Production of all steel fume hood

1Introduction

All steel fume hoodIt is an important component of the local exhaust system. Its efficiency has a significant impact on the technical and economic performance of the entire exhaust system. A well-designed local exhaust hood can achieve good control effect with a small air volume and ensure that the concentration of harmful substances in the work area meets the national health standards. According to the degree of sealing, exhaust hoods can be classified into closed exhaust hoods, semi closed exhaust hoods, and open exhaust hoods. A semi enclosed exhaust hood refers to an exhaust hood that, due to operational needs, often cannot completely or partially enclose equipment that generates pollutants, and must have a large working hole. A fume hood is a type of semi enclosed exhaust hood. There are generally three forms of fume hoods, which differ in the position of the exhaust outlet and are suitable for pollutants with different densities. When the density of pollutants is low, use exhaust ventilation; When the density is high, use lower exhaust; When the density is uncertain, it is possible to choose to exhaust both up and down simultaneously, and the upper exhaust outlet can be adjusted. The door of the fume hood can be adjusted up and down. Under operating conditions, the smaller the opening of the door, the better, which has a better effect under the same exhaust volume. The ability of fume hoods to control pollutants mainly depends on the wind speed at the opening, and it is generally recommended that the wind speed at the opening be 0.3~1.5m/s.

2Low temperature fume hood

When the temperature of harmful substances is lower than the surrounding air temperature, it is called a low-temperature fume hood. As shown in Figure 1 (a), the movement of airflow when the cold process fume hood adopts upper exhaust. The suction velocity at the upper part of the working hole is 150% of the average flow velocity, while the lower part is only 60% of the average flow velocity, and harmful gases will escape from the lower part. To improve this situation, a baffle should be added inside the cabinet and the exhaust outlet should be located at the lower part of the fume hood, as shown in Figure 1 (b).

Figure 1 Ventilation Cabinet

3High temperature fume hood

The fume hood used in processes with high heat generation is called high temperatureAll steel fume hoodThe hot air flow inside the fume hood during the hot process needs to rise upwards. If it is sucked in at the lower part like during the cold process, harmful gases will escape from the upper part, as shown in Figure 1 (c). Therefore, the fume hood for the thermal process must exhaust air from the upper part.

For processes with unstable heat generation, exhaust vents can be installed both above and below, as shown in Figure 1 (d). With the change of heat generation inside the cabinet, the ratio of upper and lower exhaust air volume can be adjusted to make the velocity distribution of the working hole more uniform. The exhaust volume of the fume hood is calculated according to equation (1):

In the formula, L₁The amount of harmful gases generated inside the fume hood, m/s；

ν - Control wind speed on the working hole, m/s；

A - Area of operating port or gap, ㎡

Figure 3 Gas supply fume hood

4Application of local exhaust

Local exhaust is a local ventilation method that directly removes pollutants from the source. When pollutants are concentrated in a certain area, local exhaust is an effective ventilation method to control the environmental hazards caused by pollutants. If comprehensive ventilation is adopted in this situation, it will instead cause pollutants to spread indoors; When the amount of pollutants generated is large, the required dilution ventilation volume is too large and even difficult to achieve in practice. There are many situations where pollutants occur at designated locations in industrial plants, such as plating tanks, dropping or operating points of bulk conveyor belts, welding workstations, chemical analysis stations, painting, grinding machines, etc. There are also some cases of fixed-point pollution generation in civil buildings, such as stoves in kitchens, hotpot in restaurants, chemical testing benches in schools, etc. It can be seen that the application of local exhaust is very extensive.