Dust Control Handbook for Industrial Minerals Mining and Processing

quantity from 50 to 70 cfm (Q I ) will cause the PF to decrease from 104 to 80, respectively, justby increasing the intake air quantity by 20 cfm.High efficiency filters are a necessity for the outside intake air. Laboratory experiments showeda 10-fold increase in protection factors when using a 99-percent efficient filter versus a 38-percent efficient filter on respirable sized particles. The balance is to have a sufficient quantityof intake air to create a positive enclosure pressurization to eliminate the wind from blowing dustinto the enclosure, but not so much airflow as to burden the filtration system beyond what isnecessary. In addition, all intake air must be conditioned for temperature, either via heating inthe winter or air-conditioning in the summer, and thus, the greater the intake air quantities, thegreater the cost for providing for this temperature component.Outside Air Inlet LocationAnother key consideration is the location of the outside inlet for the intake air. Locating the cabair inlet near major dust sources causes high dust loading on the air filtration system. This highdust loading burdens the filtration system and reduces its effectiveness by increasing the pressuredrop across the loaded filter and decreasing the quantity of air and cab pressurization. Inaddition, the increased pressure drop across the loaded filter also increases the potential for dustleakage around the filter cartridge. This requires that the filter cartridge be cleaned or changedmore frequently which also increases the filter cost. Finally, air filtration is based on relativedust capture efficiency, so filtering higher outside dust levels creates higher inside cab dustconcentrations.In an effort to minimize these effects, it is recommended to place the enclosure's air inletlocation strategically away from dust sources to reduce dust loading of the filter cartridge[NIOSH 2001a]. This can usually be accomplished by locating the outside air intake inlet athigher levels away from the ground and on the opposite side of the enclosed cab, operator booth,or control room away from dust sources. This location also enables the enclosure to shield someof the dust from the inlet.PreCleanerIt is recommended that the outside air filtration system use a precleaning and final-filteringarrangement. The precleaning should be performed to remove oversized dust particles and toincrease the life cycle and the filtering capacity of the final intake filter. This could beperformed by using some type of centrifugal design to spin out oversized particles or a backflushingcompressed air system, similar to those in a local exhaust ventilation (LEV) system.These back-flushing systems can be set to clean the filter cartridge on a predetermined time basisor when the pressure builds to a predetermined level.Effective Recirculation FiltrationThe use of the recirculation component with effective filtration is critical for an optimal design.Laboratory experiments showed a 10-fold increase in protection factors when using an 85 to94.9 percent efficient filter on respirable sized dusts as compared to no recirculation filter.Laboratory testing also showed that the time for the interior to stabilize after the door was closed(decay time) was reduced by more than 50 percent when using the recirculation filter. TheOperator Booths, Control Rooms, and Enclosed Cabs 233