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A preliminary study on the structure and performance of ventilation filters Fu Weiyuan, Li Chenglin, Xue Tian, Huang Jiangyi (Hubei Nanxing Chemical General Plant, Zhijiang, Hubei 443200) Abstract: Toxin filtration devices must be installed in the ventilation design of hazardous chemicals production and destruction plants. Currently, there are few industrial devices using poison filtration ventilation systems in China. Design methods are also rarely introduced. In the design process of the poison filtration ventilation device, the composition and redundancy of the filter are important contents and parameters in the design. Reasonable design can not only save investment, but also ensure the ability of the facility to withstand the impact of accidents, so that the device can operate safely and smoothly. During operation, the timing of filter replacement is crucial. The article introduces the structure of the ventilation filter, the calculation method of HEPA filter and activated carbon filter capacity, and finally studies the filter performance test method. Keywords: poison filtration; structure; resistance CLC classification number: For factories that are highly toxic and prone to chemical leakage, poison filtration devices must be installed in the ventilation system, such as activated carbon filters and HEPA filters, and discharged after adsorption and purification to meet standards. At present, there are very few industrial installations using poison filtration ventilation systems in the country. Design methods are also rarely introduced. In the design process of the poison filtration ventilation device, the composition and redundancy of the filter are important contents and parameters in the design. Reasonable design can not only save investment, but also ensure the ability of the facility to withstand the impact of accidents, so that the device can operate safely and smoothly. During operation, the timing of filter replacement is crucial. 1 Overview Activated carbon is mainly used for air and water purification and * * Chemical protection, suitable for adsorption of various chemical agents. HEPA filters are high-efficiency filters with various structures, efficiencies and performances developed in modern times to meet the development needs of increasingly higher requirements for air cleaning technology, which are suitable for the adsorption of particulates. In the early 1990s, the United States used activated carbon and HEPA filters in the chemical agent ventilation system at Johnston Atoll. In order to increase the safety of the original filter, the operation and maintenance contractor made additional investment in the air filter supplier; after more than half a year of debugging, on May 16, 1990, the operation and maintenance contractor project director and the Army * The officers signed a memorandum stating that all air filters had met or exceeded Army acceptance standards. In 2005, experts from China and Japan decided to use a filtration system combining activated carbon and HEPA filters in the ventilation of the Haerbaling destruction facility of Japanese heritage chemical weapons. After analyzing various expected accidents, the redundancy of the system was studied, the redundancy calculation method of activated carbon filters and HEPA filters was determined, and the structural design of the system was completed. In addition, China has also used relatively simple integrated activated carbon filters in the production of certain hazardous chemicals. Although the design and application of poison filtration ventilation systems in China are still relatively small, the production technology of activated carbon filters and HEPA filters is still relatively mature, and there are many filter manufacturers, such as Xinhua Chemical Plant in Shanxi. In the future, as people's awareness of environmental protection increases, ventilation devices that use activated carbon and HEPA high-efficiency filters to eliminate harmful substances will be more and more widely used. 2 Structure of the poison filtration system The filtration system reduces the concentration of poisonous agents in the exhaust air to a level that is harmless to workers, the environment and nearby residents. The filtration system can be composed of multiple filter units, which can be connected in series or in parallel as needed. Each filter unit consists of the following parts. The following is the structural diagram of a typical ventilation and poison filtration device, as shown in Figure 1. (1) Pre-filter, which removes coarse particles such as dust; (2) High-efficiency air filter, which removes particles larger than microns in size; (3) Activated carbon filter, which removes chemical contaminants through an adsorption process. 3. Pest-removing capability of the virus filtration system. The Pest-removing capability of the virus filtration system is represented by redundancy. The redundancy of the virus filtration system is to ensure the safety of facility operation. The actual configured filters exceed the theoretically required filters. 3.1 Capacity of HEPA filter Calculate the capacity of HEPA filter, which is reflected by the ratio of the capture capacity of HEPA filter. For example, a filtration system has N groups of units, each unit HEPA contains 16 original elements, and the amount of gas captured by each element is 1000g, then the maximum capture capacity of the system is 16 N 1000g. This value divided by the amount of chemical leakage is the ratio of the capture capacity of the HEPA filter. The larger the value, the higher the redundancy. 