How Filters Work
Filters capture contaminants through multiple physical mechanisms as fluid passes through the filtration media. Understanding these mechanisms explains why different filters perform differently even with similar specifications.
What It Is
Filtration is a mechanical separation process. Contaminated fluid enters the filter, passes through media containing millions of small passages, and exits with fewer particles. The media captures particles through physical mechanisms—not chemical reactions.
Four primary mechanisms drive particle capture: direct interception, inertial impaction, diffusion, and sieving. Most filters use a combination of these mechanisms, with different mechanisms dominating depending on particle size and flow conditions.
The Four Capture Mechanisms
1. Direct Interception
Particles following fluid streamlines pass close enough to a fiber to make contact. Once contact occurs, van der Waals forces hold the particle to the fiber surface.
Most effective for: Mid-size particles (1-10 microns) traveling at moderate velocities
2. Inertial Impaction
Larger, heavier particles cannot follow the fluid streamlines as they curve around fibers. The particle's inertia carries it into the fiber instead of around it.
Most effective for: Larger particles (>1 micron) at higher velocities
3. Diffusion (Brownian Motion)
Very small particles do not follow streamlines at all. They move randomly due to collisions with fluid molecules, eventually contacting and adhering to fibers.
Most effective for: Sub-micron particles (<0.5 microns) at lower velocities
4. Sieving (Straining)
Particles larger than the pore or passage size simply cannot fit through. They are mechanically blocked on the upstream side of the media.
Most effective for: Particles larger than the smallest pore size in the media
Why It Matters
Understanding capture mechanisms explains several practical filtration behaviors:
| Observation | Mechanism Explanation |
|---|---|
| Filter efficiency increases as it loads | Captured particles create additional interception points for subsequent particles |
| Higher flow reduces efficiency for fine particles | Less time for diffusion and interception to occur |
| Some filters work better at low temperatures | Higher viscosity slows particles, increasing capture probability |
| HEPA filters capture very small AND very large particles efficiently | Different mechanisms dominate at different particle sizes |
Depth vs. Surface Filtration
Filters are classified by where particle capture occurs: throughout the media thickness (depth) or on the upstream surface (surface).
Depth Filtration
Particles are captured throughout the media thickness. The tortuous path through the media provides multiple opportunities for capture.
Advantages:
- • High dirt-holding capacity
- • Gradual pressure drop increase
- • Good for high contamination loads
Examples:
- • Cellulose filter elements
- • Fiberglass hydraulic filters
- • Pleated synthetic media
Surface Filtration
Particles are captured on the upstream face of the media. The media acts primarily as a sieve.
Advantages:
- • Cleanable and reusable
- • Consistent efficiency throughout life
- • Sharp particle size cutoff
Examples:
- • Wire mesh strainers
- • Membrane filters
- • Sintered metal filters
Common Misconceptions
"Filters work like a net that catches particles"
This describes only sieving, which is one of four mechanisms. Most filters rely more heavily on interception, impaction, and diffusion. A filter can capture particles much smaller than its visible pore size through these mechanisms.
"New filters are more efficient than used filters"
For depth filters, the opposite is often true. Captured particles create additional collection sites, improving efficiency as the filter loads. However, pressure drop also increases, eventually requiring replacement.
"Higher flow means better filtration"
Higher flow velocity actually reduces capture efficiency for fine particles by reducing residence time and overpowering diffusion. Optimal filtration occurs within the filter's designed flow range.
How It Applies in Real Systems
| Application | Primary Mechanism | Filter Type |
|---|---|---|
| Hydraulic return line | Interception + Impaction | Depth, synthetic media |
| Suction strainer | Sieving | Surface, wire mesh |
| Compressed air coalescing | Diffusion + Interception | Depth, borosilicate glass |
| HVAC pre-filter | Impaction + Interception | Depth, synthetic |
| Sterile process filtration | Sieving + Diffusion | Surface, membrane |
When Mistakes Cause Failures
Oversized Filter Housing
Installing a filter element in a housing larger than designed reduces face velocity. While this seems beneficial, it can reduce inertial impaction efficiency for larger particles.
Result: Larger particles pass through that would have been captured at proper velocity.
Operating Beyond Flow Rating
Exceeding the filter's flow rating increases velocity through the media beyond design limits. Diffusion becomes ineffective. Particles that would be captured at proper flow pass through.
Result: Reduced efficiency for fine particles despite acceptable pressure drop readings.
Wrong Media Type for Application
Different media types optimize different capture mechanisms. Using cellulose where synthetic is specified, or depth media where surface is required, changes the filter's performance characteristics.
Result: Filter may have correct micron rating but wrong efficiency curve or dirt capacity for the application.
Related Topics
What is a Filter? →
Filter components and purpose
Filter Media Types →
Cellulose, synthetic, mesh, and specialty media
Micron Ratings Explained →
Measuring filter efficiency
Micron Ratings →
Understanding filter ratings
Note: Filter performance depends on matching the filter design to application requirements including flow rate, particle size distribution, and contamination load. For assistance selecting the correct filter for your application, contact a filtration specialist.
Call (469) 608-9877 →