Understanding the technical aspects of absolute filtration

Several types of filtration methods are suited for different applications

• Mechanical filtration
  Uses a physical barrier to remove particles from a fluid. Common examples include sieves and strainers 
• Chemical filtration
  Involves the use of chemical reactions to remove impurities, often used in water treatment processes
• Biological filtration
  Utilizes biological processes to break down contaminants, commonly used in wastewater treatment

Compressed air is important in many industrial processes. However, it often has contaminants like dust and oil aerosols. These impurities can originate from intake air, installation contaminants, and the lubrication of the compressor element. To ensure clean and efficient compressed air, technicians install one or more filters after the compressor.

Filtration is essential for removing particles from the compressor air flow. Inside these filters, there are multiple layers of thin fibres. These layers create multiple capturing points, increasing the particle capacity of these so-called depth filters.

Many people think that filters work like sieves. They hold back particles that are larger than the filter's pores. However, this is a misconception. Dust particles in compressed air vary in size and are often much smaller than the media pores of the filters they are captured with.
 

Besides sieving, three different filtration mechanisms are at work, each responsible for capturing particles of a particular size
 

• Inertial Impaction
  Occurs when unclean air flows through the filter media. Heavier particles, because of their large inertia, do not follow the gas flow streamlines. Instead, they follow a straight path and collide with a fiber, which removes them from the air. Impaction becomes more important with increasing particle size

• Interception
  Involves particles of a somewhat smaller size. These particles are light enough to follow the streamline. However, if the radius is larger than the distance to the edge, the particle hits the fiber. When this happens, it gets stuck and is removed from the air. Interception gains importance with increasing particle size

• Diffusion
  Takes care of the smallest particles. These small particles do not follow the streamlines exactly. They move randomly because they collide with gas molecules. This movement is called Brownian motion. Because of this erratic movement, they are bound to collide with a fiber. The smaller a particle is, the more it can move freely. This also means it is more likely to meet a fiber. Captured by diffusion becomes more important with decreasing particle size

The total filter efficiency is a result of the combination of these three filtration mechanisms. They are particularly good at retaining larger particles and, counterintuitively, smaller particles as well. This leaves a "weakest point" of filtration known as the MPPS point – Most Penetrated Particle Size.

For the highest air quality applications, a final filter is placed behind these depth filters overlapping the MPPS point. This final filter is usually a membrane filter. It works better because it has multiple very small pores. This filter effectively stops particles, including bacterial and viral contaminants, but has a limited contaminant holding capacity.
 

Any particle larger than the pore size will block a flow path. This will quickly increase the pressure drop, much faster than a depth filter. Having an upstream depth filter is a better way to catch most particles. This helps reduce pressure build-up in the final filter. As a result, it creates a more efficient solution for pressure drop.
 

With a membrane final filter, it is possible to reach an efficiency close to 100%, even at the MPPS point of the depth filter. The combination of a depth and membrane filter gives the best filter performance. This includes both filtration efficiency and dust holding capacity. It works well for all sizes of contaminants and particles.

Final filtration is a critical step in many industrial processes, particularly in the pharmaceutical, food and beverage, cosmetics, electronics and battery industries.

 It involves removing any leftover contaminants and it avoids them contaminating the end product. This is key to avoid recalls of whole badges.

For these key applications the good practice is to work with absolute filtration to bring the risk of a breakthrough to an absolute minimum.

It is important to understand the diverse ways to filter air and the parts used in compressed air filtration. This helps to select the right solution for your requirements and maintain high air quality.

By using depth filters and membrane filters together, we can reach the highest air quality levels. This ensures safe, reliable, and efficient operations for even the most challenging applications.