Introduction to fume hood

The cabinet type exhaust hood is commonly known as the fume hood, which is similar to the closed hood. Small parts spray cabinet and chemical laboratory fume hood are typical structures of cabinet type exhaust hood. Large room type fume hood, one side is completely open, and the operator works in the cabinet, which is mainly used for large piece 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.

Introduction to fume 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 hazardous 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 generating pollutants due to operational needs. Fume hood is a type of semi enclosed exhaust hood. There are generally three types of fume hoods. The difference lies in the different positions of the exhaust outlets, which are suitable for pollutants with different densities. When the pollutant density is small, use the upper exhaust air; When the density is high, use the lower exhaust air; When the density is uncertain, the upper and lower exhaust ports can be selected, and the upper exhaust port 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.

Low temperature fume hood

When the temperature of harmful substances is lower than the ambient air temperature, it is called low-temperature fume hood. Figure 1 (a) shows the movement of air flow when the upper exhaust is adopted for the ventilator in the cold process. The suction velocity at the upper part of the working hole is 150% of the average flow rate, while that at the lower part is only 60% of the average flow rate. Harmful gas 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).

High temperature fume hood

The fume hood used in the process with large heat production is called high temperature fume hood. The hot air flow in the fume hood in the thermal process shall rise upward. If you inhale at the lower part as in the cold process, the harmful gas will escape from the upper part, as shown in Figure 1 (c). 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 Figure 1 (d). With the change of calorific value in the cabinet, the proportion of upper and lower exhaust air volume can be adjusted to make the velocity distribution of the working hole more uniform. The exhaust air volume of the fume hood is calculated according to formula (1):

Where L1 - 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 the air conditioning, purification or heating room, in order to reduce the exhaust air 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 air 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 air flow, but also save indoor exhaust air volume and greatly reduce the consumption of indoor heat and cooling capacity.

Application of local exhaust

Local exhaust is a kind of local ventilation that directly removes pollutants from the source of pollutants. When pollutants are concentrated in a certain place, local exhaust is the most effective ventilation method to control the environmental hazards caused by pollutants. If comprehensive ventilation is adopted in this situation, the pollutants will diffuse indoors; When the amount of pollutants is large, the amount of 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 trough, blanking point or operation point of bulk material belt conveyor, welding workbench, chemical analysis platform, painting, 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.