Dust Control Handbook for Industrial Minerals Mining and Processing

Checklist for Hood EffectivenessThe following is a checklist of effective practices or considerations regarding the use of hoods inexhaust ventilation systems.The most effective hood design is one that encompasses the entire dust generationprocess. This virtually eliminates dust escaping and contaminating mine/plant air and theexposure of workers.Openings/doors/access points into the hood should be minimized as much as isreasonably possible. When access is necessary, a partial enclosure application, normallyreferred to as a booth or tunnel, is then recommended. When access points are not able tobe closed, sealing these areas with clear plastic stripping is a common and effectivetechnique.When neither total nor partial hood enclosures are possible, capture hoods should beused. The hoods need to be located as close to the dust source as feasibly possible.Remember that the distance component is a squared relationship that affects the requiredhood air volume.Hood capture velocities must be able to overcome any exterior air current between thedust source and the hood.The use of a push-pull ventilation system should be considered when the distancebetween the dust source and the hood becomes too great, or when there are significantexterior air currents in the area.It is critical that makeup air drawn past a worker before entering an exhaust hood beessentially dust-free. If the air is drawn from a contaminated area, the potential exists toincrease a worker's respirable dust exposure. Dust-laden air from a dust source shouldnever be pulled through a worker's breathing zone as it is being drawn into an exhausthood.Flanges on hoods should be used because they significantly improve the airflow from thefront of the hood area, which should be directed towards the dust source. In cases wherebell shaped hoods and ducts can be used, they provide the optimum design.All hoods should be designed to meet the criteria established by the ACGIH handbook,Industrial Ventilation: A Manual of Recommended Practice for Design [ACGIH 2010].DUCTWORK AND AIR VELOCITIESThere are three basic types of systems used to transport dust to the collector: high-velocity, lowvelocity,and modified low-velocity. Most familiar to industry is the high-velocity system,where air-carrying velocities are in the range of 3,000–4,500 fpm. Fundamentally, this is a dustcollection system, while a low-velocity system can best be described as a dust containmentsystem, with duct velocities always less than 1,800 fpm. Low-velocity systems are designed sothat they will not transport nonrespirable size dust particles (generally particles over 10 μm). Itis important to keep in mind that low-velocity transport does not imply low airflow. Capturevelocities (the air velocity at the hood opening required to capture the contaminant) and hood airvolumes are the same in all designs, be they high-velocity, low-velocity, or modified lowvelocitysystems. Figure 1.9 depicts the basic difference between high- and low-velocitysystems.Fundamentals of Dust Collection Systems 19