In recent years, China's power industry has developed rapidly and its technological level has significantly improved. With the gradual implementation and adjustment of China's energy strategy, efficient energy conservation and environmental protection have become a new concept in the construction of power plants. Evaporative cooling air conditioning technology is an energy-saving and environmental protection technology that fully utilizes renewable energy, and has been widely promoted and applied in China. National technical regulations such as HVAC design specifications, energy-saving standards, and technical measures, such as the Design Code for Heating, Ventilation, and Air Conditioning in Civil Buildings (GB50736-2012) and the Energy Efficiency Design Standard for Public Buildings (GB50189-2005), clearly stipulate that this energy-saving and environmental protection technology should be adopted as much as possible.

However, due to the poor air environment in which the power plant is located, in order to ensure the cleanliness requirements of the air supply area and alleviate the impact of dust accumulation on the heat transfer performance of the evaporative cooler, it is necessary to install air filters in the evaporative cooling ventilation air conditioning unit. With the rapid development of science and technology and modern industry, it is necessary to understand the filtering mechanism, filtering materials, and categories of air filters. Based on the application of evaporative cooling ventilation air conditioning units in power plants, the importance of correctly selecting air filters should be explained. In practical applications, designers should make reasonable choices about the category and level of air filters. Tree Bird Education HVAC Design Online Teaching, Teacher Du.

air filter

Air filters are key equipment in ventilation and air conditioning systems, and their performance directly affects the heat and moisture exchange performance of air and water in evaporative cooling ventilation and air conditioning units. In recent years, the research on the filtration mechanism of air filters, the development of filtration materials, and the introduction of relevant national standards have made significant progress in the development and application of air filters.

(1) Filtering mechanism

In classical filtration theory, the mechanism of air filters mainly has five effects:

Diffusion effect: Dust particles generate Brownian motion due to the collision of gas molecules in thermal motion, with a particle size of 0.1 at room temperature μ The particle diffusion distance of m is 17 μ m. Particles will move to the surface of the fiber layer and deposit. Particle size greater than 0.3 μ The Brownian motion of m particles weakens, and the diffusion effect weakens accordingly.

Interception effect: The arrangement of fibers in the fiber layer is intricate and complex, forming countless grids. When dust particles of a certain size move to the surface of the fiber layer, if the distance from the centerline of the particles to the fiber surface is less than or equal to the particle radius, the particles on the fiber surface are intercepted.

Inertia effect: When airflow passes through a complex arrangement of fiber layers, the direction of streamline movement will change. When the mass or velocity of particles is large and the direction of the streamline changes, particles will not have time to follow the streamline and bypass the fibers due to inertia, and will detach from the streamline and approach the fibers, thus colliding and depositing on the fibers.

Gravity effect: When dust particles pass through the fiber layer, they undergo displacement from the streamline under the action of gravity, and settle on the fiber due to gravity. For particle sizes less than 0.5 μ The gravitational effect of m particles can be ignored.

Electrostatic effect: When dust particles pass through fibers with air, both fibers and particles may carry charges, creating an electrostatic effect that attracts particles. Static electricity can cause particles to change their streamline trajectory and deposit, and it can also make particles adhere more firmly to the surface of the fiber layer, improving filtration efficiency without increasing filter resistance.

(2) Filter material

Air filter materials are the core components of air filters. The "Filter Materials for Air Filters" (JG/T 404-2013) classifies filter materials according to their filtration performance, materials used, and purpose. Filter materials can be classified into six categories based on their filtration performance: ultra-high efficiency, high efficiency, sub high efficiency, high efficiency, medium efficiency, and coarse efficiency; Filter materials are divided into glass fiber, synthetic fiber, natural fiber, composite material, and other materials based on the materials used; Classified by usage, it is used for air conditioning ventilation and purification, as well as for ventilation and dust removal.

At present, conventional filter materials are developing towards composite materials and functional filter materials. Composite materials are filter materials formed by interweaving different fibers to overcome the functional defects of a single material. The prospect of functional filter media is very broad, mainly referring to certain special filter media that are different from general filter materials and can meet the special functional requirements of specific industries and fields for air filter materials.

(3) Classification

In GB/T 14295-2008, air filters are classified according to their performance, form, and filter material replacement method. According to the artificial dust counting method, performance classification mainly includes sub high efficiency filters, high and medium efficiency filters, medium efficiency filters, and coarse efficiency filters, as shown in Table 1.

The medium efficiency and coarse efficiency filters are further divided into 3 levels and 4 levels based on their counting efficiency, as shown in Table 2.

It can be divided into six types according to its form: flat type, folded type, bag type, winding type, cylindrical type, electrostatic type, etc. According to the filter material replacement method, it can be cleaned, replaced, and used once.

In the "High Efficiency Filter" (GB/T 13554-2008), it is classified based on structure, efficiency and resistance, and fire resistance. According to the structure of the filter element, it can be divided into two types: filters with partitions and filters without partitions.

