Page 113 - Advances In Productive, Safe, and Responsible Coal Mining

Mine ventilation networks optimized for safety and productivity 97

should be kept below 28°C (82°F). Mine air often has a high relative humidity, which
makes working conditions worse as the human body cannot evaporate sweat to cool
itself.
Heat in mine air comes primarily from three sources: autocompression, rock tem-
perature, and diesel equipment. Autocompression occurs as barometric air pressure
rises with decreasing elevation as fresh air enters a mine through a shaft or slope
because gases heat up when they are compressed. Hartman et al. [15] provide the fol-
lowing estimates: Dry bulb temperature ΔT d increases 9.7°C per 1000m (5.3°F per
1000ft) and wet bulb temperature ΔT w increases 4.4°C per 1000m (2.4°F per
1000ft). For a 3000-m deep shaft, ΔT d ¼29°C and ΔT w ¼13°C.
The natural rock temperature also increases as mines are developed deeper under-
ground. The rock temperature gradient with depth varies with location depending on
geologic formation and proximity to volcanic areas. Gradients may range between 1
and 5°C per 100m depth (0.6 to 3°F per 100ft). Rock heat is transmitted to mine air via
convectional heat transfer in a rather complex mechanism, as the rock is also cooled
by the air. Water entering the mine workings after flowing through crevices in hot
mine rock may also be a significant contributor to mine air heat and may increase
humidity, increasing the perceived heat of the air.
Diesel engines reject heat of fuel combustion to the mine air. At an overall effi-
ciency of 25%, a diesel engine running at 100kW would create 400kW of heat.
On average, the engine puts out much less than its rated power, with common power
factors between 25% and 50% depending on usage [16]. Therefore, a better estimate
for total heat generated by a diesel engine is the combustion heat content of the fuel
consumed.
To cool the ventilation air to acceptable temperatures for miners, three primary
methods are used: bulk intake air cooling, spot cooling, and cab cooling. With bulk
cooling, all intake air is cooled as it enters the shaft or as it flows through an under-
ground cooling plant. With spot cooling, cooling fluid is chilled at the surface and
pumped to local heat exchangers underground, often combined with auxiliary fans.
The third method is to install local air coolers in each piece of diesel equipment so
that the operator can enjoy cool temperatures inside the cab. Enclosed cabs also
improve air quality as dust can be filtered out.

6.10 Summary and conclusions

Mine ventilation planning and close coordination between the ventilation, mine plan-
ning, and operations engineers are essential for all highly productive mining opera-
tions. Ventilation provides fresh air to all active mine workings and for internal
combustion engines and dilutes harmful gases and dusts. In underground coal mines,
ventilation air quality must be monitored primarily for methane, oxygen deficiency,
diesel exhaust, dust, and any signs of combustion, indicated by the presence of carbon
monoxide. In metal and nonmetal mines, ventilation quantities are typically lower
compared to coal mines, as explosive methane is generally not present. Still, monitor-
ing should be done for diesel particulate matter, respirable dust, and radon. Dust