Why Filtration Efficiency May Decrease at Very Low Airflow: A Mechanism-Based Explanation

It is generally believed that as airflow increases, face velocity increases, and filtration efficiency decreases. In other words, lower airflow (lower velocity) should lead to higher filtration efficiency.

However, in actual testing—such as at 500 m³/h—the opposite is sometimes observed: efficiency decreases when airflow is reduced. This article aims to analyze the underlying reasons. Notably, this phenomenon has also been observed by experienced professionals in the filtration industry, and we share our findings and discussion here.

1. Filtration Mechanisms and Their Response to Airflow

Filtration relies on several mechanisms:

In theory, lower airflow should enhance diffusion—the dominant mechanism for MPPS particles—and thereby improve efficiency.

2. Why Efficiency May Drop at Very Low Airflow

Despite the theoretical advantage, the following practical and physical factors can lead to decreased efficiency at very low flow rates:

a. MPPS Shifts to a Larger Size

• At low face velocities, the MPPS may shift toward larger particle sizes (e.g., closer to 0.3 μm).

• This reduces the effectiveness of diffusion, and if inertial mechanisms are also suppressed, total efficiency declines.

b. Particle Streamlines Bypass Fibers

• At ultra-low speeds, airflow becomes more laminar and stable.

• Particles may follow streamlines and pass between fibers without interacting, especially in filters with large pore sizes or widely spaced fibers.

c. Filter Media Not Optimized for Diffusion

• Some glass fiber materials are designed for performance under standard or moderate airflow.

• At low velocity, diffusion capture may not be sufficient to dominate overall performance.

d. Flow Channeling or Uneven Loading

• Low airflow reduces turbulence and mixing, possibly leading to uneven particle distribution across the filter.

• This can result in localized underperformance.

e. Shallow or Thin Filter Media

• If the filter media is relatively thin, longer particle residence time at low speeds does not necessarily equate to more contact with fibers.

• In some configurations, particles can travel through without meaningful deviation.

3. Conclusion

While lower airflow can theoretically improve filtration efficiency through enhanced diffusion, real-world filter behavior depends heavily on filter media structure, MPPS characteristics, and airflow dynamics. In some cases, especially with specific materials or design constraints, efficiency may actually decline at ultra-low face velocities such as 500 m³/h.

Thus, when testing filters at non-standard flow conditions, it’s essential to interpret results in the context of filtration physics—not just measurement numbers.

For more insights into HEPA/ULPA filter testing, or to discuss your specific filtration requirements, feel free to contact our technical team.