1. Dust holding capacity
The dust holding capacity is not the weight of atmospheric dust that the filter can hold when it is scrapped, but the weight of specific test dust that the filter can hold under specific test conditions. The term 'specific' here refers to:
Standard test wind tunnel, as well as related testing and measurement equipment;
Standard "road dust" that is much larger than actual atmospheric dust particles;
The testing method and calculation method agreed upon by the commissioning party and the testing party, or specified in the standard;
The termination test conditions agreed upon by the client and the filter.
There is no direct correlation between the dust holding capacity and the actual weight of the dust contained by the filter, and the encouraged dust holding capacity data has no meaning for users. For example, a filter with a test dust capacity of 600g may hold 2.5kg of atmospheric dust in actual use; The other one has a dust holding capacity of 900g, and it may only be able to hold 1.5kg of dust in your hand. Only when the test conditions and test dust are the same, can we estimate which filter will have a longer service life than the other based on the dust holding capacity data.
When evaluating general ventilation filter products, professional laboratories and filter manufacturers need to conduct destructive dust emission tests on the filters. The purpose is to evaluate the average efficiency of the filters throughout the entire test process according to relevant test standards, and the dust holding capacity is one of the data obtained from such tests. How to persist in conducting dust emission tests on filters in the laboratory for a long time? Testers can use historical data to compare the advantages and disadvantages of a certain filter, and it is difficult for outsiders to understand the actual significance of the dust holding capacity.
The termination conditions for tests specified in some European and American standards are:
When the resistance reaches twice or more of the initial resistance;
The instantaneous filtration efficiency is lower than 85% of the maximum efficiency value. The resistance of most filters only increases without decreasing, and only low efficiency filters made of fluffy coarse fiber (≥ 10 μ m) materials may experience the second situation. Obviously, the higher the final resistance is set during the experiment, the greater the dust holding capacity value obtained. So, the principal and the experimenter need to clearly define the conditions for terminating the experiment, otherwise, the dust holding capacity data will be of little significance.
The termination test resistance specified in the European Eurovernt 4/9 standard is 450Pa, which is much higher than twice the initial resistance, but it is an efficiency test rather than a dust holding capacity test.
The Chinese standard only stipulates the dust emission test for "coarse efficiency" filters, with the aim of obtaining weight efficiency but dust holding capacity. When discussing dust tolerance with others, you need to know which language they are speaking.
The test dust specified by European and American standards is commonly known as ASHRAE dust, which is mainly composed of "AC fine ash". It is a floating dust (Arizona RoadDust) from a specific location in the desert area of Arizona, USA. After mixing a specific proportion of fine carbon black and short fibers into the AC fine ash, it becomes ASHRAE dust. China once stipulated the use of floating dust from the Loess Plateau, while Japan stipulated the use of its own "Guandongya clay".
2. The force that adheres to dust
When two objects come into contact, there is a tiny gravitational force between their surfaces. It is not the familiar universal gravitation, magnetic force, or electrostatic force. It is a force between molecules, or between molecular clusters. In textbooks, it is called "van der Waals force" or "van der Waals force".
On a microscopic level, a negatively charged electron cloud in a single atom or molecule has a center, and a positively charged atomic nucleus also has a center. When these two centers do not coincide, there is an electric dipole. A single dipole may undergo rapid changes or remain relatively stable, but when multiple dipoles come together, they can exert gravitational force on surrounding objects. This kind of gravity is very weak, with a magnitude 1-2 orders of magnitude smaller than chemical bonds. His range of action is also very narrow, and as the distance increases, gravity disappears; When the distance is too close (>7?), the electron clouds overlap and adjacent molecules repel each other
Chalk powder remains on the blackboard because van der Waals forces can deal with those powders; The chalk head will not stick to the blackboard because van der Waals force is negligible compared to gravity.
Dust particles in the air collide with each other and merge into large particles due to van der Waals forces. Indoor dust hits the wall and remains there, causing the wall to "fade" or form black stains. Rough surfaces are more prone to dust adhesion due to their larger contact area with dust.
The filtering medium in the filter is a maze, and the dust trapped in it has more opportunities to collide with obstacles than usual, and is left behind due to the ubiquitous van der Waals force in the filtering medium.
