Fume and dust purification in welding workshop-comparison of ventilation methods
Welding workshops are divided according to work content , generally can be divided intotwo parts: preparation and assembly welding . There are few harmful substances in the preparation process. The assembly welding part generally includes welding, fitter assembly, temporary spot welding for assembly, testing and inspection, acceptance, cleaning, painting, drying and other processes.
In the welding process, manual welding, automatic welding, semi-automatic welding, carbon dioxide gas shielded welding, argon arc welding, electroslag welding, etc. are used respectively according to the product structure. The main chemical hazards produced during welding are welding fumes, followed by harmful gases. Welding fumes are the main chemical hazards in welding workshops.
Welding shop welding fumes maximum allowable concentration of 6mg / m ³ , . 8 hour average allowable concentration of 4mg / m ³ . Long-term inhalation of high concentrations of welding fumes can cause welders' pneumoconiosis. Therefore, welding fume in the welding workshop is an urgent problem to be solved.
The treatment of welding fumes should begin with the implementation of cleaning processes. Adopt no-smoke or less-smoke welding process; develop and use low-dust and low-toxic welding materials; improve the automation degree of welding engineering mechanization. Secondly, reasonable ventilation design is an effective measure to solve welding fumes in the welding workshop.
Maximum concentration zone of welding fume
A large amount of harmful fumes and gases are generated during welding, and they spread evenly throughout the workshop along the height of the plant. However, there is a maximum concentration zone for welding fumes in the welding workshop. This height can be called the "appropriate height". "Appropriate height"
If the welding fume exhaust system is discharged outdoors, it must meet the local pollutant emission requirements. If the system exhaust air circulates indoors, purification measures must be taken, and the content of harmful substances in the circulating air should not exceed 30% of the maximum allowable concentration in the workshop .
Comparison of ventilation methods
1. Local exhaust
As far as possible, the welding workshop conducts local exhaust in places where harmful substances are generated. The local exhaust system is mainly composed of exhaust hood, air pipe, purification device and fan. The key to its design is to determine the form of the exhaust hood according to the process requirements and site conditions, understand the distribution of the flow field and concentration field near the hood mouth, and formulate the corresponding Control of wind speed. Different types of exhaust hoods control different wind speeds.
Local ventilation can process welding smoke from the source, prevent welding smoke from spreading, better protect the health of employees, and at the same time can meet the exhaust requirements with a smaller amount of air, reduce the investment and operating costs, and is the most cost-effective exhaust method. However, because many workshops are limited in space or the welding points are not fixed, it is not possible to install equipment or arrange air ducts indoors.
2. Comprehensive ventilation
When the welding position of the welding workshop is not fixed and the local exhaust hood cannot be used, full ventilation shall be provided. Comprehensive ventilation includes natural ventilation and mechanical ventilation , and mechanical ventilation includes dilution ventilation and displacement ventilation .
Conventional mechanical ventilation generally uses dilution ventilation. The outdoor harmful air is used to dilute indoor harmful substances and then discharged to the outdoor. The effect of dilution ventilation depends on sufficient ventilation and reasonable airflow organization. The study pointed out that the actual ventilation efficiency of diluted ventilation is 50% to 70% , and the maximum is not more than 80% .
Displacement ventilation uses clean air from the lower part of the room. The air velocity is low and the disturbance is small. A laminar clean area is formed in the working area in the lower part of the room. The air gradually rises, and the fresh air below the sub-level completely replaces the dirty air, ensuring the temperature, humidity and cleanliness of the air within the working height of the personnel. The ventilation efficiency of the replacement ventilation system is greater than 1 , usually between 100% and 200% .