Page 159 - Advanced Mine Ventilation

Respirable Dust Control 139

Figure 10.1 Collection efficiency as a function of k.

The relationship between E and k is nearly linear. Some of the theoretical conclu-
sions that can be derived from Eq. (10.3) are as follows:
1. Efficiency of collection goes down rapidly with decrease in particle size.
2. Efficiency goes up with higher waterdrop velocity.
3. Efficiency improves with smaller water particle size.
The combination of the above items (2) and (3) suggests water discharged at higher
pressures will have higher collection efficiencies. Table 10.1 shows some of their data
[1] that confirm the above conclusions.
Other studies [2,3] have shown that decreasing the water droplet size excessively is
not always beneficial. If a water droplet is too small, it may evaporate quickly and cap-
ture no dust. Cheng [2] calculated the optimum size of water droplets for various dust
particle sizes and concluded that the maximum collection efficiencies for 1 and 2 mm
particles is 100 and 200 mm, respectively. Woffinden [3] obtained slightly lower
values for optimum droplet sizes. His results are shown in Fig. 10.2. His results indi-
cate that the optimum droplet size for 1 mm dust particle is between 20 and 40 mm,
depending on the water droplet velocity.
Table 10.1 provides a basis for designing an optimum spray system for all mining
machinery in underground coal mines.