Causes Of Low Efficiency Of Combined Ventilation System In Coal Mines In Resolving The Problem Of Air Leaks (Inflows) Between Levels And SurfaceValeriy Popov, Yuriy Filatov, Hee Lee and Anatoliy GolikE3S Web Conf., 21 (2017) 01033DOI: https://doi.org/10.1051/e3sconf/20172101033