Filtration works by separating solid particles from a fluid (liquid or gas) using a filter medium, based on the principles of particle theory which describe matter as tiny, moving particles.
The Core Principle
At its heart, filtration leverages the fact that matter is composed of discrete particles. The filter medium acts as a physical barrier or a structure with specific characteristics designed to interact with these particles. When the fluid containing particles passes through the filter, the particles are captured or retained while the fluid passes through. This separation happens because the particles, as individual entities, behave differently from the fluid molecules relative to the filter structure.
Particle Interaction with the Filter
Different mechanisms, all explained by how particles behave, contribute to filtration:
- Sieving/Straining: The most straightforward mechanism where particles larger than the pores or openings in the filter medium are physically blocked.
- Capture by Interaction: Smaller particles, even those smaller than the filter's openings, can be captured through various interactions with the filter material.
Fibrous Filtration and Particle Capture
In many filtration systems, particularly for air or gas, the filter medium consists of a mat of fibers. Particle theory helps explain how even very small particles are captured by these fibers:
- Small particles follow the air flow around the fibre and are captured if they come close enough to the fiber. This happens because even tiny particles have mass and size. As they are carried by the fluid flow lines, they can be intercepted if their path brings them into close proximity with a filter fiber.
- The reference highlights that this capture effect increases with increased particle size, as larger particles are less likely to perfectly follow tight curves in flow lines around fibers and are more likely to impact the fiber.
- It also increases with smaller fibre diameters and/or smaller clearance between (i.e. closer spacing of) the fibres, as these conditions present more surfaces and denser structure for particles to encounter and be captured by.
These interactions—becoming close enough to a fiber due to flow patterns (interception) or deviating from flow lines due to inertia and hitting a fiber (inertial impaction)—are fundamental ways particles are removed from the fluid stream in fibrous filters, all based on their behavior as distinct physical entities.
Factors Affecting Filtration Efficiency
Based on particle theory and the mechanics of capture, several factors influence how effective a filter is:
- Particle Size: Generally, very large particles are easily sieved. For smaller particles, efficiency depends on interaction mechanisms.
- Filter Structure: The size, shape, and arrangement of the filter's pores or fibers are critical. As the reference notes, smaller fiber diameters and closer spacing improve capture.
- Fluid Velocity: The speed of the fluid flow can affect how particles interact with the filter medium (e.g., higher velocity can increase inertial impaction).
- Particle Properties: The density and shape of the particles also play a role in how they move and interact within the fluid stream and with the filter.
Understanding filtration through the lens of particle theory allows for the design and selection of appropriate filter media for specific applications, ensuring efficient removal of contaminants based on their physical characteristics and behavior.
| Filtration Mechanism (Example) | How Particle Theory Applies | Influencing Factors (from reference) |
|---|---|---|
| Sieving/Straining | Particles are discrete entities larger than pores | Pore size, Particle size |
| Capture by Fibers (Reference) | Particles follow flow, interact with fiber surface | Particle size, Fibre diameter, Fibre spacing/clearance |
