Comparison of Ventilation Mode of Welding Plants
The welding shop, according to the work, can be divided into two parts: feed preparation and assembly welding. There are few harmful substances generated in the feed preparation process. Assembly welding generally include welding, fitter assembly, temporary spot welding, 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 depending on the product structure. The main chemical hazards generated during the welding process are welding fumes, followed by harmful gases. Welding fumes are the main chemical hazards in the welding shop.
The chemical composition of common structural steel electrode fume are shown in Table 1:
The maximum allowable concentration of welding fumes in the welding shop is 6 mg/m3, and the average allowable concentration in 8 hours is 4 mg/m3. Long-term inhalation of high concentrations of welding fumes can cause welders to pneumoconiosis. Therefore, welding fume in the welding shop is an urgent problem to be solved.
The control of welding fumes should begin with the implementation of cleaning processes. Use the fumeless or fume-free welding process; develop and utilize low-fume and low-toxic welding materials; improve the mechanization and automation of welding engineering. Secondly, reasonable ventilation design is an effective measure to solve welding fumes in welding shops.
Maximum concentration of welding fumes
A large amount of harmful fumes and gases are generated during welding, and they spread evenly throughout the plant along the height of the plant. However, the welding fumes in the welding shop have a maximum concentration band, which can be called the “appropriate height”. The “appropriate height” are shown in Table 2.
Emission Standards
The welding fume exhaust system must meet local pollutant emission requirements if it is discharged outdoors. If the system exhaust air circulates indoors, purification measures must be taken and the content of hazardous substances in the circulation shall not exceed 30% of the maximum allowable concentration in the welding shop.
Comparison of ventilation methods
1, local exhaust
The welding shop performs local exhaust as much as possible at the location where the hazardous materials are generated. The local exhaust system is mainly composed of exhaust hood, air duct, purification device and fan. The key to its design is to determine the form of the exhaust hood according to the process requirements and the site conditions, to understand the distribution of the flow field and concentration field near the hood and to formulate corresponding wind speed to control. See Table 3 for the controlled wind speeds of different types of exhaust hoods.
Local ventilation can be controlled from the source of welding fumes, prevent the spread of welding fumes, better protect the health of employees, and at the same time achieve requirements by smaller air volume, to reduce investment and operating costs, which is the most cost-effective way of exhausting air. However, due to the limited space or the insufficient solder joints in many workshops, it is impossible to install equipment or arrange air ducts indoors, and only other methods can be used.
2、Full ventilation
When the welding position of welding shop is not fixed and the local exhaust hood cannot be used, full ventilation should be set. Full ventilation includes natural and mechanical ventilation, and mechanical ventilation includes both dilution and displacement ventilation.
Conventional mechanical ventilation generally uses dilution ventilation. After diluting indoor harmful substances with outdoor clean air and discharging it to the outside, the effect of dilution ventilation depends on sufficient ventilation and reasonable airflow organization. Studies have shown that the actual ventilation efficiency of dilution ventilation is 50% to 70%, and the maximum is not more than 80%.
The displacement ventilation adopts the clean air through the lower part of the room, the wind speed of the air supply is low, the disturbance is small, and a laminar flow clean area is formed in the lower working area of the room. The pollutants are gradually raised under the promotion of the laminar airflow, the suction of the upper exhaust vent and the thermal effect of the heating equipment. The fresh air completely replaces the dirty air below the sub-level, ensuring the air temperature, humidity and cleanliness requirements at the personnel working height. The ventilation efficiency of the displacement ventilation system is greater than 1, usually between 100% and 200%. Therefore, the displacement ventilation has less air volume than the dilution ventilation, and has a good energy saving effect. (See the figure below: Comparison of dilution ventilation and displacement ventilation)
The full ventilation air volume is determined by the consumption of the welding rod, but in the absence of this data, the exhaust air volume can be calculated at 3500 m3/h per welding point. In terms of solving the dust hazard in the welding shop, if the ventilation measures can meet the maximum allowable concentration of welding fumes, the concentration of various harmful gases generated by the welding operation can also be reduced below the maximum allowable concentration. The exhaust air volume of domestic welding electrodes is shown in Table 4.
