Dust Control Handbook for Industrial Minerals Mining and Processing

This table highlights a number of critical factors. First, it shows how the effectiveness of eachsystem deteriorates as the leakage into the enclosure increases. Leakage occurs from either alack of cab integrity or an insufficient intake component. The second critical factor is the needfor a recirculation system. When one evaluates an enclosure with no leakage, the PF is increasedfrom 20 to 104 by the addition of a 300 cfm recirculation system with a 70 percent efficientfilter. This highlights the critical need of the recirculation component. A high efficiencyrecirculation filter is not critical as long as a filter within reasonable filter efficiency range (70–95 percent) is used and maintained. This is demonstrated by viewing the first column in Table9.3 where l = 0 (leakage equals zero) and the PF only increases by 24 points when going from a70 to a 90 percent efficiency recirculation filter. Since the air is constantly being recirculated,any respirable dust inside the cab will be removed within a few passes through the recirculationsystem.Table 9.3. Calculated PF derived from mathematical model(Q I ) = 50 cfm(Q R ) = 300 cfm(η I ) = 0.95(η R ) = 0(Q I ) = 50 cfm(Q R ) = 300 cfm(η I ) = 0.95(η R ) = 0.70(Q I ) = 50 cfm(Q R ) = 300 cfm(η I ) = 0.95(η R ) = 0.90l 0 l 0. 05 l 0. 10 l 0. 1520.0 10.3 6.9 5.2104.0 53.3 35.9 27.0128.0 65.6 44.1 33.2OTHER METHODS TO LOWER RESPIRABLE DUST IN ENCLOSURESSmall In-Unit HEPA FiltersIn some operator booths and control rooms, stand-alone air purifier units containing a HEPAfilter have been installed. These stand-alone systems are small and portable units that areavailable at a fraction of the cost of permanent systems. However, these systems can only beeffective if they are sized to handle the volumetric capacity of the booth or control room, and ifthe filters are replaced when necessary [Logson 1998/1999]. An obvious and significantshortcoming with these types of systems is that they do not provide any pressurization to keepdust from leaking into the booth or room. A few studies have shown that in many instances, theydo not provide sufficient protection and companies would be better served to invest their moneyand time into improving their existing roof-mounted unit [Cecala and Zimmer 2004a,b].Therefore, stand-alone air purifier units containing a HEPA filter are not recommended in mostcases and should be replaced with more comprehensive and properly sized recirculation systems.238 Operator Booths, Control Rooms, and Enclosed Cabs