Local Ventilation

When it is impossible to totally or partially encapsulate the source of air pollution, local exhaust ventilation (LEV) systems are used that are combined with a dust removal system or an air cleaning device [ACGIH 2019]. The task of local ventilation is to capture air pollution directly at the source of the emission and to prevent it from spreading in the room. The type of LEV used depends on both the location of the emission source and the direction and velocity of the air pollution spread. A disadvantage associated with the use of LEV is the need to place them directly in the area of dust emission sources. This is due to the small area of effective operation of the elements that capture air pollution (e.g. hoods, suction nozzles).

Placing the hood in the area where there is a threat of dust emission allows the removal of pollutants with a density lower than the density of air.

To eliminate or reduce the risk of chemical and particulate pollutant emissions, actions should be taken by:

• • placing the hood directly above the emission source,
• • choosing the shape and dimensions of the hood so that it can cover the entire pollution zone,
• • avoiding the conduct of interference processes in the area of extracting pollutants from the emission source to the hood,
• • the breathing zone of a machine operator should not be located on the pollutants’ removal line from the source to the hood.

The above can be achieved by checking the following technical parameters:

• • distance of the inlet plane of the hood from the surface of the source of pollution,
• • geometrical dimensions of the hood,
• • air flow velocity distribution in the area between the hood and the surface of the source of pollution,
• • value of capture velocity,
• • air velocity distribution in the inlet plane of the hood,
• • volume of air flow extracted from the hood.

Suction nozzles can be placed close to the source of air pollution. This allows capturing most air pollutants in the workplace.

Suction nozzles, as well as housing, should be designed during the manufacture of machinery and planning of technological processes. It is necessary to provide verified data in the operating instructions concerning the value of the extracted air flow rate in relation to flow resistance.

When using suction nozzles, it is necessary to take into account the movability of the emission source. This is related to the effectiveness of the LEV, which mainly depends on the location relative to the source of air pollution. Therefore, two cases should be considered during the design of the suction nozzles:

• • immovable source with constant extracted direction - immovable LEV is used,
• • movable source - then the LEV is selected from three types:
• • immovable system with a high volume flow of exhaust air,
• • system moving along with the source without the ability to recognize the direction of particle ejection,
• • movable system positioning itself according to the direction of particle ejection.

A serious disadvantage associated with the use of suction nozzles is the occurrence of the phenomenon of rapid decrease in air velocity with an increase in distance from the plane of the inlet opening of the LEV.

Hence - local ventilation (suction nozzles) does not replace general ventilation of the room, but complements it. The key to high efficiency in capturing air pollution is a proper interaction between general and local ventilation.

The work to date is primarily related to increasing the effectiveness of local ventilation solutions and their adaptation to the threat of dust and chemical substances at workplaces. Currently, a number of studies focus on supporting the operation of local ventilation through the use of covers, tables with variable air flow direction (upper, lower and side extraction), general push-pull ventilation or positive displacement ventilation [Wang et al. 2012; Albuquerque et al. 2015].

As part of the research conducted by Jankowski [2019], it has been demonstrated that the side air supply support allows increasing the capture efficiency and reducing the time of air pollution flow from the working zone to the upper zone and the air filtration zone. A supportive flange diffuser was used at the suction nozzle. The air supply was directed directly into the area of effect of the LEV. Such a supply in the form of a flow supplied from the annular gap around the suction opening acts as a dynamic flange. Thus, it not only increases the range of the suction nozzle’s effect as a capturing hood, but a directional effect also appears (Figure 6.3).

The use of the principles and control of technical parameters given below may eliminate or reduce the risk of air pollution emissions in the workplaces by: ^[1]

FIGURE 6.3 Smoke flow visualization in the measuring plane of supply air device supporting the LEV.

• • determining the air velocity distribution in the inlet plane of the suction nozzle,
• • determining air velocity distribution in the area between the suction nozzle and the surface of the source of pollution.

• [1] utilizing the flow of pollutants by placing the LEV by means of particleemissions, • covering a maximum calcium emission zone of air pollution, • matching the shape and dimensions of the suction nozzle to the area of theemission source, • forcing sufficient particle capture velocity, adapted to the velocity of particles formed during material processing, • determining the required value of the volume air flow extracted from theLEV, • reducing the distance between the suction nozzle and the source of airpollution, • lowering the required value of the extracted air volume by using covers andflanges,