Heat Sources _ Effects in Mines by hcj

VIEWS: 96 PAGES: 20

									University of South Australia                           School of Engineering



L1 - Heat Sources in Mines

Need for air conditioning
Temperature - humidity control (controlling latent and sensible
heat)
      a. Cooling
      b. Heating
      c. Humidification/Dehumidification

Is one of the three functions of total mine air conditioning. It is
akin to quality control in that it pertains to the physical quality of
the air, whereas quality control pertains to the chemical quality:-
purifying & removing contaminants ie. gas control & dust control.

The usual reason for employing air conditioning in mines, is
comfort rather than for product or process purpose. The heat
content of the mine air is maintained within limits prescribed for
comfort, safety and working efficiency of human beings.

Occasionally, product air conditioning is employed, as in coal
mines where slaking of the roof in warm, moist, summer air, or in
salt mines where excessive absorption of moisture by the mineral
product may constitute environmental problems.

Mine air conditioning becomes necessary when ventilation alone is
inadequate to maintain acceptable atmospheric-heat standards.
This aspect of total air conditioning supplements rather than
replaces ventilation and quality control.

Air conditioning can be expected to play an increasingly important
role in mining under the increasingly hostile environmental
conditions now being encountered underground.



Mine Vent. Notes                   1                                   BVC
Air Conditioning                                                    13/07/10
University of South Australia                         School of Engineering



Cooling
The ultimate and eventual limitation in all mining is depth; and,
along with rock pressure, the most serious problem with depth is
heat.

Cooling and dehumidification processes are necessary in providing
bearable environmental conditions in mines of great depth [> 1.5
km] and in some of only moderate depth, especially those that are
extensively mechanised or in zones of high wall rock temperatures.

The simplest applications are small portable units called spot
coolers, but large central cooling plants are becoming more
common.

All told, deep mining districts in over 15 countries have had to
resort to cooling systems, the best known and most extensive
consisting of the deep [ 3.7 km] gold mines of Sth. Africa.
In Europe, practically all air conditioning occurs in deep coal
mines.
Other mines worldwide, that make use of cooling and
dehumidification include nickel, potash, and salt.

Heating
Relatively shallow mines in cold climates find it necessary to heat
air being taken underground, for comfort reasons as well as for
prevention of freezing in intake openings.

Mines employing heating are located in countries in high latitudes
and/or at high elevations, including metal, nonmetal, and coal
mines in USA, Canada, Scandinavia, & Russia.




Mine Vent. Notes                  2                                  BVC
Air Conditioning                                                  13/07/10
University of South Australia                          School of Engineering



Sources of Heat in Mines
Cooling and dehumidification are the most critical needs in mine
air conditioning, underground heat sources have to be identified
and quantified.
There are 9 potential sources of heat in mines, the first 4 of which
are considered major and capable of creating intolerable
environmental conditions:

                   1.   Autocompression
                   2.   Wall rock
                   3.   Underground water
                   4.   Machinery & lights
                   5.   Human metabolism
                   6.   Oxidation
                   7.   Blasting
                   8.   Rock movement
                   9.   Pipelines

The above heat sources are ranked in approximate order of
importance. Since any or all may be operative, however, it is
important in mine temp.-humidity control both to understand the
nature of the heat sources and to be able to calculate or estimate
the magnitude of the heat flow from each, broken down into
sensible and latent components.

Autocompression
Similar to the way a gas in a compressor reacts, air entering a mine
through a shaft is compressed and heated as it flows downward.
Autocompression occurs when potential energy is converted to
thermal energy. If no interchange in the heat or moisture content of
the air takes place in the shaft, the compression occurs
adiabatically, with the temperature rise following the adiabatic
law:
                  T2/T1 = (p2/p1)(-1)/

Mine Vent. Notes                        3                             BVC
Air Conditioning                                                   13/07/10
University of South Australia                          School of Engineering



        Where    T = absolute dry-bulb temp
                 p = atmospheric pressure
                  = ratio of specific heats of air @ constant
                       volume & pressure
subscripts 1 & 2 denote initial & final conditions.
Values of  are 1.402 for dry air and 1.362minimum for saturated
air.
The exponent (-1)/ = 0.287 for dry air & 0.266 for saturated.
However because of the non-adiabatic airflow that prevails in
mine shafts (direct calculation of wet-bulb temperature increases is
even more complicated). Pickup of both heat & moisture from the
wall rock can occur.
Autocompression may also be masked by the presence of other
heating or cooling sources located in or near the shaft, such as air
& water lines, hoisting equipment, other machinery or electrical
facilities

