Physico-chemical studies on dusts: Dust suppression by water sprays

This study is specially concerned with combating, by using water sprays, the incidence of pneumoconiosis in coal mines. Health hazard in coal mines is discussed and existing methods of dust control are surveyed. Formation of water sprays is explained and classification of spray nozzles is given. Experiments were performed on moving dust clouds in an experimental wind-tunnel of 18 in. diameter and 63 ft, length under controlled conditions. Simultaneous dust sampling was made by operating two thermal precipitators located before and after the spray nozzle. Dust clouds were produced using Hattersley's laboratory type dust cloud generator. Coal dust was specially prepared for producing a dust cloud with 95 per cent particles below 5 mu size. Distribution and deposition of dust particles in the wind tunnel were studied. A photoelectric device was built for monitoring dust cloud concentration instantaneously. It was proved that short interval variations in the dust cloud concentration were not significant for the planned work on dust suppression. A system for counting thermal precipitator slides using an Automatic Particle Counting Machine, was developed, and found to give fairly reproducible results. This instrument was extensively used for evaluating most of the thermal precipitator slides taken in the course of dust suppression work. The dust suppression work is concerned with:- (a) Dust suppression with small high pressure spray nozzles. (b) Dust suppression with full size swirl-spray nozzles as used by the National Coal Board. The high pressure spray variables tested included droplet size, throughput, dust concentration, position of the spray nozzle, type of spray - hollow and solid cone, two stage spraying, and effect of turbulence. (These were studied under conditions of dust flow in the wind tunnel. It was found that applied pressure played a more important part in removing dust particles from air than the amount of water sprayed. There is however a certain minimum flow rate required to achieve good dust suppression. In general more dust suppression was obtained when the dust cloud concentration was high. Solid cone sprays removed more dust than hollow cone sprays when used at the same pressure. In two stage spraying, 86.0 per cent dust was removed in the 1/2 - 5 mu size-range, this being the best result achieved. Position of the spray nozzle is of relatively low importance for experiments carried out in the wind tunnel. The sprayed droplets did not appear to be selective in suppressing any particular size-range of dust particles. Using full size spray nozzles the characteristics affecting dust removal were studied in the laboratory, e.g. droplet size, spray uniformity, spray penetration etc. It was found that sprays produced from most of these nozzles were conical in shape with hollow cone and that the distribution of droplets in the spray cone was uniform. Spatial dispersion of these nozzles was more uniform if the target distance was greater than 4 feet. Dust removal up to 70 per cent was achieved by some of these nozzles in the wind tunnel when used at 40 p.s.i. applied pressure. The nozzle efficiency increases with the increase in applied pressure and increase in the relative velocity between the spray droplet and the dust particle. When these nozzles were used for suppressing dust produced during the transportation of coal, reduction in the dust concentration up to 50.0 per cent was achieved with a Hayden-Nilos spray operating at 50 p.s.i, applied pressure. When the sprays were working, the relative humidity of the mine atmosphere was increased by 6-10 per cent