"Axial Fan" by fan working pressure (total pressure) size classification ① fan: the standard state, the fan rated pressure range p < 98Pa (10 mmH2O). This fan is not the case, also known as free fan, commonly used in the ventilation of buildings. ② Fan: design condition, fans rated pressure range 98Pa < p < 14710Pa (1500 mmH2O). Usually refers ventilator fan, the fan is also discussed in this chapter. Fan is the most widely used fan. Air pollution control, ventilation, air conditioning and other works are mostly the use of such fans. ③ Blower: working pressure range is 14710Pa < p < 196120Pa. High pressure, is commonly used in waste-water treatment aeration equipment. ④ Compressor: Working pressure range p > 196120Pa, or a gas turbine compression ratio greater than 3.5, such as the commonly used air compressor.