Air filter is a kind of air filtering device. Its main function is to filter the dust in the air and purify the gas entering the room. It is generally used in clean workshops, clean rooms and laboratories.
At present, air filter is the most important means of air purification, its quality can be said to determine whether the indoor cleanliness is up to standard.
The performance of air filter mainly includes air volume, filtration efficiency, air resistance and dust capacity, which are also the four main indexes affecting the filter.
Air filter is mainly divided into primary filter, medium filter, sub high efficiency particulate air filter, high efficiency particulate air filter and ultra low penetration air filter.
The following is a brief introduction to the functions of these filters.
1. The primary filter mainly filters dust particles above 5.0μm, which is generally used as a pre filter to protect the service life of medium efficiency and high efficiency filters.
2. The medium efficiency filter filters the dust particles above 1.0μm, which are generally placed in front of the high efficiency filter and behind the fan, so as to reduce the load of the high efficiency filter and extend the service life of the accessories in the air conditioning box and the high efficiency filter.
3. The sub high efficiency particulate air filter can filter dust particles above 0.5μm, which can be used as terminal filter in the purification system with low requirements.
4. High efficiency filter is mainly used to filter dust particles above 0.3μm. It is the terminal filter in the air conditioning purification system, and must be used in the high-grade clean room (0.3μm clean room).
5. Ultra high efficiency filter (UHF) filters out dust particles larger than 0.1μm and is installed at the end of air conditioning ventilation system, i.e. high efficiency air supply outlet. It is terminal purification equipment that must be used in high-grade clean rooms (0.1μm clean rooms).
This article focuses on the four main indexes affecting the air filter.
Air volume, filtration efficiency, air resistance and dust holding capacity are the four main indexes affecting air filter. In addition, supply air volume and filter life are also very important parameters.
1. Air volume: the air volume passing through the filter = air velocity of filter section × cross sectional area of filter (M2) × 3600m3
2. Filtration efficiency: under the rated air volume, the percentage of the difference between the dust concentration N1 and N2 before and after the filter and the dust concentration in the air before the filter is called the filtration power A.
It is more intuitionistic to evaluate the final effect of filter by penetration rate. The penetration rate K refers to the percentage of dust concentration in the air behind the filter and in front of the filter.
3. Air resistance: the resistance of air flowing through the filter is a part of the total resistance of the system. The resistance increases with the increase of filtration rate. The resistance of the filter must be evaluated on the premise of rated air volume, and the resistance of the filter will increase with the increase of dust holding capacity. The resistance of a new filter in use is called the initial resistance, and the resistance when the dust capacity reaches the specified maximum value is called the final resistance.
Generally, the final resistance of medium efficiency and high efficiency filters is about 2 times of the initial resistance.
4. Dust capacity: it is the amount of dust accumulated inside the filter when the final resistance is reached under the rated air volume.
The main source of dust in clean rooms is personnel flow, which accounts for more than 80% of the total dust. Besides, it comes from buildings, about 10% - 15%. Generally speaking, the dust brought by the clean air supply system should be very small.
It is worth noting that the function of the air filter is to filter dust particles, at the same time also the main measure of sterilization.
This is because dust particles are often used as carriers of bacteria. If there are more dust particles in the air, bacteria will easily attach to them. The reduction gear box and steering gear box to filter dust particles are equivalent to filtering a large number of bacteria.
In addition, the HEPA filter should be replaced in any of the following cases.
1. When the air flow speed is reduced to the minimum, the air flow velocity cannot be increased even after the primary and medium efficiency air filters are replaced.
2. The resistance of high efficiency air filter is 1.5 ~ 2 times of the initial resistance.
3. Leakage of high efficiency air filter is irreparable.
