Atlas Copco's portfolio of compressed air filters by visiting www.atlascopco.com/air-usa!