Publish Time: 2022-07-15 Origin: Site
Cabinet type exhaust hood is commonly known as fume hood, which is similar to closed hood. Small parts spray cabinet and chemical laboratory fume hood are typical structures of cabinet type exhaust hood. Large room type fume hood, with one side fully open, and operators working in the cabinet, is mainly used for large-scale painting, powder bagging, etc. The working port of the fume hood has a great influence on the air distribution in the fume hood, and the air distribution directly affects the working effect of the cabinet exhaust hood.
Local exhaust hood is an important part of local exhaust system. Its efficiency has a very important impact on the technical and economic performance of the whole exhaust system. The well-designed local exhaust hood can obtain the best control effect with a small air volume, and ensure that the concentration of harmful substances in the working area meets the provisions of national health standards. Exhaust hoods are divided into closed exhaust hoods, semi closed exhaust hoods and open exhaust hoods according to the degree of tightness. Semi closed exhaust hood refers to an exhaust hood with a large working hole, which is often unable to completely or partially close the equipment that produces pollutants due to the needs of operation. Fume hood is a type of semi closed exhaust hood. There are generally three types of fume hoods. The difference lies in the different positions of exhaust outlets, which are suitable for pollutants with different densities. When the density of pollutants is small, use upper exhaust air; When the density is high, use the lower exhaust air; When the density is uncertain, you can choose to exhaust up and down at the same time, and the upper exhaust outlet can be adjusted. The cabinet door of the fume hood can be adjusted up and down. Under the condition of operation permission, the smaller the opening of the cabinet door, the better. In this way, it has a better effect under the same exhaust air volume. The ability of the fume hood to control pollutants mainly depends on the wind speed at the opening. Generally, the wind speed at the opening is recommended to be 0.3~1.5m/s.
When the temperature of harmful substances is lower than the surrounding air temperature, it is called low-temperature fume hood. As shown in the figure, the movement of air flow when the upper exhaust is adopted for the ventilator in the cold process. The suction speed of the upper part of the working hole is 150% of the average flow rate, while the lower part is only 60% of the average flow rate, and harmful gases will escape from the lower part. In order to improve this situation, a baffle shall be added in the cabinet, and the exhaust outlet shall be set at the lower part of the fume hood, as shown in Figure 1 (b).
The fume hood used in the process with large heat production is called high temperature fume hood. During the thermal process, the hot air flow in the fume hood should float upward. If you inhale in the lower part like the cold process, the harmful gas will escape from the upper part, as shown in the figure. Therefore, the fume hood of the thermal process must exhaust air at the upper part.
For the process with unstable calorific value, exhaust outlets can be set up at both the top and bottom, as shown in the figure. With the change of the calorific value in the cabinet, the proportion of the 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 formula (1):
Where L1 - the amount of harmful gas in the fume hood, m/s;
ν— Control wind speed on the working hole, m/s;
A - area of operation port or gap, m2;
β— Safety factor, β= 1.1~1.2。
For the fume hood used in the chemical laboratory, the control wind speed on the working hole can be determined according to figure 2.
If the fume hood is placed in an air-conditioned, purified or heated room, in order to reduce the exhaust volume of indoor air, an air curtain can be set above the cabinet door. The air sent by the air curtain can be taken from outdoor or adjacent rooms, and the air volume is about 70% - 75% of the exhaust volume. This kind of fume hood is called air supply fume hood, as shown in Figure 3. It can not only prevent the interference of indoor transverse airflow, but also save indoor exhaust air volume and greatly reduce the consumption of indoor heat and cooling capacity.
Local exhaust is a kind of local ventilation that directly removes pollutants from the source of pollutants. When pollutants are concentrated somewhere, local exhaust is the most effective ventilation method to control the environmental hazards of pollutants. If comprehensive ventilation is adopted in this situation, the pollutants will spread indoors instead; When the amount of pollutants is large, the dilution ventilation required is too large, even difficult to achieve in practice. There are many cases of pollutants at fixed points in industrial plants, such as electroplating tanks, blanking points or operation points of bulk belt conveyor, welding workbench, chemical analysis platform, paint spraying, grinding machine, etc. There are also some fixed-point pollutants in civil buildings, such as stoves in kitchens, hot pots in restaurants, chemical test benches in schools, etc. It can be seen that local exhaust is widely used.