Ventilation and Cooling in Underground Mines


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VENTILATION AND COOLING IN UNDERGROUND MINES
     

M.J. Howes

The main objective of mine ventilation is the provision of
sufficient quantities of air to all the working places and travel ways in an
underground mine to dilute to an acceptable level those contaminants which
cannot be controlled by any other means. Where depth and rock temperatures are
such that air temperatures are excessive, mechanical refrigeration systems may
be used to supplement the beneficial effects of ventilation.
The Mine Atmosphere
The composition of the gaseous envelope encircling the earth
varies by less than 0.01% from place to place and the constitution of “dry" air
is usually taken as 78.09% nitrogen, 20.95% oxygen, 0.93% argon and 0.03% carbon
dioxide. Water vapour is also present in varying amounts depending on the air
temperature and pressure and the availability of free water surfaces. As
ventilation air flows through a mine, the concentration of water vapour may
change significantly and this variation is the subject of the separate study of
psychrometry. To define the state of a water vapour and dry air mixture at a
particular point requires the three measurable independent properties of
barometric pressure, dry bulb and wet bulb temperatures.
Ventilation Requirements
The contaminants to be controlled by dilution ventilation are
primarily gases and dust, although ionizing radiations associated with naturally
occurring radon may present problems, especially in uranium mines and where the
background uranium concentrations of the host or adjacent rocks are elevated.
The amount of air required for dilution control will depend on both the strength
of the contaminant source and the effectiveness of other control measures such
as water for dust suppression or methane drainage systems in coal mines. The
minimum dilution air flow rate is determined by the contaminant requiring the
greatest dilution quantity with due cognizance of the possible additive effects
of mixtures and synergism where one contaminant can increase the effect of
another. Overriding this value could be a minimum air velocity requirement which
is typically 0.25 m/s and increasing as air temperatures also increase.
Diesel-powered equipment ventilation
In mechanized mines using diesel-powered mobile equipment and
in the absence of continuous gas monitoring, exhaust gas dilution is used to
determine the minimum ventilation air requirements where they operate. The
amount of air required normally ranges between 0.03 and per kW of rated power at the point of operation
depending on the type of the engine and whether any exhaust gas conditioning is
being used. Continuing developments in both fuel and engine technology are
providing lower engine emissions while catalytic converters, wet scrubbers and
ceramic filters may further reduce the leaving concentrations of carbon
monoxide/aldehydes, oxides of nitrogen and diesel particulates respectively.
This helps in meeting increasingly stringent contaminant limits without
significantly increasing exhaust dilution rates. The minimum possible dilution
limit of per kW is determined by the carbon dioxide
emissions which are proportional to engine power and unaffected by exhaust gas
conditioning.Diesel engines are about one-third efficient at converting
the energy available in the fuel to useful power and most of this is then used
to overcome friction resulting in a heat output which is about three times the
power output. Even when hauling rock up a decline in a truck, the useful work
done is only about 10% of energy available in the fuel. Higher diesel engine
powers are used in larger mobile equipment which require bigger excavations to
operate safely. Allowing for normal vehicle clearances and a typical diesel
exhaust gas dilution rate of per kW, the minimum air velocities where diesels
operate average about 0.5 m/s.
Ventilation of different mining methods
Although the setting of general air quantity requirements is
not appropriate where detailed mine and ventilation planning information is
available or possible, they are supportive of the criteria being used for
design. Deviations from normal values generally can be explained and justified,
for instance, in mines with heat or radon problems. The general relationship
is:


where t is the annual production rate in million tonnes per
annum (Mtpa), is a variable air quantity factor which is
directly related to production rate and is the constant air quantity required to
ventilate the mine infrastructure such as the ore handling system. Typical
values of are given in table
74.1.__________________________________________________________________________
Table 74.1.    Design air quantity factors




Mining method
 


Block-caving

 50


Room-and-pillar (Potash)

 75


Sub-level caving

120


Open stoping  large >.5 Mtpa  small
<.5 Mtpa

160240


Mechanized cut-and-fill

320


Non-mechanized mining

400
__________________________________________________________________________     The
constant air quantity is mainly dependent on the ore handling system
and, to a certain extent, on the overall mine production rate. For mines where
rock is transported through a decline using diesel powered truck haulage or
there is no crushing of the mined rock, a suitable value of is . This typically increases to when using underground crushers and skip
hoisting with underground maintenance areas. As the ore handling system become
more extensive (i.e., using conveyors or other ore transfer systems), can further increase by up to 50%. On very large
mines where multiple shaft systems are used, the constant air quantity is also a multiple of the number of shaft
systems required.
Cooling Requirements     Design
thermal conditions
The provision of suitable thermal conditions to minimize the
dangers and adverse effects of heat stress may require mechanical cooling in
addition to the ventilation necessary to control contaminants. Although the
applied heat stress is a complex function of climatic variables and
physiological responses to them, in practical mining terms it is the air
velocity and wet bulb temperature that have the greatest influence. This is
illustrated by the clothing-corrected air cooling powers given in table 74.2. Underground the radiant
temperature is taken to be equal to the dry bulb temperature and 10 higher than the wet bulb temperature. The
barometric pressure and the clothing regime are typical for underground work
(i.e., 110 kPa and 0.52 clothing
units).__________________________________________________________________________
Table 74.2.     Clothing-corrected air cooling powers





Air velocity (m/s)

Wet bulb temperature (°C)


 

20.0

22.5

25.0

27.5

30.0

32.5


0.1

 176

 153

 128

 100

 70

 37


0.25

 238

 210

 179

 145

 107

 64


0.5

 284

 254

 220

 181

 137

 87


1.0

 321

 290

 254

 212

 163

 104
__________________________________________________________________________     An
air velocity of 0.1 m/s reflects the effect of natural convection (i.e., no
perceivable airflow at all). An air velocity of 0.25 m/s is the minimum normally
allowed in mining and 0.5 m/s would be required where the wet bulb temperature
exceeds 25. With respect to achieving thermal equilibrium,
the metabolic heat resulting from typical work rates are: rest, ; light work, 115 to , medium work, 150 to ; and hard work, 200 to . Design conditions for a specific mine
application would be determined from a detailed optimization study. Generally,
optimum wet bulb temperatures are between 27.5 and 28.5 with the lower temperatures applicable to less
mechanized operations. Work performance decreases and the risk of heat-related
illness increases significantly when the wet bulb temperature exceeds 30.0 , and work should not normally continue when the
wet bulb temperature is greater than 32.5 .
Mine heat loads Next part of the document