When determining the ventilation plan of the welding shop, it must be carried out flexibly according to the specific conditions. Several common comprehensive ventilation plans are as follows:
2.1 Natural Ventilation
Natural ventilation does not require power consumption. It is an economical ventilation method. Natural ventilation is generally used for welding outdoor or in open space. For example, some industrial plant roof has natural ventilators, roof skylights, which are natural ventilation applications. However, since natural ventilation is easily affected by outdoor meteorological conditions, especially in the case of unstable wind power, it is not suitable for plants that generate pollutants such as dust and harmful gases.
2.2 Axial fan and natural ventilator (skylight) exhaust on the side wall
For the low-single-span workshop with small area, the axial flow fan can be installed on the upper part of each welding station on the outer wall to achieve good ventilation effect; the roof natural ventilator (skylight) can enhance effect of ventilation. This method is suitable for places where the height of the plant is low and the welding station is relatively fixed.
2.3 Set the induction fan, natural ventilator (skylight or roof fan) exhaust
By installing the induction fan to ventilate at a certain injection angle, drawing the indoor welding flue gas flows upward, and finally exhausting outside through a natural ventilator (sunroof or roof fan), this method is suitable for places with high-height plant and sites without interference from crane.
2.4 Roof Ventilator
The exhaust fan can be arranged on the roof of the welding shop, and the welding fumes in the workshop are discharged to the outside to achieve the purpose of reducing the concentration of the fumes in the workshop. In this way, the airflow organization is unreasonable, and the ventilation efficiency of the work area is low, which is not recommended.
2.5 Push-Pull Ventilation
Push-pull ventilation is a ventilation airflow composed of a single blow-off airflow and a single indraft airflow. Therefore, it is a ventilation method that can effectively control the diffusion of pollution sources. This ventilation method is to set a blow-off nozzle on the two-span middle column or single-span side, to exhaust fresh air introduced from the outside, and an axial fan is arranged on the side wall opposite to the air supply to exhaust the air. It is divided into two categories: small-range push-pull ventilation system and large-scale push-pull ventilation system. Since the characteristics of welding fumes are naturally spread upwards within a certain height, in order to ensure the ventilation effect, the ventilation required by this method is very large and has lower economic efficiency.
2.6 Overall Ventilation Dust Control Method
The ventilation volume of the welding shop is generally large. For the air-conditioning or heating room, the welding fumes in the workshop are purified by the purifier and then recycled in the workshop by setting the air purifying device, so that the concentration of the fumes in the workshop can be reduced. It also solves the purpose of energy loss in the workshop. MEGAUNITY’s overall ventilation and dust control system for high-space workshops adopts displacement ventilation, doubles ventilation efficiency, and uses secondary air return technology to further reduce operating costs, making the investment in the overall dust removal control system of the workshop acceptable to most factories.
Summary
Through the comparison of the above various ventilation methods, the reasonable design of the airflow organization of the workshop has a great influence on the dust removal effect and investment cost of the welding fume. The existing “Specifications for Design of Heating, Ventilation and Air Conditioning of Industrial Buildings” GB50019-2015 have only provided a guiding description for heating ventilation and air conditioning of general industrial buildings. Due to the lack of research on the treatment characteristics of a large number of toxic and harmful pollutants such as welding dust, some welding enterprises have ventilation facilities, but they are virtually useless. The indoor environment design of the welding plant lacks a normative basis, resulting in many indoor health and sanitation failures, endangering workers’ health, and nonstandard high energy consumption unqualified project will appear.
In view of the current lack of research on the application of ventilation methods in the welding plant, the Ministry of Housing and Urban-Rural Development has introduced a new industry standard: “Specifications for Heating, Ventilation and Air Conditioning Design of Welding Plant” JGJ353-2017. The development of new specifications will provide basis for welding companies to build, rebuild and enhance the design of heating, ventilation and air conditioning of welding plant.
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