Classification and design principles of local exhaust hoods

According to different principles and methods, local exhaust hoods are mainly divided into three basic types: closed exhaust hoods, receiving exhaust hoods and external exhaust hoods. Most exhaust hoods can be classified according to this. Exhaust hoods vary widely in their ability to control pollutants.

1. Classification and characteristics of various types of exhaust hoods

(1) Airtight cover

All sources of harmful substances are sealed in the hood, and air is sucked from outside the hood to maintain negative pressure inside the hood. It can effectively control harmful substances with a small exhaust air volume, and the airtight cover used for the dust removal system is also called a dust-proof airtight cover.

(2) Cabinet-type exhaust hood (exhaust cabinet)

The structure is similar to the closed cover, except that due to the requirements of the process or operation, one side of the cover needs to be fully opened. When used in painting operations, bagging powder materials, etc., the operator needs to directly enter the cabinet to work, and a large fume hood is used.

(3) External suction hood

Due to the limitation of process conditions, an external suction hood can be used when the production equipment cannot be sealed. Using the action of the exhaust air flow, a certain inhalation speed is created at the emission point of the harmful substances, so that the harmful substances are sucked into the hood. Such exhaust hoods are collectively referred to as external suction hoods.

(4) Slot side exhaust hood

Slot side exhaust hood is a special form of external suction hood (side suction hood), which is specially used for various industrial troughs.

(5) Blow-suction exhaust hood

Using the characteristics of dense jet energy, slow velocity decay, and fast inspiratory airflow velocity decay, combining the two is a method to effectively control harmful substances. It has the characteristics of small air volume, good control effect, strong anti-interference ability, and does not affect the process operation.

(6) Receiving exhaust hood

The production process and the equipment itself will generate or induce certain airflow movements, such as convective airflow above the high temperature heat source. The exhaust hood is set in front of the polluted airflow, and the harmful substances will enter the hood directly with the airflow. This type of exhaust hood is collectively referred to as the receiving hood.

(7) Air supplement cover

Use the supplementary air device to directly send the outdoor air to the exhaust hood at the exhaust outlet, such as the supplementary air fume cupboard.

2. When designing a local exhaust hood, the following principles should be followed:

(1) The exhaust hood should be able to capture the harmful substances released by the source of harmful substances, so as not to disperse them into the working environment, so that the concentration of harmful substances in the working area can meet the national health standards, and the harmful substances can be captured with less energy consumption. .

(2) For the sources of harmful substances that can be sealed, sealing measures should be adopted first, and they should be sealed as much as possible, and the best control effect should be achieved with the smallest exhaust air volume. When determining the position, structure and wind speed of the air inlet of the airtight hood, the negative pressure in the hood should be uniform to prevent harmful substances from escaping. For pollution sources that emit dust, excessive dust extraction should be avoided.

(3) When the source of harmful substances cannot be completely sealed, an external exhaust hood can be set up. The hood of the outer hood should surround or be close to the source of harmful substances as much as possible, so that the source of harmful substances is limited to a small local space. The suction range should also be minimized to facilitate the capture and control of harmful substances.

(4) When the exhaust hood cannot be set near the source of harmful substances or the distance between the hood and the source of harmful substances is large, a suction hood can be set up. Objects should be used with caution.

(5) The mouth of the exhaust hood should follow the movement direction of the harmful airflow, so that the harmful airflow can directly enter the hood, but the exhaust line is not allowed to pass through the worker's breathing belt.

(6) The external hood, receiving hood and blowing and suction hood should be arranged in places to avoid disturbing the airflow, and the setting of the exhaust hood should be convenient for workers to operate and maintain equipment.

(7) The exhaust hood must be sturdy and durable, and should be simple in structure, low in cost, and easy to install and maintain.

(8) In the place where the exhaust hood is used for ventilation, it is necessary to avoid or reduce the influence of interfering air flow (such as hall ventilation and supply air flow, etc.) on the inhalation air flow as much as possible.