1、 Shenyang ventilation pipeline Improper selection of local exhaust hood type

The survey results show that most of the users can select the correct exhaust hood type, but there are also some wrong exhaust hood types. For example, a push table saw generates wood dust when sawing wood. Because of the large particle size and proportion of wood dust, the wood dust generated when sawing wood by the push table will fall to the ground after moving a short distance along the sawing streamline. In general, the lower suction hood should be used to control the wood dust generated by the push table saw, but the upper suction hood is used in the design, and the control effect is poor.  

Under the condition of adopting the same exhaust air volume, the lower suction hood was changed. The detection results showed that the wood dust concentration at the operating position was 5.95 times lower than that when the upper suction hood was set.

It can be seen that the control effect can be effectively improved by selecting the correct local exhaust hood for operation.

2. The position of local exhaust hood and the design of hood mouth wind speed are unreasonable

The position of the local exhaust hood and the wind speed at the hood mouth have a great influence on the control effect of the local exhaust hood. It was found in the investigation that the hood opening of the local exhaust hood was far away from the source of harmful substances, and was not aligned with the airflow direction of harmful substances. The hood opening of the local exhaust hood was blocked, the expansion angle of the hood was too small, and the wind speed at the hood opening and the control point was less than the wind speed that should be reached in the design. Next, the upper suction cover and the side suction cover are analyzed.  

On the premise of not affecting the operation, the distance between the exhaust hood and the harmful substances can be shortened by 0.6m and 0.3m respectively; The measured average wind speed at the hood is 0.3m/s, which is lower than 70% of the designed average wind speed at the hood. The concentration of harmful substances at the operation position exceeds 1.6 and 2.0 times of the national standard occupational exposure limit respectively.  

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