Under specific circumstances, electrostatic forces also participate in the process of capturing dust. If the filtering medium carries static electricity, the filtering effect will be significantly improved. One of the reasons is that electrostatic force causes dust to change its trajectory and collide with obstacles, and the other is that electrostatic force is more likely to stick dust than van der Waals force. Dust can also be artificially added with static electricity, making them easily adsorbed by other objects, such as household electrostatic filters and industrial electrostatic precipitators.
3. Filter resistance
Filters create resistance to airflow. The filter accumulates dust and resistance increases. When the resistance increases to a certain specified value, the filter is scrapped
The resistance of a new filter is called the "initial resistance", and the resistance value corresponding to the scrapped filter is called the "final resistance".
When designing, a representative resistance value is often required to calculate the design air volume of the system. This resistance value is called the "design resistance", and the common method is to take the average of the initial resistance and the final resistance.
Determine final resistance
The choice of final resistance is directly related to the service life of the filter, the range of changes in system air volume, and system energy consumption.
In general, the selection of final resistance is the responsibility of the designer. Experienced engineers can change the final resistance value of the original design according to the on-site situation.
Some designers may forget to tell users the final resistance value they have chosen; Sometimes users may switch to other models of filters or suppliers. At this point, the on-site engineer had to determine the final resistance value on their own.
In most cases, the final resistance of the on-site filter is 2-4 times the initial resistance.
The dirtier the filter, the faster the resistance increases. A high final resistance value does not necessarily mean that the service life of the filter will be significantly extended, but it will cause a sharp decrease in the air flow of the air conditioning system. Therefore, there is no need to set the final resistance value too high. Some test standards stipulate that the condition for terminating the test is when the resistance is "twice the initial resistance or higher", but this regulation has little to do with actual usage conditions.
Low efficiency filters often use coarse fiber filter media with a diameter of ≥ 10 μ m. Due to the large gaps between fibers, excessive resistance may blow away the accumulated dust on the filter. At this point, the resistance no longer increases, but the filtration efficiency decreases to zero. Therefore, it is necessary to strictly limit the final resistance value of filters below G4.
Here are some suggestions for final resistance, which come from experience and have little reason to explain.
Table 1-1 Recommended final resistance values for filtration efficiency specifications: Recommended final resistance (Pa)
G3 (coarse effect) 100-200
G4150-250
F5-F6 (medium efficiency) 250-300
F7-F8 (high school grade) 300-400
F9-H11 (sub efficient) 400-450
Efficient and Very Efficient 450-600
The final resistance of the filter may mean that it needs to be replaced immediately, or it may mean planning to replace the filter tomorrow, next week, or next month. The user's perception of final resistance depends on the specific regulations on site and the operator's experience.
Resistance monitoring
Each filtering section should be equipped with a resistance monitoring device. The final resistance depends on the instrument to determine, not just the operator's feeling.
The cheapest resistance monitoring device is a U-tube differential pressure gauge. The inclined tube differential pressure gauge is more aesthetically pleasing and has higher accuracy than the U-shaped tube. The pointer type differential pressure gauge has a higher grade and price. Differential pressure transmitters can convert resistance into current or voltage signals, which are then transmitted to the control system.
In addition to a resistance monitoring device that can read, a final resistance alarm device should also be added. A convenient way is to mark a red bar on the differential pressure gauge; The safe method is to use a differential pressure switch.
4. Filtration efficiency
The "filtration efficiency" of an air filter is the ratio of the amount of dust captured to the original dust content in the air
The meaning of efficiency may seem simple, but its meaning and value vary greatly depending on the experimental method used.
Among the factors that determine filtration efficiency, the meaning of "quantity" of dust is diverse, resulting in a wide range of calculated and measured filter efficiency values. In practice, the total weight of dust and the number of dust particles; Sometimes it refers to the amount of dust of a typical particle size, or the amount of dust of all particle sizes; There are also specific methods to briefly reflect the concentration of light (colorimetric method) and fluorescence (fluorescence method); There are instantaneous quantities under certain conditions, as well as weighted average efficiency values of the entire process of dust emission testing.
