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Summary of commonly used detection methods for high-efficiency filters
As environmental problems become more and more severe, air governance has become a topic of discussion worldwide. The primary issue of air pollution control is the control of dust (PM2.5). At present, the conventional dust control work is to filter the flue gas, which is also a large-scale bag filter used in modern industry. This article focuses on the effect of dust filtration test results and methods. Sodium Flame Method Sodium Flame: Originated in the United Kingdom, common in China, and implemented in some European countries in the 1970s and 1990s. The test dust source is monodisperse phase sodium chloride salt spray. 'Quantity' refers to the brightness of the hydrogen flame when the salt spray is contained. The main instrument is a photometer. The salt water splashes under the agitation of compressed air, and after drying, it forms a tiny salt mist and enters the air duct. Separate samples before and after the filter, and the salt mist gas samples make the color of the hydrogen flame blue and increase the brightness. The brightness of the flame is used to judge the concentration of salt fog in the air, and to determine the filtering efficiency of the filter on the salt fog. The national standard stipulates that the average diameter of the salt spray particles is 0.4mm, but the actual measurement result of the existing domestic devices is 0.5mm. The European measurement result of the actual test salt spray particle diameter is 0.65mm. With the popularization of the scanning method, the sodium flame method is no longer used in Europe. The relevant domestic departments are revising the original national standards, whether to abolish or continue to use the sodium flame method, there is no conclusion between the two opinions. Related standards: British BS3928-1969, European Eurovent 4/4, and China GB6165-85. Photometer scanning: The dust source is generally polydisperse phase droplets, such as DOP smoke produced by Laskin nozzles. Use a photometer to scan the entire surface of the filter for leak detection. This scanning method can quickly and accurately find the leaking point of the filter. Since the dust source is a polydisperse phase, and the photometer cannot determine the particle size of the dust, the 'filtration efficiency' given by this scanning method has no practical meaning. Some manufacturers and users believe that as long as the quality and specifications of the filter paper are strictly controlled, the efficiency of the filter has been determined. Therefore, the quality of the filter can be guaranteed only by scanning for the purpose of leak detection. There is no corresponding standard for the method of photometer scanning leak detection, but this method is very effective for the quality control of the production process, and the test equipment used is relatively simple, so some manufacturers currently use this method. The photometric scanning test bench can easily be changed to a counting scanning bench, and you can just buy a laser particle counter for a little bit of money. DOP law: originated in the United States, internationally accepted, China has never implemented it. The test dust source is 0.3mm monodisperse DOP (plasticizer commonly used in the plastics industry) droplets. 'Amount' is the degree of turbidity of the air containing DOP. The instrument for measuring dust is a photometer. The turbidity difference of the gas sample is used to determine the filtration efficiency of the filter for DOP particles. The DOP liquid is heated to steam, and the steam condenses into tiny droplets under certain conditions. After removing the excessively large and small droplets, particles of about 0.3mm are left, and the mist-like DOP enters the air duct. Measure the turbidity of the gas sample before and after the filter, and judge the filter efficiency for 0.3mm dust. The DOP method has a history of more than 50 years. This method was once the most commonly used method for measuring high-efficiency filters in the world. In the early days, people thought that the filter was the most difficult to filter 0.3mm dust, so it was stipulated to use the 0.3mm dust measurement high efficiency filter. DOP contains a benzene ring, which is suspected to be carcinogenic, so many laboratories switch to substitutes with similar performance but no benzene ring, such as DOS, but the test method is still called the 'DOP method.' By changing the dust generation parameters, DOP droplets of other particle sizes can be obtained. So there is the 0.1mm DOP method for measuring ultra-high efficiency filters in European and American countries 20 years ago, and sometimes the measuring instrument is also changed to a condensation nucleus laser particle counter. Some foreign manufacturers have marked the filtration efficiency of 0.05mm or 0.03mm DOP, which are unconventional in commercial business. The DOP method for measuring high efficiency filters is also called the 'thermal DOP method'. The corresponding 'cold DOP' refers to the polydisperse DOP dust produced by the Laskin nozzle (bubble bubbles in the liquid with compressed air, splashing to produce foggy artificial dust). People often use it when scanning the filter. Cold DOP. Related standards: US military standard MIL-STD-282. Counting scanning method: common in Europe, similar to the United States, and other countries follow closely. The current international mainstream test method for high efficiency filters. The main measuring instrument is a large-flow laser particle counter or a condensation nucleus counter (CNC). Scan and inspect the entire air outlet surface of the filter with a counter. The counter gives the number and particle size of each point of dust. This method can not only measure the average efficiency of the filter, but also compare the local efficiency of each point. European experience shows that for high-efficiency filters, the most easily penetrated dust particle size is between 0.1 and 0.25mm. First determine the most easily penetrated dust particle size under test conditions, and then continuously scan and measure the filter pair. The filtering effect of this particle size dust, Europeans call this method MPPS
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