Reasonable accuracy can be obtained if increases in both dry-bulb
wet-bulb temperatures are estimated from the following
relationships:
Taken from the steady flow energy equation per unit of mass:

            q + w = [H2-H1] + [V22 – V12] + g[Z2-Z1]
and since the process is adiabatic q = 0 and
no work is done hence w = 0 and
if X sectional area A is constant then V2 = V1

             then H2-H1 = g[Z1-Z2]
              and H2-H1 = Cp * T &
                   T/Z = g/ Cp
        Cp for dry air = 1005J/kg*K
                    T/Z = g/Cp  9.76C/1000m,



Mine Vent. Notes                 4                                    BVC
Air Conditioning                                                   13/07/10
University of South Australia                                School of Engineering



similarly for moist air Tw/Z  4.37 C/1000m when surface wet
bulb temps average out at 15 C, 4 C/1000m @ 20 C surface wet
bulb [Env. Eng. In S.A. Mines, Mine Vent. Soc. Sth. Africa]

        hence:
                   td/Z = 9.76 0C/1000m
                   tw/Z = 4.37 0C/1000m
        where td = dry-bulb temp. incease
        and tw = wet-bulb temp. increase
                Z = is elevation change [+ve for increasing depth]

Example:
                   1. Compute the dry-bulb temperature increase by
                      formula,
                   2. Estimate both dry-bulb and wet-bulb temperature
                      increase due to autocompression of air flowing
                      adiabatically to the bottom of a 2000m deep shaft if
                      surface temperatures are 30C dry bulb and 20C
                      wet-bulb. Elevation of the shaft collar Z1 = 300m
                      above sea level with atmospheric pressure of 98kPa.

Solution:
a) Using the formula
                 T2/T1 = (p2/p1)(-1)/

                          Where T2 = T1 (p2/p1)(-1)/
                          T2 = [30+273] (121.5/98)0.287
                             = 303 * 1.06
                             = 320
                   i.e. td = 320-273 = 47C

[p2 calculated from p=gh & p @ STP = 100kPa, w/- = 1.2
kg/m^3]

Mine Vent. Notes                         5                                  BVC
Air Conditioning                                                         13/07/10
University of South Australia                              School of Engineering



b) Estimate increases in dry-bulb and wet-bulb from

                          td/Z = 9.76 C/1000m
                          tw/Z = 4.37 C/1000m

                           td = 2000*9.76/1000
                                  = 19.52C
                             td2 = 30+19.52 = 49.5C

                          and tw = 2000*4.37/1000
                                   = 8.64C
                                  tw2 = 20+8.64 = 28.6C

The value for td determined in a) agrees reasonably well [5%]
with that estimated in b)

If the heat flow q in watts (W) is required for cooling &
dehumidification calculations the values may be read from
psychrometric charts [preferred] or calculated from formulae.

Wall Rock
Geothermal Gradient the temperature of subsurface rock rises
steadily with depth. In most climates, the so called virgin-rock
temperature tr ceases to be affected by surface temperature
changes and is taken as a constant for reference purposes at about
15m beneath the surface. It then increases with depth at
approximately a uniform rate in a given locale and rock formation;
the rise is termed the geothermal gradient, or the temperature
change per unit depth t/Z. The age of the rocks, their thermal
properties, and their proximity to recent igneous activity or hot
springs largely determine the gradient

Although it varies in different mining districts and can be
determined accurately only by field measurements, the earth's heat
Mine Vent. Notes                        6                                 BVC
Air Conditioning                                                       13/07/10
University of South Australia                            School of Engineering



flux q/A ranges from 0.04-0.06 W/m^2 and averages 0.05 W/m^2.
Knowing the heat flux and the thermal conductivity k in W/m.C
of the various rock formations encountered, the geothermal
gradient C/100m for a given formation can be calculated from
conductive heat-transfer theory

                                q = kAt/Z

              Rearranging t/Z = q/A
                                       k
        The overall gradient can then be determined by cumulating
        the gradients for the individual formations.
        Although the heat flow varies directly with conductivity, the
        geothermal gradient varies inversely with k.