Testing the same filter using different methods will result in different efficiency values. The testing methods used by different countries and manufacturers are not uniform, and the interpretation and expression of filter effectiveness vary greatly. Without testing methods, filtering efficiency cannot be discussed.
In history, there were a group of people who worked on ventilation and clean room filters, another group who worked on car filters, as well as those who worked on dust collectors and liquid filters. They each used their own methods and talked about their own efficiency. When air filter factories are involved in the buying and selling of car filters, or when dust collector factories develop air filters, they themselves are inevitably confused by their claimed filtration efficiency.
In order to save time and reduce misunderstandings, some methods of using codes to represent efficiency have emerged in foreign countries. These codes specify the experimental methods and efficiency indicators, such as Section 2.1 "Filter Efficiency Identification" and Section 2.2 "Filter Efficiency Specification Comparison" in rural areas. Nowadays, filter manufacturers from all over the world are occupying the Chinese market, and domestic manufacturers are also using efficiency labels freely in order to promote their products to users from various backgrounds. Various efficiency values and terms make users, designers, and manufacturers confused.
In addition, there is a "single fiber efficiency" in filtration theory, which can have a geometric meaning greater than 100%. Please refer to the relevant content on filtration theory in the next part of this book for details. Even if one is not careful, those who are engaged in theory may become confused about efficiency issues.
In this chaotic situation, if you must know the specific efficiency values, please do not forget to specify the specific experimental methods and methods for calculating efficiency.
5. Filtration mechanism
⑴ Collision and adhesion
Dust particles in the air either undergo inertial motion with the airflow, irregular motion, or move under the influence of a certain field force. When particles in motion collide with obstacles, the van der Waals forces between the particles and the obstacle surface cause them to stick together.
⑵ Fiber filter material
The filter material should be able to effectively intercept dust particles without creating excessive resistance to the airflow. Non woven fiber materials and specific paper meet this requirement. The disorderly interweaving of fibers forms countless barriers against dust, and the wide space between fibers allows airflow to pass smoothly.
⑶ Inertia principle
Large particles undergo inertial motion in the airflow. The airflow encounters obstacles and detours, causing particles to deviate from the direction of the airflow due to inertia and collide with the obstacles. The larger the particle, the stronger the inertial force, and the greater the possibility of colliding with obstacles, therefore the better the filtering effect.
⑷ Diffusion principle
Small particles undergo irregular Brownian motion. When dealing with irregular motion mathematically, the "diffusion" theory in mass transfer is used, hence the concept of diffusion principle. The smaller the particles, the more intense the irregular motion, and the more opportunities they have to collide with obstacles, so the filtering effect is better.
Efficiency varies with the size of dust particles
The ratio of the amount of dust captured by the filter to the amount of dust in the unfiltered air is called the "filtration efficiency". Particles smaller than 0.1 μ m (micrometers) mainly undergo diffusion motion, and the larger the particle, the higher the efficiency; Particles larger than 0.5 μ m mainly undergo inertial motion, and the larger the particle, the higher the efficiency.
Between 0.1~0.5 μ m, there is a lowest point in efficiency, and dust of this particle size is the most difficult to filter.
⑹ Resistance
Fibers cause airflow to bypass and generate smile resistance. The sum of the resistance of countless fibers is the resistance of the filter.
The resistance of the filter increases with the increase of air flow rate. By increasing the area of the filter material, the relative wind speed of the filter material can be reduced to reduce the resistance of the filter.
Dynamic performance
The captured dust creates additional resistance to the airflow, so the resistance of the filter gradually increases during use. The captured dust combines with the filtering medium to form new obstacles, resulting in a slight improvement in filtration efficiency.
The captured dust fights and gathers on the windward side of the filter material. The larger the filtering area, the more dust it can accommodate, and the longer the service life of the filter.
⑻ Filter scrap
The more dust accumulates in the filter, the greater the resistance. When the resistance reaches a level that is not allowed by the design, the lifespan of the filter comes to an end. Sometimes, excessive resistance can cause the dust already captured on the filter to scatter. When this danger occurs, the filter should also be scrapped.
