What influence do the air velocity and air volume of a box-type filter have on its filtration effect?

The air velocity and air volume of the box-type filter are the core parameters that affect its filtration effect, directly related to the filtration efficiency, resistance stability, filter material life, and system operation safety. The interaction between the two (air volume = wind speed × effective area) will affect the filtration effect through multiple dimensions, such as air flow state, particulate matter capture mechanism, and filter material load, as follows:
I. Impact on Filtration Efficiency: Closely related to the particle size of particulate matter
Filtration efficiency (i.e., the ability to retain particles of different sizes) is significantly affected by wind speed, and due to different particle capture mechanisms (such as inertial collision, diffusion, interception, etc.), the directions of influence on particles of different sizes may be opposite.
The influence on large particles (≥1μm)
Large particles are mainly captured through "inertial collisions" and "interception effects" :
When the wind speed is high, the large particles carried by the airflow have a large inertia and are difficult to be carried around the filter material fibers by the airflow. They are more likely to collide with the fibers and be retained. At this time, the filtration efficiency slightly increases with the increase in wind speed.
However, when the wind speed is too high (exceeding the design value by more than 50%), it may cause the large particles that have been trapped to be "re-blown" by the high-speed airflow (i.e., "secondary dust lifting"), which instead increases the dust content in the downstream air and reduces efficiency.
The influence on small particles (≤0.5μm)
Small particles are mainly captured through "Brownian diffusion" (the smaller the particles, the more intense the Brownian motion and the easier it is to come into contact with the filter material fibers) :
When the wind speed is low, the airflow stays in the filter material for a longer time, and small particles have more sufficient time to diffuse and contact the fibers. The filtration efficiency significantly increases as the wind speed decreases.
When the wind speed is too high: the airflow velocity is fast, and small particles have no time to diffuse before passing through the filter material, resulting in a significant decrease in efficiency (for example, when the wind speed exceeds 1.5m/s, the efficiency of a high-efficiency box filter for 0.3μm particles may drop from 99.99% to below 99.9%).
The indirect impact of excessive air volume
If the actual air volume far exceeds the rated air volume of the filter (for example, by more than 20%), even if the average wind speed does not significantly exceed the standard, it may still cause "efficiency shortcomings" due to uneven air distribution (local wind speed being too high) - in some areas, the penetration rate of small particles in the filter material increases due to the excessively high wind speed, and the overall filtration efficiency is reduced.
Ii. Impact on Filtration Resistance: It directly determines the system's energy consumption and stability
Filtration resistance is the pressure loss when air passes through a filter and is nonlinearly and positively correlated with wind speed (resistance ≈ is proportional to the square of wind speed). The specific influences are as follows:
Resistance increases sharply with the rise of wind speed/air volume
When the wind speed rises from the design value (such as 1m/s) to 1.5m/s, the resistance may increase to 2.25 times the original (in a square relationship). If the air volume doubles, the resistance may increase fourfold. Excessive resistance can lead to:
The system energy consumption has soared (the fan needs more power to overcome resistance).
Insufficient air pressure from the fan leads to a decrease in the actual air supply volume, which instead affects the air change rate in the clean area.
Resistance fluctuations affect the stability of filtration.
Frequent fluctuations in wind speed/air volume (such as the system air volume going up and down) will cause frequent changes in resistance. The filter material fibers will become fatigued due to repeated forces and may break prematurely (especially brittle filter materials, such as glass fibers), which in turn leads to "leakage" - unfiltered air directly passes through the damaged area, resulting in a sudden drop in filtration effect.
Iii. Impact on Dust Holding Capacity and Service Life: Determines the continuous effective time of the filter
Dust holding capacity refers to the maximum amount of dust that a filter can retain. Wind speed and air volume indirectly affect the dust holding capacity and service life by influencing the "dust accumulation speed" and the "filter material carrying capacity".
High wind speed/high air volume accelerates dust accumulation and shortens the lifespan.
The higher the wind speed, the more dust passes through the filter material per unit time, and the dust accumulation rate accelerates (for example, when the wind speed rises from 0.8m/s to 1.2m/s, the dust accumulation rate may increase by more than 50%), causing the filter to reach "dust saturation" earlier (the resistance reaches the replacement threshold), and its service life is shortened by 30% to 50%.
Excessively high wind speed causes uneven load on the filter material.
Even if the average wind speed is within the design range, if the local wind speed is too high (such as when the air flow is concentrated at the edge or center of the filter), it will cause the local filter material to accumulate dust too quickly, resulting in blockage first. Meanwhile, other areas still have dust-holding space, and the overall dust-holding capacity is wasted. The filtration effect will decline due to local failure.
Iv. Impact on Filter Material Integrity: Avoiding "Physical Failure"
The mechanical strength of the filter material is limited. When the wind speed and air volume exceed the design limit, it may directly lead to damage to the filter material, completely rendering the filtration effect ineffective.
When the wind speed is too high, the impact force of the airflow on the filter material increases. Especially for filter materials with pleated structures (such as V-shaped box filters), the pleats may rub against each other due to the impact of the airflow, leading to fiber breakage.
If the air volume is chronically overloaded (such as exceeding the rated value by more than 30%), the sealant between the filter material frame and the box body may crack due to excessive pressure, resulting in "bypass leakage". Unfiltered air directly passes through the gap, and at this time, the filtration efficiency is almost zero.
Summary: Wind speed and air volume need to "match the design value."
The best filtration effect of the box-type filter should be achieved within the range of the design air velocity (typically 0.8-1.5m/s for medium-efficiency filters and 0.3-0.8m/s for high-efficiency filters) and the rated air volume.
Below the design value: Although it can enhance the filtration efficiency of small particles, insufficient air volume may lead to inadequate ventilation in the clean area. Moreover, low air velocity may cause large particles to settle on the surface of the filter material, thereby increasing the resistance and reducing the upward speed.
Higher than the design value: The efficiency of large particles slightly increases, but the efficiency of small particles decreases; the resistance suddenly increases, the filter material is prone to damage, and the overall filtration effect is unstable.
Therefore, in actual operation, it is necessary to control the wind speed and air volume within the rated range marked by the manufacturer through methods such as fan frequency conversion and air valve adjustment, to balance the filtration efficiency, energy consumption, and service life.