Example

A limestone has a thermal conductivity of 2.08W/mC, a relatively
low value. Calculate the geothermal gradient for the formation and
the virgin-rock temperature at 2000m depth if the surface rock
temperature to = 19C. assume an average heat flux

                   From the equation
                             t/Z = q/A
                                            k
                                       = 0.05/2.08*100
                                       = 2.4C/100m
        Temperature gain           = 2.4/100*2000
                                        = 48C
        temperature @ 2k           = 19 +48
                                        = 67C



Mine Vent. Notes                         7                              BVC
Air Conditioning                                                     13/07/10
University of South Australia                           School of Engineering



Refer to figure below of average geothermal gradients of
worldwide mining districts. cf North Broken Hill, extrapolated to
2k temp approx. 70C
Superimposed on the figure for comparison purposes is the
autocompression effect based on surface wet-bulb temp.= 18C &
a thermal gradient of 4.4C/1000m.
Considering only autocompression and a cut off temp. of wet-bulb
temp = 26C, it is seen that at an approximate depth of 2000m
[critical depth] autocompression alone dictates that air
conditioning must be installed.
Since the cooling potential of the ventilation air has been
exhausted/consumed.
 The critical temperature of the rock and its associated depth; is
also shown on the figure, at which rock heat alone governs and air
conditioning becomes essential for the hottest and poorest
ventilated workings of the mine. This temperature is considered to
be 41C.
It is evident from the figure that this critical temperature is reached
in many mines at depths considerably less than the critical depth
because of autocompression alone.




Mine Vent. Notes                   8                                   BVC
Air Conditioning                                                    13/07/10
University of South Australia       School of Engineering




Mine Vent. Notes                9                  BVC
Air Conditioning                                13/07/10
University of South Australia                          School of Engineering



Sensible-Heat Flow
Heat transfer from the wall rock to the ventilation air in a mine
opening is unsteady and complex.
Refer to the accompanying idealised diagram.




Initially when the opening is being advanced, heat flow into the
mine is at a very high rate.
Subsequently , the layer of rock adjacent to the opening cools to
essentially the temperature of the air in the opening, or within 1-
2C, thus retarding the heat transfer.
In mechanised mining, with rapidly advancing workings, heat flow
remains high at the face where the greatest number of miners are
exposed.


Mine Vent. Notes                  10                                  BVC
Air Conditioning                                                   13/07/10
University of South Australia                               School of Engineering



To minimise the heat flow from the rock to air, it is essential that:
            1. Length of airway
            2. Temperature differential between rock and air
            3. moisture
Be kept to an absolute minima.
Moisture, if present, exacerbates heat transfer by lowering the heat
transfer resistance at the interface and lowering air dry-bulb
temperature.

Airway size & shape, air velocity, surface irregularities, and nature
of air-rock interface is secondary.

Insulation has not proven successful, it is more beneficial to reduce
the wetness of the rock surface.
In addition to heat flow from the wall rock, there is also a heat
addition from the rock broken in mining. It may be substantial, up
to 60% of the total rock heat flow into an airway.

Heat transfer into mine openings, because it is unsteady, exact
mathematical solution is not possible.
The irregular shape of most mine openings, the complexity of the
air-rock interface, the usual presence of water, and unknown and
continually changing temperature distributions in both rock & air
account for the difficulty of the analysis.

Rock thermal properties must also be determined with some
precision and involves the determination of two coefficients, one
for rock thermal properties and the other for rock heat flow.

The resulting equation for sensible heat flow is expressed in the
form
                 q = S[k/re](tr – td) 

Where              S is surface area of the opening = O*L
                   O is perimeter
Mine Vent. Notes                        11                                 BVC
Air Conditioning                                                        13/07/10
University of South Australia                                School of Engineering



            L is length
            re is hydraulic radius of the opening [A/O]
             = heat flow coefficient [dimensionless]
the hydraulic radius is modified for roughness by multiplication by
2 is calculated as

                   re = 2 A/O where A is cross sectional area

To obtain the heat flow coefficient, reference is made the Goch-
Patterson data where values of  are presented as a function of the
rock thermal properties coefficient , also dimensionless:

            = /r2e
where  is rock thermal diffusivity in m2/s &  is time in seconds.
The diffusivity, in turn, reflects other thermal properties and is
found from the relation

                    = k/wrcr

where wr is rock specific weight in kg/m^3 & cr is rock specific
heat in kJ/kgC.
Thermal properties of rock are determined by laboratory or field
tests or approximated from measured values of similar locations
reported in the attached table.
After calculating the diffusivity  and the coefficient , the
corresponding value  is selected by interpolation from the table
of Goch-Patterson. The heat flow is then calculated using equation

                   q = Sk/re (tr – td) 




Mine Vent. Notes                           12                               BVC
Air Conditioning                                                         13/07/10
University of South Australia                        School of Engineering




Example:
Calculate the rock heat flow in a 50m segment of development
drive driven in Broken Hill Gneiss given the following conditions.
      Virgin rock temperature 60C
      Drive size 4.5m*5m
      Entering air temp 30C dry, 24C wet
      Barometric pressure 120kPa
      Time following development 3 months

