Design and field trials of water-mist based venture systems for dust mitigation on longwall faces
Dust generation from longwall chock movement and the Beam Stage Loader/crusher (BSL) is a major source of air contamination on modern longwall faces. If not controlled effectively, much of these respirable dust particles would disperse quickly into the longwall due to high face ventilation velocities, contributing significantly to higher dust levels. A new water mist based venturi system has been developed for the purpose of suppressing respirable dust from longwall chock movements close to the maingate (MG). The unit is powered by compressed air and water using an ultrasonic nozzle embedded in a venturi body. The ultrasonic nozzle is capable of producing ultra fine water mist with droplet sizes ranging from 1 to 100 μm. Laboratory tests indicate that the ultrasonic nozzle (MAL-1300-B), when combined with a 70 mm (diameter) x 143 mm (length) venturi body, was can produce an optimum spray covering a distance over 10 m. Further tests show that a combination of air supply at 6 bar and water at 4 bar produces the optimum water mist thrust with inducted air velocity over 8 m/s. The venturi system was built as a stand alone unit using fire resistant and antistatic materials and can be easily hooked under the chock canopy with a magnetic base. The system can be powered by compressed air and water supplied to the longwall face and adjusted with the spray angle to achieve the droplet size and velocity needed for dust suppression and diversion. Computational Fluid Dynamics (CFD) modelling was undertaken to gain a better understanding of face ventilation and dust flow patterns to optimise the spray orientation of the venturi system for field trial installation. CFD modelling results show that the operating conditions of sprays with the best mitigation performance vary according to the source of dust, a better dust mitigation effect can be achieved when the venturi units on longwall chock are installed at 20o down towards the floor and tilted 45o along the face. Field trials were conducted at two underground longwall mines in QLD and NSW. Three venturi units were installed on Chock No 6 on the longwall with an additional unit trialled at the BSL to mitigate dust from longwall outbye. Dust measurements with real time monitoring Personal Dust Monitor (PDM) and gravimetric samplers indicate dust mitigation efficiency up to 30% has been achieved in both trials.