Description
Class E21F : SAFETY DEVICES, TRANSPORT, FILLING-UP, RESCUE, VENTILATION, OR DRAINAGE IN OR OF MINES OR TUNNELS
Subclass 1/18: Ventilation of mines or tunnels; Distribution of ventilating currents (ventilating rooms or spaces in general F24F) Gravity-flow ventilation (E21F 1/02-E21F 1/16 take precedence)