The filter is an indispensable device on the conveying medium pipeline, usually installed at the inlet end of the pressure reducing valve, pressure relief valve, fixed water level valve and other equipment.Filters are widely used in metallurgy, chemical, petroleum, paper, medicine, food, mining, electric power, etc.The function of the filter is to filter out various impurities such as solid particles, moisture and oil contained in the air.The main performance indicators of the air filter are flow characteristics, water separation efficiency and filtration accuracy. The flow characteristics indicate the relationship between the pressure difference between the inlet and outlet and the standard flow rate through the element at the rated flow rate. It is a measure of the resistance of the filter. Under the condition of satisfying the filtration precision, it is desirable that the resistance is as small as possible. The water separation efficiency is an index to measure the water separation capacity of the filter. The filtration precision indicates that the size of the smallest particle of dust can be filtered out. The accuracy of the filtration is directly related to the size of the vent hole of the filter element. The larger the aperture, the lower the filtering accuracy, but the lower the resistance loss.