Tunnels and underground railways are dozens of metres below the surface, tunnels and underground railway stations rely solely on the import and export of natural wind is far from enough, it must be installed tunnel fans to transport and regulate the air or to exclude smoke and dust (smoke from cars passing through the tunnel), for staff and passengers need.

     Tunnel to the exhaust fan to axial flow type, the requirements are based on the concentration of smoke and dust in the tunnel can automatically run, to move the leaves adjustable type is appropriate, can improve the fan operating efficiency, and in the event of a fire can be reversed, so that it quickly exhaust smoke. The underground railway to the exhaust fan is available centrifugal or axial flow type, can also be universal tunnel ventilation fan. As set up in underground tunnels, mostly small. As the exhaust port is on the ground in the city, it is required that the fan noise should be low. In short tunnels, long tunnels or underground railways, jet or axial flow fans can be used. These fans are suspended from the canopy. Due to the high noise level, an anechoic enclosure is required and can be reversed according to the wind direction.

      The tunnel fan (jet fan) is a special type of axial fan, mainly used in the longitudinal ventilation system of tunnels such as roads, railways and subways. Tunnel fans (jet fans) are generally suspended on the top or both sides of the tunnel, do not occupy the traffic area, do not need to build additional ducts, civil construction costs are low; fans are easy to install, operation and maintenance is simple, is a very economic way of ventilation. When the tunnel fan (jet fan) is in operation, part of the air in the tunnel is sucked from the top of the fan, accelerated by the impeller and then shot out at high speed from the other end of the fan. This part of the high-speed airflow with high kinetic energy will transfer energy to the other gases in the tunnel, generating a pressure rise to overcome the tunnel resistance, thus driving the air in the tunnel to flow in the direction of the blower jet airflow. When the flow rate decays to a certain level, the next group of fans continues to work. In this way, fresh air is sucked in from one end of the tunnel and dirty air is discharged from the other.

       Classification of tunnel fan types

     Tunnel fans have a variety of names, the first called tunnel axial fan, later developed into a tunnel jet fan, and inclined flow fan, multi-stage axial fan (jet fan), axial flow compressor, etc., all belong to the axial fan derived from; tunnel fans from the use of up to distinguish: construction-specific tunnel fan, to SDF type on behalf of the axial flow fan, and then subdivided into variable frequency tunnel fan, three-stage SDF type tunnel fan, in the absence of inclined shaft, the configuration of 1.5 m diameter wind bag long-term transport distance up to 4000 m; and after the tunnel is built, in order to maintain good air in the tunnel, and to prevent the need to discharge smoke in case of fire, and the use of two-way jet type mainly The SDS tunnel fan is required to switch direction of rotation within 30 seconds and to maintain a 2 hour operating time in case of fire or other emergency. In order to reduce noise pollution to the environment, mufflers must be installed at the inlet and outlet of the fans, while the noise of the fans can be controlled to within 80dB.

       According to the different forms of ventilation, tunnel fans (tunnel jet fans) are divided into two main types: one-way tunnel fans (tunnel jet fans) and two-way reversible tunnel fans (tunnel jet fans). Unidirectional tunnel jet fans, generally used as one-way ventilation; in special cases the fan reverses and can provide 50% to 80% of the forward thrust. Two-way reversible tunnel jet fans, can be used as two-way ventilation, to facilitate user control of the direction of airflow, the fan forward and reverse rotation of the thrust is basically equal.