Dilution versus Entrainment in High Air Volume Longwall Environments

Abstract

Abstract The age-old question, “How much is enough?” needs to be answered with regard to the practice of adding higher and higher air velocities in an effort to lower respirable coal dust concentrations in longwall installations. What is the level at which a further increase in air velocities no longer dilutes, but entrains and suspends respirable dust? At some point entrainment becomes the dominant factor and respirable dust levels will become constant, or even increase with further increases in air velocity. When the problem of entrainment exceeds the benefit of dilution, any further increase in air velocities is a waste of available air that could be used elsewhere in the mine where it is needed. This article presents a case study, approximately 2 years in length, done by Jim Walter Resources' engineering staff, which evaluates the effect of wide variations in air velocity (770 to 1500 ft/min) on respirable dust concentrations on seven longwalls in the Blue Creek coal seam in Central Alabama. Production levels are incorporated into the analysis to establish that no reasonable mathematical relationship exists between further increases in already high air velocities and reductions in respirable dust levels.