The mission of HEPA andULPA filters seems simple: to remove those extremely small particles from flowing air. However, achieving this protection is a sophisticated hunt orchestrated by four fundamental mechanisms: interception, inertial impaction, diffusion, and electrostatics.
Principles of the Four Capture Mechanisms
1. What is Interception? This is the most intuitive mechanism, targeting relatively large particles (typically larger than 0.6 micrometers in diameter). Their large mass and inertia make it difficult for them to closely follow the airflow streamline. When the airflow carrying these particles passes through the fibers, because the distance between the particle’s center and the fiber surface is less than its radius, they “collide head-on” and are directly captured by the fibers. This is like a log floating in a river, unable to pass under a bridge pier due to its size, and eventually getting stuck. Interception is the main force in handling larger suspended particles.
2. What is Inertial Impaction? For medium-sized particles (approximately 0.3 to 1 micrometer), whose mass and momentum are more significant, inertia is the dominant force. These particles, due to inertia, cannot adjust their orientation in time to follow the sharply curved streamlines and avoid the fibers, thus deviating from the mainstream airflow. They are then “thrown” towards the fiber surface and adhere due to their own momentum. The more complex the fiber arrangement and the more tortuous the airflow path, the higher the capture efficiency of inertial collisions. This is a key “kinetic energy trap” in the filter layer.
3. What is diffusion? When particle size is extremely small (typically less than 0.1 micrometers), their mass is extremely light, and the random thermal motion (Brownian motion) of air molecules has a far greater impact on them than the overall inertia of the airflow. These ultrafine particles no longer smoothly follow the airflow but instead perform a random, trembling “dance” in the air. This random motion greatly increases their chances of contacting the fibers. During their flow through the fibrous medium, they “stroll” to the fiber surface and are captured due to continuous random deviations. Diffusion is the core mechanism for capturing ultrafine particles such as viruses and smoke.
4. What is electrostatic adsorption? Unlike the previous three mechanisms that rely on mechanical action, electrostatic adsorption is an active, enhanced force. Many high-efficiency filter media (especially charged meltblown materials) have fibers with a persistent electrostatic charge. When electrically neutral particles pass through these charged fibers, they are induced to polarize, generating an opposite charge, and are thus strongly adsorbed by the fibers. This Coulomb force extends far beyond mechanical contact, efficiently capturing various charged or polarizable particles, especially those particles whose mechanical filtration efficiency is at a low point due to their “awkward” size (approximately 0.1-0.3 micrometers). It’s like creating an invisible energy net around the fibers.
Four Mechanisms and the “Most Penetrating Particle Size“
These four mechanisms do not work independently; their combined effect determines the legendary efficiency of HEPA/ULPA filters. It is worth noting that each mechanism has a different capture efficiency curve for particles of different sizes. The diffusion mechanism is extremely effective for small particles, inertial interception is significantly effective for large particles, and there is an efficiency “valley” between the two (approximately 0.1-0.3 micrometers). Particles in this size range are neither small enough to undergo intense Brownian motion nor have sufficient mass and inertia to escape the airflow, making them the most difficult to capture and thus known as the “most penetrable particle size.”
Therefore, HEPA/ULPA filters are not only guardians of clean air but also ingenious users of the laws of physics. Interception, inertia, diffusion, and electrostatics are their invisible weapons, constructing a reliable defense in the microscopic world. However, the pinnacle of technology lies not only in the purity of its function but also in its ability to integrate with the operating logic of the world. In Munich, Germany, Trenntech‘s philosophy resonates with this: the ultimate significance of any cutting-edge technology transcends its functional peak and lies in its ability to coexist harmoniously with the larger environmental system and resource cycle.