Dust Control Handbook for Industrial Minerals Mining and Processing

Figure 1.10. Sawtooth design of a low-velocity system with a dropoutduct back into the process at the base of each leg.The particle-carrying ability of a low-velocity system is confined to particles smaller than 10 μm.Because of the absence of larger particles, as well as the lower velocities involved, abrasion islow even at points of air direction change. This allows for the use of short radius or miteredelbows without fear of extreme wear.From a design standpoint, dust control systems are engineered for expected pressure losseswithin the system. These losses are due to air friction and pressure losses across the dustcollection unit. By reducing the air velocity, frictional losses in the ductwork and fittings arereduced, lowering overall power requirements.Because of the behavior of low-velocity air, moderate orifice size changes can be made withoutcompletely upsetting the system. Even opening a branch completely will not drastically changethe airflow in other branches because the pressure drop change is minimal. Should one branch ofthe network fail or change airflows dramatically, the system tends to stay in balance and otherbranches do not lose effectiveness. Therefore, the airflow can vary substantially on two identicalpieces of equipment while maintaining excellent dust containment. Thus, overall losses throughthe dust collection system are lower.The disadvantages of low-velocity systems are the higher initial cost and more complex design.In these systems, the ability to pass a certain volume of air (cfm) through the duct at apredetermined velocity (fpm) dictates the diameter of the duct, requiring larger diameterductwork. Also, because ductwork cannot be run horizontally, a sawtooth design is used, whichadds expense and complicates installation.22 Fundamentals of Dust Collection Systems