Dust Control Handbook for Industrial Minerals Mining and Processing

Effective FiltrationFigure 9.4. Respirable dust concentrations inside enclosed cabfor three days of testing with cab door closed and open.In order to achieve effective filtering and provide clean air to an operator's enclosure, the systemneeds to be composed of both an effective outside intake air and a recirculation filtrationcomponent.Effective Outside Air Filtration and Enclosure PressurizationA minimum quantity of at least 25 cfm of intake/outside air per person is necessary to dilute CO 2quantities exhaled by each worker [ASABE 2003]. A typical range for most enclosed cabs onsurface equipment is 40 to 70 cfm. The outside air delivered into an enclosure is the onlycomponent that creates pressurization, which is critical for an effective system. The amount ofintake air delivered to create this pressurization must be carefully controlled and optimized.Once the optimal point is determined, increasing the air quantity degrades the system byincreasing particle penetration and decreasing filter efficiency by allowing more contaminants toflow through the filter media.With outside air, the optimal design is to provide enough air to achieve an acceptable level ofpressurization, but further increasing airflow only decreases the air quality in the enclosure. Themathematical model derived from the laboratory study to determine the PF can be used toemphasize this point. The following is a hypothetical situation: a recirculation system thatprovides 300 cfm (Q R ), a recirculation filter efficiency of 0.7 (70 percent) (η R ), an intake filterefficiency of 0.95 (95 percent) (η I ), and assuming no air leakage (l=0)—increasing the intake air232 Operator Booths, Control Rooms, and Enclosed Cabs