According to the classification of efficiency and resistance, the filtration efficiency detected by the sodium flame method specified in GB/T 6165 shall not be less than 99.9%, which is classified as a high-efficiency air filter. It is classified into three categories: A, B, and C based on the filtration efficiency and resistance performance; According to the counting method specified in GB/T 6165, if the filtration efficiency of the filter is not less than 99.999%, it is classified into three categories: D, E, and F based on the filtration efficiency and resistance performance. According to GB 8624, filters are classified into three levels based on the fire resistance rating of the materials used: 1, 2, and 3.

Application status of air filters for evaporative cooling ventilation air conditioning units in power plants

(1) Evaporative cooling ventilation air conditioning unit

At present, evaporative cooling ventilation air conditioning technology has been widely applied in power plants and nuclear power plant steam rooms, electrical equipment rooms, centralized control rooms, and electronic equipment rooms both domestically and internationally. The ventilation and traditional air conditioning cooling methods commonly used in power plants have problems during operation, such as high outdoor temperature that cannot meet the temperature requirements of the factory building solely relying on ventilation, large natural ventilation and sandstorms affecting the normal operation of equipment, high power consumption of traditional air conditioning cooling, and high operating costs.

The application of evaporative cooling ventilation air conditioning technology is mainly aimed at improving the indoor temperature of various ventilation rooms in summer, and providing a safe and stable operating environment for process equipment.

The outdoor air enters through the air inlet, filters out pollutants in the air through the filtering section, and undergoes heat and moisture exchange with water in the evaporative cooling section, causing a decrease in air temperature. At present, power plants that have already used or are determined to use evaporative cooling ventilation air conditioning units generally use direct evaporative coolers or direct indirect evaporative coolers for the evaporative cooling section. The commonly used fillers for direct evaporative cooling include organic fillers, inorganic fillers, natural plant fiber fillers, metal fillers, and fillers - high-pressure micro mist composite evaporative cooling ventilation air conditioning units are also used.

Plate fin indirect evaporative cooler and tube type indirect evaporative cooler are the main application forms of indirect evaporative cooling. The air treated by the evaporative cooling section is sent to the ventilation and air conditioning area through the supply fan. The structural diagram of the evaporative cooling ventilation and air conditioning unit is shown in Figure 1.

(2) Problems that arise in the application

Evaporative cooling ventilation air conditioning units are generally placed outdoors. In order to protect the cleanliness of the evaporative cooler and supply air, it is necessary to set up air filters. At present, many evaporative cooling ventilation air conditioning units in engineering projects require the replacement of filters in a short period of time during use, greatly increasing user operating costs and causing waste of raw materials. In addition, if the filter is not cleaned and replaced in a timely manner, more and more dust will accumulate on the filter material, leading to increased resistance and insufficient air supply.

The direct evaporative cooler (section) cools the air by directly contacting the treated air with the water filling layer; The core of an indirect evaporative cooler is an air air heat exchanger, which has two disconnected channels for heat exchange through the walls between the two channels, allowing the primary air to be cooled. If the outdoor air is not effectively filtered, dust will accumulate on the surface of the evaporative cooler, and after mixing with the spray water, scale will form, seriously affecting the heat and humidity exchange performance of the unit.

A frequency conversion equipment room in a power plant in Xinjiang uses a direct evaporative cooling ventilation air conditioning unit to eliminate indoor waste heat. The unit is placed on the roof of the equipment room, and the actual unit is shown in Figure 2. The evaporative cooling team of Xi'an Engineering University conducted tests on the unit in August 2012. A large amount of dust accumulated on the surface of the air filter of the unit, and the dust situation on the surface of the filter is shown in the figure. The test air volume of the unit was 35000m/h. After testing, it was found that the supply air volume did not meet the design requirements. After dismantling the air filter, it was tested that the indoor supply air volume met the design requirements.

Therefore, designers need to have a scientific and reasonable understanding of the selection of filters based on the environmental conditions of the project location, the performance of filters, and the cleaning and replacement cycle.

(3) Selection and Introduction of Application Forms

In general, the final level of filter determines the degree of air purification, while the upstream levels of filters only play a protective role. They protect the lower end filter to extend its service life, or protect the air conditioning unit to ensure its normal operation. In the design process, the efficiency of the last level filter should be determined first based on the user's cleanliness requirements, and then the filter with protective effect should be selected. If this level filter also needs protection, a filter should be added at its windward end.

According to the author's research, the filtration section of evaporative cooling ventilation air conditioning units is generally equipped with coarse and medium efficiency two-stage filters. The commonly used forms of coarse efficiency filters include plate type and winding type, while the commonly used form of medium efficiency filters is bag type filters. The filtration section of the direct evaporative cooling ventilation air conditioning unit used in the turbine room and deoxygenation room of a nuclear power plant in Hainan consists of two stages: coarse efficiency and medium efficiency. The coarse efficiency filter is a corrugated plate filter, and the medium efficiency filter is a bag filter.

At present, the coarse effect filter is generally made of medium and fine hole foam plastic, non-woven fabric, and glass fiber as the filter material, and is generally made into plate type, winding type, drawer type and other shapes. Plate filters are widely used as coarse efficiency filters in air conditioning units due to their advantages such as low cost, light weight, and compact structure.

