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1. Two Main Categories of Particle Separation

In cleanroom applications, airborne particle concentrations are extremely low—far lower than in typical industrial dust‑collection systems—and the particles themselves tend to be very fine. To remove them reliably, we primarily rely on barrier‑type filters, which physically block contaminants and guarantee a secure final filtration stage. A secondary approach often used is electrostatic separation, where charged particles are drawn out of the airflow.

Barrier‑type filters are sorted by where they trap particles:

• Surface filters, which capture particles on their face. Common examples include metal meshes, perforated plates, and chemically formed microporous membranes.

• Depth filters, in which particles become embedded throughout the media. These come in two varieties based on packing density:

  • High‑density depth filters, where research remains sparse.

  • Low‑density depth filters, such as fiber beds, nonwoven fabrics, and filter papers. Although their internal fiber networks are complex, their high porosity lets us treat each fiber as an individual filtering element—simplifying both design and analysis. Low‑density depth filters also offer modest airflow resistance alongside excellent collection efficiency, making them a mainstay in cleanroom technology.

2. The Two Phases of Filtration

The filtration process unfolds in two distinct phases:

1. Stable Phase
   During this initial stage, both particle‑capture efficiency and pressure drop remain essentially constant over time. They depend solely on the filter’s inherent structure, the particles’ properties, and the airflow characteristics. Any thickening of the media from captured particles is negligible—especially when dealing with the very low particle loads typical of cleanroom air. For these conditions, the stable phase largely defines how the filter will perform.

2. Unstable Phase
   In this longer second stage, efficiency and pressure drop begin to drift as a function of time. This drift is driven not by the particles’ original properties but by factors such as continued particle deposition, chemical interactions with the carrier gas, and moisture effects. While this phase is critical for most industrial filters—where heavy dust loads and varied environmental conditions prevail—it plays a far smaller role in advanced air‑cleaning systems designed for ultra‑low particle concentrations.