Local exhaust ventilation is used when the places of harmful emissions in the room are localized and it is possible to prevent their spread throughout the room. Local exhaust ventilation in industrial premises ensures the capture and removal of harmful emissions: gases, smoke, dust and heat partially released from the equipment. To remove hazards, local suctions are used (shelters in the form of cabinets, umbrellas, side suctions, curtains, shelters in the form of casings near machine tools, etc.). The main requirements that they must satisfy:
- The place of formation of harmful emissions should, if possible, be completely covered.
- The design of the local suction must be such that the suction does not interfere with normal operation and did not reduce labor productivity.
- Harmful emissions must be removed from the place of their formation in the direction of their natural movement (hot gases and vapors must be removed upwards, cold heavy gases and dust - downwards).
- The designs of local exhausts are conditionally divided into three groups.
- Semi-open exhausts (hoods, umbrellas). Air volumes are determined by calculation.
- Open type (onboard suction). Removal of harmful emissions is achieved only with large volumes of sucked air.
Exhaust ventilation device
When installing local exhaust ventilation to capture dust emissions, the air removed from the workshop, before being released into the atmosphere, must be preliminarily cleaned of dust.
The most complex exhaust systems are those that provide for a very high degree of air purification from dust with the installation of two or even three dust collectors (filters) in series.
Local exhaust systems, as a rule, they are very effective, as they allow the removal of harmful substances directly from the place of their formation or release, preventing them from spreading indoors. Due to the significant concentration of harmful substances (vapours, gases, dust), it is usually possible to achieve a good sanitary and hygienic effect with a small amount of air removed.
However, local systems cannot solve all the problems facing ventilation. Not all harmful secretions can be localized by these systems. For example, when harmful emissions are dispersed over a large area or volume; air supply to individual areas of the room cannot provide the necessary conditions for the air environment, the same is true if the work is done on the entire area of the room or its nature is associated with movement, etc.