Measurement of inhalable dust in wind conditions pertaining to mines. Final report on CEC Contract 7256-21/029/08

The project set out first to obtain a quantitative definition of inhalable dust, to develop an improved framework on which to base the design of new samplers for inhalable dust according to that definition, and to investigate the effects on sampling of the physical properties (other than aerodynamic) of the dust particles. The second – and main – objective was to investigate how the performances of inhalable dust samplers are influenced by the turbulent nature of the airflow found in coal mines where it is anticipated that such samplers might be used.A new quantitative definition of inhalable dust was derived from the results of studies (both at the Institute of Occupational Medicine in Edinburgh and at Bergbau-Forschung GmbH in Essen) in wind tunnels using model human heads (both with and without the torso). Thus it was found that, for a range of wind speeds and breathing parameters corresponding to miners and their working environments, inhalability may be described in terms of a single curve expressed as a function of particle aerodynamic diameter (d ), falling initially from unity for the smallest ae particle sizes and levelling off to about 0.5 for particles with d exceeding about 40�m.A study of the aerodynamic factors which control the basic performance characteristics of dust samplers has provided a theoretical basis for interpreting the performances of existing inhalable dust samplers (like the ORB) and developing new and improved ones (like the rotating inhalable dust entry). In fact, experimental tests in the wind tunnel suggested that the performance of the new sampler matches the quantitative definition of inhalable dust more closely than the ORB sampler. A prototype practical sampler based on this entry is now in the process of being developed.Studies of dust sampler performance, initially in the wind tunnel and then later in coal mines, showed that sampling errors due to particle blowoff from external surfaces can be substantial and therefore can be the source of considerable uncertainty (and hence variability) in the measurement of airborne dust by aspiration methods. The ORB inhalable dust sampler was found to be prone to such problems (along with the MRE Type 113A sampler, especially when used for collecting ‘total’ dust). The new rotating inhalable dust entry was not.In order to investigate the complex turbulent structure of air flows in coal mines, both on the face and in the roadways, a 1/10-scale model of a longwall face was built. The turbulence properties of the mine airflow (intensity and length scale in three dimensions) were obtained by hot-wire anemometry, and referred to full-scale. Using a system of square-mesh, biplanar grids in a large wind tunnel, an appropriate range of representative turbulence conditions was simulated at full-scale for the purpose of examining the performances in turbulent air flow of dust samplers.The performances of a number of dust samplers were investigated for a range of turbulence conditions representing those where dust sampling may take place on the coal face or in the return roadway. For sharp-edged isokinetic probes over the ranges of turbulence conditions examined, there was found to be no significant effect due to turbulence, consistent with theory based on turbulent diffusion. On the other hand, in a basic study using idealised disc-shaped blunt samplers facing the wind, there was found to be a marked effect of turbulence which increased with particle, aerodynamic size in a manner also consistent with theory. However for the two inhalable dust samplers, the ORB and the rotating inhalable dust sampler, any such effect for particles in the size range of practical interest was found to be insignificant. For the present, therefore, the results of this study do not indicate that any changes in sampling practice (on account of turbulence effects) should be recommended at this stage. “”