The automatic winding air filter converts the pressure difference before and after the filter into a sensing electrical signal and automatically replaces the filter material. The filter material is often made of chemical fibers, which can be washed, regenerated, and reused. As a medium efficiency filter, bag type air filters are generally installed after the coarse efficiency filter to be protected. The filter bags can be made of different filtering materials and of different lengths, and can be replaced and cleaned.

In the frequency conversion machine room of Xiegou Coal Mine in Xishan, Shanxi Province, traditional precision air conditioning was used to handle the temperature and humidity in the machine room. However, due to the strong wind and sand in the Shanxi region and the high amount of coal ash and sand dust in the air of the mining area, the filter screen of the precision air conditioning was severely blocked shortly after use, directly affecting the cooling effect of the equipment.

In order to solve this problem, Xi'an Engineering University and Xi'an Jingshang Artificial Environment Co., Ltd. jointly developed an indirect direct two-stage evaporative cooling air conditioning unit with a coiled air filter (as shown in Figure 3). In order to reduce the maintenance, operation and management workload of the air filter, an automatic coiled air filter (as shown in Figure 4) was designed, and a direct current fresh air air conditioning supply method was adopted, Thoroughly improved the working environment of the variable frequency machine room and achieved good results.

In fact, there are similar problems in the frequency conversion rooms of thermal power plants in northwest and other regions of China. This technology can also be promoted and applied, which can not only improve the working environment of the frequency conversion room in power plants, but also reduce the maintenance workload of the air filtration section of the air conditioning unit. It can also protect the heat and humidity exchange functional sections such as the direct (indirect) evaporative cooler, and improve the heat exchange efficiency of the evaporative cooling air conditioning unit.

In areas with strong sandstorms such as the northwest and middle east of China, inertia filters can be installed in front of evaporative cooling ventilation air conditioning units to remove large particles of sand and dust. The utility model patent "Inertia Filter" (patent number: 201220214971.2) is a filtering device equipped with an external shell, an inlet duct, and an outlet duct.

The internal structure of the intake duct is a spiral blade structure, and the outlet duct is an inverted cone. The tail end of the intake duct is connected to the front end of the outlet duct with a gap, and a dust exhaust port is also set at the bottom of the inertia filter. Generally, the dust outlet is connected to the dust exhaust duct, and sand and dust are discharged by the dust exhaust fan. The direct evaporative cooling ventilation air conditioning unit used in the steam turbine room and desalination water workshop of a nuclear power plant in Pakistan was jointly developed by Xi'an Engineering University and Nantong Kunlun Air Conditioning Co., Ltd. The unit uses inertial filters and bag filters, and the physical and inertial filters of the unit are shown in Figures 5 and 6.

The evaporative cooling ventilation air conditioning unit used in the second phase of the Chashma Nuclear Power Plant in Pakistan adopts integrated inertial filters and plate filters, and the integrated inertial filters have a similar structure to the aforementioned. Recently, there is also an inertial filter for windy and sandy areas on the market, which consists of an external shell, blades, and a dust outlet, with a certain angle between adjacent blades.

When selecting air filters during the unit design process, while meeting resistance requirements, it is necessary to comprehensively consider factors such as the environmental conditions of the project location and initial investment based on the above application forms.

(4) Operation and maintenance

During the operation of the unit, it is necessary to regularly check whether the resistance of the filter exceeds the initial resistance, which requires the installation of a resistance detection device to determine when to clean or replace the filter. The replacement cycle of commonly used air filters for evaporative cooling ventilation air conditioning units in power plants is shown in Table 3 for reference only. If necessary, the cleaning or replacement cycle should be determined based on the actual situation.

conclusion

Evaporative cooling technology has initial investment, maintenance costs, and operating costs that are 1/2, 1/3, and 1/4 of mechanical refrigeration, in line with the concept of efficient energy conservation and environmental protection in power plants. In order to alleviate the impact of dust accumulation on the heat transfer performance of the evaporative cooler, it is necessary to install air filters in the evaporative cooling ventilation air conditioning unit. Through the introduction of the filtration mechanism, filtration materials, and classification of air filters in the article, designers need to scientifically and reasonably choose air filters:

(1) When setting up air filters in a graded manner, the specifications of the two filters should not differ too much, and the first filter should protect the second filter;

(2) According to research findings, the filtration section of evaporative cooling ventilation air conditioning units in power plants is generally equipped with coarse and medium efficiency filters;

(3) In power plants with large sandstorms, it is advisable to install inertia filters and wound filters in front of the air inlet for evaporative cooling ventilation air conditioning units;

(4) During the operation of the unit, it is necessary to conduct resistance checks and periodic cleaning and replacement of the air filter.

Disclaimer: The author of this article, Zhang Xin, Huang Xiang, and Dong Xiaojie, reproduced the content for learning and communication purposes only. The opinions expressed do not represent a position, and the copyright belongs to the original author. If there is any infringement, please contact us for deletion