Dust Control Handbook for Industrial Minerals Mining and Processing

Orifice plates should also not be installed near hood inlet openings. The higher velocities nearthe orifice tend to capture unwanted material. Hood capture velocities below 500 fpm should bemaintained. On fine-grind materials, a target capture velocity of 200 fpm is desirable.Minimizing the Use of Flexible HosesFlexible hoses should not, as a rule, be used as part of the ductwork in low-velocity systems. Afailure could compromise the dust control ability of the branch lines in the general area. Flexiblehoses also increase the frictional losses in the system. If flexible hoses are used, they should beplaced between the orifice plate and the inlet hood where their failure would have the leastimpact.Modified Low-Velocity (MLV) SystemThe objective of the modified low-velocity (MLV) system is to combine the advantages of thehigh- and low-velocity systems, while avoiding the disadvantages of both. The key to thesuccess of the MLV resides at the pickup hood. Here the first duct run must be vertical for atleast 6 feet, and preferably 8 to 10 feet. The velocity in this first vertical run is extremely low,even compared to the standard low-velocity system (i.e. 1,000 to 1,200 fpm). As in othersystems, dust is contained, but only the smaller micron particles are transported. The 6- to 10-foot vertical rise is necessary to reduce turbulence, to achieve a smooth laminar flow, and tooptimize elutriation (separation of lighter and heavier particles) at the first bend.Because there are minimal particles in the airstream after this first vertical rise, the need tomaintain high velocities in subsequent horizontal runs is not necessary. Similarly, there is noneed for an overall sawtooth design or mid-run discharge points. Velocity in the first horizontalrun following a 1,200-fpm vertical duct should be about 2,000 fpm.Another important design consideration with MLV systems is that as additional lateral lines areconnected to the main trunk line, the exit velocity of the trunk line after the initial lateralconnection should be increased by about 100 fpm (see Figure 1.13). Even in long runs withmany lateral lines, it is not likely that the line velocity upon reaching the collector will exceed2,500 to 3,000 fpm. Given the low pressure drops in such a system, it is not necessary toconstruct expensive elbows, mitered bends, or laterals. Instead, box elbows and fittings can beused.26 Fundamentals of Dust Collection Systems