The fan is divided into an axial flow fan, a centrifugal fan and a diagonal flow (mixed flow) fan according to the flow direction of the airflow after entering the impeller. The fan can also be divided into single-stage, two-stage or multi-stage pressurized fans according to the form of pressure. For example, 4-72 is a single-stage pressurization, and a high-end fan is a multi-stage pressurized fan. Fans can be divided into: axial flow fans, mixed flow fans, roof fans, air conditioning fans, etc. according to the purpose. The fan can be divided into a negative pressure fan, a low pressure fan, a medium pressure fan and a high pressure fan according to the pressure. According to the outlet pressure (boost), it is divided into: fan (â?¤ 15,000 Pa), blower (1.5 to 350,000 Pa), compressor (â?¥ 350,000 Pa) [7]. Sort by layer There are many classification methods for ventilators, which can be roughly classified by the following types: (1) According to the level of pressure generated, it can be divided into: volumetric type: reciprocating type and rotary type; turbine type: centrifugal type, axial flow type, mixed flow type and cross flow type, and jet type. Fans generally refer to turbine type, ie, centrifugal, axial, mixed flow, cross flow, and the like. [1] Its main features are: Centrifugal fan: higher pressure, but less air. Axial fan: Higher air volume, but lower pressure. Mixed flow fan: The air volume and pressure are between the centrifugal and axial fans. Cross-flow fan: It has a high dynamic pressure and can get a flat air flow. (2) According to the different materials used, it can be divided into: iron shell fan (common fan), fiberglass fan, plastic fan, aluminum fan, stainless steel fan, etc. (3) According to the direction of gas flow, it can be divided into centrifugal type, axial flow type, diagonal flow type (mixed flow type) and cross flow type. (4) The flow direction after entering the impeller according to the airflow can be divided into: axial flow fan, centrifugal fan and diagonal flow (mixed flow) fan. (5) According to the application, it can be divided into: the push-in partial fan and the explosion-proof motor are placed outside the flow channel or in the flow channel, and the explosion-proof motor is placed in the explosion-proof sealed cavity. (6) It can also be divided according to the form of pressure: single-stage, double-stage or multi-stage pressure fan [1].