Dust Control Handbook for Industrial Minerals Mining and Processing

A common misconception when designing a low-velocity system is that an exhaust hood is notrequired due to the low-velocity ducting design. To the contrary, a properly designed exhausthood on a low-velocity system is still a requirement for a good dust containment system. Aproperly designed hood will help maintain lower system pressure losses by minimizing the shockloss that occurs from the airflow entering the system. Hood design is essentially the same forboth high- and low-velocity systems. However, all exhaust hoods should be located away fromthe point of material impact to lower the likelihood of material being drawn into the hood. Theprinciple of low-velocity dust containment is to exhaust air from a well-enclosed area. This willimpart a negative inward pressure, trapping fugitive dust within the enclosure. The shape of theexhaust hood is also important for two reasons:1. A well-designed hood has a lower resistance to air movement.2. A well-designed hood prevents material from being vacuumed into the dust containmentsystem. Even with low-velocity ducting, coarse material can still be drawn into thesystem. It is therefore important to have a hood with a face velocity of 200 to 300 fpm orless in order to avoid entraining saleable product.Finally, the design and layout for a low-velocity system requires more engineering. Because ofthe constraint that runs cannot be made horizontally, it is sometimes difficult to physically findroom for the required sawtooth design of the ducting. However, a properly designed andbalanced low-velocity system provides a virtually nonplugging and low-maintenance dustcontrol system. More specific engineering principles for low-velocity systems are detailedbelow.Airflow Target of 1,800 fpmBecause of the nature of a low-velocity system, it is not necessary to maintain a specific airvelocity within the ductwork. However, the desired target of 1,800 fpm should move most of therespirable dust particles (minus 10 μm) through the system to the filter equipment whilecontaining it within the confines of the individual pieces of dust-producing equipment/hood. Thebasic goal of low-velocity dust control is to minimize the amount of contaminants (respirabledust) that escape the system, thus becoming a hazard or nuisance. The 1,800 fpm targetmaintains a slightly negative pressure within the piece of equipment, while providing adequateairflow to discourage dust particles from escaping the unit. This ensures that any airflow at theequipment is moving inwards rather than outwards.Avoidance of Horizontal DuctworkAs a result of the system's low-velocity airflow, it is imperative that ductwork is not runhorizontally. This is because the heavier dust particles drawn into the airstream may drop outand attach themselves to the sides of these ducts. The net result is an eventual narrowing of theduct opening, leading to an increase in velocity and ductwork erosion. Therefore, ductworkshould be designed for a minimum upflow angle of 45 degrees and a minimum downflow angleof 30 degrees (Figure 1.11). This allows any particles drawn into the airstream to slide back totheir source. This design also requires that a dropout point be located at the low point of all ductruns to allow these particles to fall out, as shown in Figure 1.10.Fundamentals of Dust Collection Systems 23