Local exhaust is a kind of local ventilation mode that directly removes pollutants from the pollution source. When pollutants are concentrated in a certain place, local exhaust is the most effective way to control the environmental hazards of pollutants. If comprehensive ventilation is adopted in this situation, pollutants will diffuse indoors; When the amount of pollutants is large, the required dilution ventilation is too large, even difficult to achieve in practice.
There are many situations where pollutants occur at fixed points in industrial plants, such as Electroplating bath Blanking point or running point conveyed by bulk belt Welding workbench Chemical analysis workbench, painting, grinding machine, etc. There are also some cases of pollutants generated at fixed points in civil buildings, such as those in kitchens 
Local exhaust Figure 1 is a schematic diagram of a local mechanical exhaust system. The system is made of exhaust hood, fan, air purification equipment, air duct and exhaust outlet. Exhaust hood - equipment used to catch pollutants, is an essential part of the local ventilation system. Air duct - the channel for air transmission. According to the nature of pollutants, it can be steel plate, glass plate PVC board , concrete slab, brick board, etc. Air purification equipment - used to prevent air pollution. When pollutants in the exhaust exceed the national allowable emission standards, purification must be carried out; If the concentration does not exceed the standard, the equipment may not be set. Air outlet - the outlet for air exhaust, which can be divided into hood and shutter. The division of local exhaust system should follow the following principles:
1. When the pollutants have the same or similar properties, the working hours are the same, and the pollutant emission points are not far away, they can be combined into a system.
2. When different pollutants can be mixed to produce combustible, explosive, toxic, corrosive, or easy to condense and accumulate dust, they should not be combined into one system, but should each become an independent system, and the system should also have flame retardant, explosion-proof, anti-corrosion, anti condensation and condensate drainage measures.
