Leak detection methods and standards for HEPA filters

27/07/2020

    As the end filter of the air filter system, the HEPA filter is the last layer of protection for air purification. When the high-efficiency filter leaves the factory, it will be leak-tested to ensure the quality of the product.

    Next, introduce several leak detection methods for high-efficiency filters

    1. SodiumFlame

    The sodium flame method originated in the United Kingdom in 1969 and was implemented in some European countries in the 1970s and 1990s. It is one of the current national standard methods in my country. Its test dust source is polydisperse phase sodium chloride salt fog, and the "quantity" is the brightness of the hydrogen flame when the salt fogs burn. The saltwater splashes under the agitation of compressed air, and after drying, it forms tiny salt crystal particles and enters the air duct. Samples are taken before and after the filter. The salt mist samples make the color of the hydrogen flame blue and increase the brightness. The air is judged by the brightness of the flame. The main detection instrument is a flame photometer. The relevant standards of the sodium flame method are British BS3928-1969, European Eurovent 4/4, and China GB6165-85. This method can only detect sponges with low sensitivity and cannot detect ultra-high efficiency filters.

    2. OilMist

    The oil mist law originated in Germany and was also implemented in China and the former Soviet Union. The test dust source is oil mist, and the "quantity" is the turbidity of the air containing oil mist. The turbidity difference of the gas samples before and after the filter is used to judge the filter efficiency of the oil mist particles. Germany stipulates the use of paraffin oil, and the particle size of the oil mist is 0.3~0.5 microns. The Chinese standard stipulates that the average weight diameter of the sponge oil mist is 0.28~0.34 microns, and there is no specific regulation on the type of oil. Related standards are China GB6165-85, Germany DIN24184-1990. When the oil mist method detects the filter, it is easy to cause damage to the filter, and the value cannot be read directly, which wastes time. At present, the German oil mist law has become history. Germany first promulgated the national standard with the counting methods as the detection method in 1993. The European standard EN-1822 was formulated on the basis of the German standard. At present, only a few military units in our country use this method.

    3. DOP

    The DOP method originated in the United States in 1956 and has been adopted by many countries, as well as in Chinese national standards. This method was once the most commonly used method for testing high-efficiency filter sponges in the world. Its test dust source is 0.3-micron monodisperse dioctyl phthalate (DOP) droplets, also known as "hot DOP", and "quantity" is the degree of turbidity of the air containing DOP. The DOP liquid is heated into steam, and the steam condenses into tiny droplets under specific conditions. After removing the excessively large and small droplets, the particles of about 0.3 microns are left, which enter the air duct and measure the turbidity of the gas sample before and after the filter. , And judge the filter efficiency for 0.3-micron dust. The measuring instrument is mainly a light scattering photometer. The relevant standards are MIL-STD-282-1956.

    4. ParticleCounter

    This method is commonly used in Europe, and the test method for ultra-high efficiency air filters in the United States is also relatively similar. It is currently the mainstream sponge test method in the world. The dust source is polydisperse phase droplets or solid dust with certain particle size. Sometimes, filter manufacturers have to use atmospheric dust or other specific dust in accordance with the special requirements of users. If a condensation nucleus counter is used in the test, a monodisperse test dust source with a known particle size must be used. 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 of dust at each point and can also compare the local efficiency of each point.