What is a Fume Hood?

Cabinet exhaust hoods are commonly called fume hoods and are similar to closed enclosures. Small parts spray paint cabinet and chemical laboratory fume hood are typical structures of cabinet exhaust hood. Large room fume hood, one side is completely open, the operator works in the cabinet, mainly used for large pieces of paint, powder bagging, etc. The working opening of the fume hood has a great influence on the airflow distribution in the cabinet, and the airflow distribution directly affects the working effect of the cabinet exhaust hood. ^[1]

The local exhaust hood is an important part of the local exhaust system. Its efficiency has a very important impact on the technical and economic performance of the entire exhaust system. The well-designed local exhaust hood can obtain the best control effect with small air volume, and ensure that the concentration of harmful substances in the work area meets the requirements of national health standards. The exhaust hood is classified according to the degree of airtightness, including a closed exhaust hood, a semi-closed exhaust hood, and an open exhaust hood. A semi-closed exhaust hood refers to an exhaust hood with a large working hole that often cannot be completely or partially closed due to operational requirements. Fume hoods are a type of this type of semi-closed exhaust hood. There are generally three types of fume hoods. The difference lies in the location of the exhaust vents, which is suitable for pollutants with different densities. When the density of the pollutants is small, the upper exhaust air is used; when the density is high, the lower exhaust air is used; when the density is uncertain, the upper and lower exhaust air can be selected at the same time, and the upper exhaust vent is adjustable. The door of the fume hood can be adjusted up and down. Under the conditions of operation permission, the smaller the door opening is, the better, so it has a better effect under the same exhaust volume. The ability of a 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. ^[1]

When the temperature of the harmful object is lower than the temperature of the surrounding air, it is called a low temperature fume hood. Figure 1 (a) shows the movement of the air flow when the upper exhaust air is used in the fume hood in the cold process. The suction speed at 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. Harmful gases will escape from the lower part. In order to improve this situation, a baffle should be added in the cabinet, and the exhaust vent should be set in the lower part of the fume cabinet, as shown in Figure 1 (b). ^[1]

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 hot process should rise upward. If it is sucked in the lower part like the cold process, harmful gas will escape from the upper part, as shown in Figure 1 (c). Therefore, the fume hood of the thermal process must be exhausted in the upper part.

For the process of unstable heat generation, air outlets can be set up and down, as shown in Figure 1 (d). With the change of the heat generation in the cabinet, the proportion of the upper and lower exhaust air volume can be adjusted to make the speed distribution of the working holes more uniform. The exhaust air volume of the fume hood is calculated according to formula (1): ^[1]

Local exhaust is a method of local ventilation that removes pollutants directly from the source. When the pollutants are concentrated in one place, local exhaust is the most effective way to control the environmental hazards of pollutants. If full ventilation is used in such occasions, the pollutants will diffuse indoors; when the amount of pollutants is large, the required dilution ventilation will be too large, and it is even difficult to achieve in practice. Pollutant spotting occurs in many industrial plants, such as electroplating tanks, blanking or operating points conveyed by bulk material belts, welding tables, chemical analysis tables, paint spraying, and grinders. There are also some cases of fixed-point pollution in civil buildings, such as stoves in kitchens, hot pots in restaurants, chemical test benches in schools, and so on. It can be seen that local exhaust ventilation is widely used. ^[1]