I. Core Principle: Resistance and Dust Holding Capacity
1. Lower Initial Resistance: The resistance (airflow resistance) of a filter is roughly proportional to the airflow speed passing through it. The lower the airflow speed, the slower the air moves through the filter media fibers, resulting in lower initial resistance.
- Simplified formula: ΔP ∝ v (where ΔP is the resistance and v is the airflow speed)
2. Slower Resistance Increase: As filters are used, they continuously capture dust (accumulated dust), which gradually increases the resistance. The lower the initial resistance, the longer it takes to reach the terminal resistance (usually twice the initial resistance) that indicates the need for replacement.
3. Analogy to Running: Imagine resistance as running. Starting with a slow jog (low airflow speed) allows you to run farther and longer before feeling the same level of fatigue (reaching terminal resistance) compared to starting with a sprint (high airflow speed).
4. Higher Dust Holding Capacity Utilization: The rated dust holding capacity of a filter refers to the weight of dust it can hold when it reaches terminal resistance. At low airflow speeds, dust particles are more likely to be captured deeply and evenly within the filter media, rather than being concentrated and clogging the surface. This allows the filter to more effectively use its entire structure to hold more dust, thereby extending its life.
II. Changes in Collection Efficiency
1. For High-Efficiency/HEPA Filters: The main collection mechanisms are inertial impaction, interception, and diffusion.
2. Diffusion Effect: For very small particles (mainly <0.3μm), Brownian motion causes them to move erratically. At lower airflow speeds, air stays longer in the filter media, increasing the probability that small particles will collide with fibers due to diffusion and be captured. Therefore, at low airflow speeds, the collection efficiency of HEPA filters for tiny particles may even slightly improve.
3. Inertial Impaction and Interception Effects: For larger particles, these effects are stronger at higher airflow speeds. However, the most critical MPPS (Most Penetrating Particle Size) @0.3μm efficiency of HEPA filters is more influenced by the diffusion effect. Thus, low airflow speed operation does not reduce the efficiency of HEPA filters; it may even make them more efficient.
III. Physical Stress on Filter Media
Lower airflow speeds mean that the pulling force and vibration of air on filter media fibers are reduced, physically decreasing the fatigue and damage risks of the filter media. This is beneficial for long-term operational stability.
Summary and Analogy: You can understand it this way:
Imagine a high-efficiency filter as a very dense mesh sponge.
- High airflow speed = Using a high-pressure water gun to quickly flush the sponge. Water will force its way through, mostly passing through the surface and the easiest paths, quickly clogging the surface and rapidly increasing resistance, with much of the sponge's interior space remaining unused.
- Low airflow speed = Allowing water to slowly seep into the sponge. Water has enough time to evenly diffuse into every tiny pore of the sponge, enabling it to hold more water and causing resistance to increase very slowly.
IV. Considerations in Practical Applications
Although low airflow speed operation is beneficial for extending filter life, trade-offs must be made in system design.
1. Airflow Requirement: The system's airflow (cubic meters/hour) is predetermined. Airflow = airflow speed × filter area. The most effective way to reduce airflow speed is to increase the filter area.
2. Method: Use larger-sized filters or adopt designs such as "V-shaped" or "pocket-type" to provide a larger effective filter area within the same installation space. This is why many high-efficiency supply air outlets use "V-shaped filter screens" or "multi-pocket" designs.
3. Cost Trade-off: Increasing the filter area means higher initial investment costs (larger and more expensive filters), but this results in longer replacement cycles and lower operating resistance (saving electricity). A life cycle cost assessment is necessary.
4. System Design: Fans must be capable of operating at lower resistance to ensure operation at the designed airflow.
Running high-efficiency filters at airflow speeds below their rated speed is one of the most effective and scientific methods to extend their service life. This is usually achieved by increasing the effective filter area and is an important principle in modern air purification systems and cleanroom design.