Wind speed is one of the most important factors for the effectiveness of air filters. In daily use, we will say that the lower the wind speed, the better the filtering effect! The following is a brief analysis of this concept:
The diffusion of small particle size dust (Brownian motion) is obvious, the wind speed is low, the airflow retention time in the filter material is longer, and the dust will have more chances to impact obstacles, so the filtration efficiency is high. Experience has shown that for a high-efficiency filter when the wind speed is reduced by half, the dust transmittance will be reduced by nearly an order of magnitude (the efficiency value will increase by a 9), the wind speed will be doubled, and the transmittance will increase by an order of magnitude (the efficiency will be reduced by a 9).
Similar to the effect of diffusion, when the filter material is electrostatic (electret material), the longer the dust exists in the filter material, the greater the possibility of being adsorbed by the material. As the wind speed changes, the filtration efficiency of electrostatic materials will change significantly. If you know that there is static electricity in the material, you should minimize the airflow through each filter when designing the air conditioning system.
According to traditional theory, when the wind speed decreases, the collision probability between dust and fibers will decrease, and the filtration efficiency will decrease. But in practice, this effect is not obvious, because the wind speed is small, the elasticity of the fiber to dust is also small, and the dust is more likely to adhere.
When the wind speed is high, the resistance is high. If the life of the filter is based on the ultimate resistance, the life of the filter will be shortened when the wind speed is higher. It is difficult for ordinary users to observe the influence of wind speed on filtration efficiency, but it is easier to observe the influence of wind speed on resistance.
For HEPA filters, the air velocity through the filter material is usually 0.01-0.04m/s. Within this range, the resistance of the filter is proportional to the airflow. For example, the initial resistance of a 484*484*220mm high-efficiency filter is 250Pa at a rated air volume of 1000m3/h. If the actual air volume used is 500m3/h, the initial resistance can be reduced to 125Pa. For the general ventilation filter in the air conditioning box, the airflow rate through the filter material is in the range of 0.13-1.0m/sec. Resistance is no longer a linear relationship with the amount of air, but an upward arc. The air volume is increased by 30%, and the resistance can be increased by 50%. If the resistance of the filter is a very important parameter for you, you need to ask the air filter supplier for the resistance curve.
