How does the HEPA filter work?

1.What is HEPA filter? 

HEPA(High efficiency particulate air Filter)，filters that meet HEPA standards have an efficiency of 99.7% for 0.1 micron and 0.3 micron, and the characteristics of HEPA nets are that air can pass through, but fine particles cannot pass through.

It can remove more than 99.97% of particles with a diameter of 0.3 micron (1/200 of the diameter of a hair), and is the most effective filtration medium for pollutants such as smoke, dust and bacteria.

HEPA is divided into PP filter paper, glass fiber, composite PP PET filter paper, melt-blown polyester non-woven fabric and melt-blown glass fiber five materials. Features: large wind resistance, large dust capacity, high filtration accuracy, can be processed into various sizes and shapes according to customer needs, suitable for different models.

2. How does the HEPA filter work?

HEPA filters filter in four forms: interception, gravity, airflow and van der Waals forces.

1. The interception mechanism is commonly understood as the screen, and generally large particles of 5μm and 10μm are intercepted and "screened" down.

2. Under the influence of gravity, the speed of dust particles with small volume and high density decreases when they pass through HEPA, and they naturally settle on the HEPA filterscreen just like sand settling on the bottom of a river.

3. The uneven weaving of the filter screen forms a large number of air vortices, and small particles are adsorbed on the HEPA filter screen under the action of the airflow cyclone.

4. The ultrafine particles hit the HEPA fiber layer by Brownian motion and were purified by van der Waals force. If the virus carrier is less than 0.3μm, it is purified under the influence of this force.

3. HEPA filter grade

I always hear someone say, "The filter level I use is H13", then what is the "H13" evaluation standard here?

Speaking of HEPA filters, we will introduce MPPS, which is the most penetrating particle problem. Known as the most penetrating particle, PM0.3 penetrates HEPA filters more easily than smaller particles such as PM0.1 (less susceptible to cyclones and van der Waals forces). That is to say, the filtration capacity of HEPA for PM0.3 is 99.97%, and the filtration capacity of PM0.1 may be 99.99%, which is a misunderstanding in our previous cognition of HEPA filter.

According to the EU EN1882 standard, according to the filter efficiency, we divide the HEPAl filter into 5 grades: coarse filter, medium filter, high efficiency filter, HEPA high efficiency filterand super high efficiency filter. A filter with a filtration efficiency higher than 99.97% for 0.3μm particles is called H13.

4. HEPA common misunderstanding

Misconception 1: The larger the volume of particulate matter, the easier it is to be removed by HEPA?

Analysis: The purification principle of HEPA filter is not simply like a sieve to filter out particles larger than the mesh to purify the air. Instead, it relies on the van der Waals force between fine particles and the filter to form an adsorption effect, and its filtration efficiency is very good for particles above 0.5μm and below 0.1μm.

Particles below 0.1μm do Brownian motion, the smaller the particle, the stronger the Brownian motion, the more times it is hit, and the adsorption effect is good.

The particles above 0.5μm do inertial motion, the greater the mass, the greater the inertia, and the filter effect is good.

In contrast, particles with a diameter of 0.1-0.3μm become difficult to remove HEPA. This is why the industry defines the HEPA filter grade with a filtration rate of 0.3μm particles.

Misconception 2: HEPA can achieve more than 99.97% purification efficiency for 0.3μm micro-particles, then its purification effect on 0.1μm micro-particles is not sure?

Analysis: In the same way as the misunderstanding, PM0.3 is easier to break through the protection of the HEPA filter, because it is less susceptible to the influence of van der Waals forces, therefore, the effect of the filter on PM0.3 of 99.97% May be better for PM0.1, or even 99.99%.

Misconception 3: The higher the HEPA filtration efficiency, the better?

Analysis: The higher the filtration efficiency of HEPA, the greater the resistance, and the actual ventilation volume will be reduced. As the air volume decreases, the number of purification per unit time will also be reduced, and the purification efficiency will be reduced.

Therefore, only the most reasonable combination of fan, filter and air circulation design can achieve an excellent model.

5. How often do you change the HEPA filter?

The core index to judge the service life of the filter is the dust capacity. The core data affecting the dust capacity is the extended area of the filter screen, the larger the extended area of the filter screen, the higher the dust capacity, the more durable the filter screen. Dust capacity refers to the amount of dust accumulation when the resistance due to dust accumulation reaches the specified value under the action of a certain air volume.

But for ordinary users, the basis for judging the replacement of the filter is to observe with the naked eye, the filter "seems" to be dirty, and the "feeling" should be changed... It is very unscientific to judge whether the filter should be replaced by naked eye observation, which may make the filter overuse, causing secondary pollution, and may also make the filter "retire" in advance, without maximizing its use value.

The advantage of HEPA is that the technology is mature, and because of the physical filtration method, no secondary pollution will be formed. The disadvantage is that the HEPA wind resistance is larger, the fan requirements are higher, the more advanced HEPA filter, the greater the wind resistance value.

It is recommended to replace the pre-filter every 2 months and the HEPA filter and activated carbon filter every 6-8 months.