Dust Control Handbook for Industrial Minerals Mining and Processing

The airflow characteristic for an exhaust system is substantially different. The air exhausted, orpulled into the duct, is captured from all directions around the duct opening and thus forms anearly spherical shape, as opposed to the conical shape of the blowing system. Another majordifference is the air velocity. The air velocity for an exhaust system is approximately 10 percentof the intake velocity at the duct opening at only 1 diameter away, as compared to 30 diametersaway at the 10 percent level for the blowing system. These ventilation principles underscorehow critical it is for an effective hood design to be very close to the dust generation source.Figure 1.3. Comparison of ventilation characteristics for blowing versus exhausting system[adapted from Hartman et al. 1997].Hood TypesHoods have a vast range of different configurations, but usually fall into three differentcategories: enclosing, capturing, and receiving.Enclosing hoods are those in which the source is either partially or totally enclosed to provide therequired airflow to capture the dust and prevent it from contaminating the work environment.The most effective way to capture dust generated is a hood that encompasses the entire dustgeneration process. Openings into the enclosure (hood) are minimized with doors and accesspoints into the contaminated work process. This situation is normally used when worker accessis not necessary and openings are only necessary for the product to enter and exit a piece ofmachinery or a work process. These types of enclosing hoods can have numerous applicationsthroughout the mining and minerals processing sequence, and are most often used in crushing,grinding, milling, and screening applications.When access is necessary into the dust generation process or area, it is then common to use sometype of booth or tunnel—a type of partial enclosure application. In these partial enclosuresystems, the key is to provide sufficient intake airflow to eliminate, or at least minimize, anyescape of dust from the enclosed area. This is best accomplished by enclosing the dustgeneration area or zone as much as possible. One common method to do this is with clear plastic12 Fundamentals of Dust Collection Systems