3.2 Calculation of the capacity of activated carbon filters Because the structures of activated carbon filters and HEPA filters are different, the form of examining the capacity can also be different. The capacity of the activated carbon filter can be expressed by the ratio of the length of the activated carbon layer used (50mm) to the length of the necessary activated carbon layer; the larger the ratio, the stronger the capacity. The length of the activated carbon layer is calculated as follows: If an accident occurs, the chemical agent will turn into leakage gas and be adsorbed by the activated carbon. Its concentration distribution is shown in Figure 2: The necessary activated carbon layer length defined in the article refers to the length of the activated carbon layer when the total amount of adsorbed chemicals is assumed to be adsorbed in the saturated adsorption zone. This length is required to be at least less than the thickness of the activated carbon layer. The calculation method for the necessary activated carbon layer length is as follows: Necessary activated carbon layer length (m) = GT/Gs/A where, G: filter inlet chemical dose (g/h) is calculated from the chemical gas leakage amount and the gas air volume at the activated carbon filter inlet. Gs: Saturated adsorption capacity (g/m3) indicates the maximum amount of chemical agent that activated carbon can adsorb per unit volume, which depends on the concentration of chemical agent gas at the entrance of the activated carbon filter. The saturated adsorption capacity varies depending on the type of activated carbon and gas conditions. The results of the "Adsorption of Chemical Agents with Activated Carbon Experiment" are calculated from the relationship between the adsorption capacity and concentration. T: Adsorption time (h) The discharge time is calculated based on the assumption that the chemical agent fills a certain room instantly and then is discharged from the room by ventilation gas piston circulation. For example: Assume that the room volume is 1,680m3 and the ventilation volume is 166,900m3/h, so the time to discharge all chemicals is 1,680/166,900 60 minutes = 0.6 minutes. A: Cross-sectional area of the activated carbon filter (m2) 4. If the performance of the filter can be continuously monitored online in the middle of the filter, the safety of facility operation can be fully guaranteed; because this investment is too large, the filter resistance is generally tested (mainly for HEPA filters) or sampling analysis (for activated carbon filters). Determining the on-site filter resistance is of great significance. Generally, the final resistance of the filter is 2 to 3 times the initial resistance, and the filter must be replaced. On-site detection of filter resistance is an important basis for determining whether to replace the HEPA filter. The efficiency and initial resistance of ventilation filters are measured one by one in the test detection system. However, the actual conditions on site cannot match the test conditions of the production plant. In order to obtain accurate test data in the test system, the system pipes are specially designed in relatively stable air flow pipe sections according to standards, and the data obtained are relatively accurate. Due to the requirements of the site, the data collection points are often not set in the stable air flow stage, but are set in the turbulent flow pipe section. This results in the data collected on site being not very accurate. Moreover, the requirements of the measurement itself vary with different testing methods, such as the stability and uniformity of air volume and wind speed, the phase difference parameters of the upper and lower test pipelines, and the correction of temperature and humidity, which are often different, all of which will cause different test results. In this way, operation and maintenance personnel cannot correctly judge the use status of the filter based on the pressure difference of the filter. This causes the resistance of the pre-filter and high-efficiency filter to rise rapidly and cause them to blow out, causing pollution to production and the surrounding environment. During the operation and debugging of the filter, in view of the fact that the on-site filter pressure difference indication is inaccurate and the exact time of filter replacement cannot be correctly determined, long-term testing and exploration are required. Based on the actual experience on site, after installing the filter, read the reading P1 of the differential pressure gauge as the initial resistance, thereby determining the filter replacement resistance P2: P2 = 2P1. Starting from the installation and operation of the filter, the final resistance of the filter replacement is first determined according to the above method. When the required value is reached, maintenance personnel check whether the filter needs to be replaced. After it is confirmed that it needs to be replaced, the future replacement value of the filter is determined to be this fixed value. If it does not need to be replaced, monitoring and inspection of the filter resistance must be strengthened in future operations. Once it needs to be replaced, record the current final resistance of the filter and use this value as the reference value for replacement of the filter. 5 Conclusion There is no precedent for the combined use of activated carbon and HEPA filters in poison filtration ventilation devices in China. The construction of the Haerbaling destruction facility for Japanese chemical weapons will undoubtedly provide valuable experience for the design and operation of China's poison filtration and ventilation devices. References: Guo Jianfen, Xu Xiaohao, Zhang Kai, et al. HEPA filter. Cleanliness and Air-conditioning Technology, 2001(1):16-19. Shi Min, Nie Xueli, et al. Experience in ventilation filter operation and management. China Science and Technology Information, 2006(13):165. Yuan Delong. Review of HEPA/ULPA filter performance testing. Cleanliness and Air Conditioning Technology, 2003(4):56-58.