Introduction
Hoods are specifically designed to meet the characteristics of the type of ore or product being processed. An effective hood is a critical part to any system because if the hood does not capture the dust, the rest of the exhaust ventilation system becomes meaningless. A properly designed hood will create an effective flow rate and airflow pattern to capture the dust and carry it into the ventilation system. The effectiveness of the hood is determined by its ability to induce an inward airflow pattern for the dust-laden air in the work environment.
Hoods and Blowing versus Exhausting Ventilation
The limitations of exhausting systems need to be considered when determining the effectiveness of a hood at capturing dust. This issue is most evident when comparing the characteristics of blowing versus exhausting air from a duct.
With a blowing system, the air delivered from the fan maintains its directional effect for a substantial distance once exiting the duct. With a blowing system, at a distance of 30 diameters (dimension of the exiting duct), the air velocity is reduced to approximately 10 percent of the exiting velocity (Figure).
This blowing air tends to maintain its conical shape and actually entrains additional air, a process commonly referred to as induction. When one compares a blowing system to an exhaust system, the air velocity reaches this approximate 10-percent level at only one duct diameter from the exhaust inlet.
The airflow characteristics for an exhaust system are substantially different. The air exhausted, or pulled into the duct, is captured from all directions around the duct opening and thus forms a nearly spherical shape, as opposed to the conical shape of the blowing system.
Another major difference is the air velocity. The air velocity for an exhaust system is approximately 10 percent of the intake velocity at the duct opening at only 1 diameter away, as compared to 30 diameters away at the 10-percent level for the blowing system. These ventilation principles underscore how critical it is for an effective hood design to be very close to the dust generation source.
Conclusion
Effective hood design is crucial for controlling dust in industrial environments. Differences between blowing and exhausting systems demonstrate the importance of selecting the right ventilation strategy. Exhaust systems, with their spherical airflow and quick velocity reduction, capture dust best when placed near the source. Proper hood placement enhances dust capture, supporting air quality and safety.
Reference
Dust Control Handbook for Industrial Minerals Mining and Processing