Solution:
From the table of measured rock thermal props.
k = 3.18W/mC and  = 1.65*10-6 m2/s

             re = 2[4.5*5]/[2*4.5+2*5] = 1.7m
        and  = 1.65*10-6 * 3*30*24*3600/1.7^2
                = 4.5 therefore from the table
              = 0.6449


Mine Vent. Notes                13                                  BVC
Air Conditioning                                                 13/07/10
University of South Australia                      School of Engineering



        hence using
             q = Sk/re (tr – td) 
               = [19*50]*3.18/1.7*(60-30)*0.6449
               = 34381W
               = 34.4kW




Mine Vent. Notes                14                                BVC
Air Conditioning                                               13/07/10
University of South Australia                            School of Engineering




now consider that we want to find the air temperature at the end of
the drift.
Using the heat flow equation from rock to air
            q = mcvt
then the td1 = td0 + t
            = td0 + q/mcv
now from psychrometric tables for air at 30dry,24wet & 120kPa,
the apparent specific volume is 0.741m^3/kg and apparent specific
humidity [ASH] is 13.5g/kg
If providing an air velocity of 0.6m/s, the flow rate of air then is
13.5m^3/s

calculation for             cv = 1.005 + 1.884W [where W = ASH kg/kg]
                               = 1.005 + 0.001*13.5*1.884
                               = 1.03kJ/kgC

 td1 = 30 + [34.4*0.741]/13.5*1.03
       = 31.8C

& tw1 = 24.5C reading from the psychrometric chart for no change
in latent heat or moisture content.
Underground Water
Two sources are encountered:
                   Groundwater
                   Mine water
All groundwater, but especially that from hot fissures and natural
rock reservoirs, is a prolific source of heat in mine workings.
Since water and heat are both derived from the surrounding rock or
geothermal sources, the water temperature will approach or even
exceed the rock temperature.

The water transfers its heat to the mine air, mainly by evaporation
increasing the latent heat of the air.

Mine Vent. Notes                        15                              BVC
Air Conditioning                                                     13/07/10
University of South Australia                          School of Engineering



Evaporation in airways should, and can be minimised by:
                   Grouting or sealing the rock
                   Isolating the water inflow
                   Constructing covered ditches
                   Laying pipe to transport large flows of water.
It is crucial to maintain separation of th conditioned air and
groundwater if the water temperature is high or exceeds the air
wet-bulb
An additional hazard is if the water temperature exceeds 52C
there is a risk of scalding or burns to the body.

Equally large amounts of latent heat may be added to the mine air
through the evaporation of service water supplied for drilling and
wetting down and of drainage water from filling operations, so-
called mine water, which is also heated by the rock. As a result,
careful control over the open channel flow of waste service water
as well as groundwater is necessary in hot mines.

On the other hand, the chilling of service water supplies partial air
conditioning to working places and is an alternative receiving wide
application in South African mines.
The gain in latent heat content due to evaporation of moisture from
a free-water surface in a mine opening follows a psychrometric
process.
The evaporation rate is:
            Proportional to the differences in vapour pressures or
               temperatures of the air & water.
            The velocity.
            The water surface area
Depending on the differences in temperature, the air may gain or
lose sensible as well as latent heat.

The total heat gain from hot underground water in open channel
flow can be calculated from the relationship

Mine Vent. Notes                  16                                  BVC
Air Conditioning                                                   13/07/10
University of South Australia                           School of Engineering




                          q = Gwcw(t1-t2) kW

        where Gw is weight flow rate of water in kg/s
              cw is specific heat of water = 4.187 kJ/kgC
          and t1 & t2 are water temperatures in C at points of
        emission & exit from the mine airway, respectively.

Example:
A mine produces 20L/s of underground water on the deepest level.
The water enters the workings at 45C and exits the airway at say
35C. Determine the total heat gain to the ventilation system.

                   Heat gain q = 20*4.187(45-35)

                        = 837.4 kW
For estimating purposes, at least 90% may be considered latent
heat, thus the approximate distribution would be.
                       qL = 0.9*837.4 = 754 kW
                       qs = 0.1*837.4 = 84kW

Machinery and Lights
Nearly all the energy consumption of machinery underground adds
heat to the mine air, since power losses and most of the work done
are converted directly to heat or indirectly through friction.

This is true of electrical, compressed air, or internal combustion
(diesel) machinery, although compressed-air machines also exhibit
a local cooling effect at discharge.

Only that portion of the load of materials-handling equipment used
to elevate and raise the potential energy of broken ore or coal
accomplishes useful work, and it is sufficiently minor to be
neglected.

Mine Vent. Notes                         17                            BVC
Air Conditioning                                                    13/07/10
University of South Australia                          School of Engineering




Hence, in practice, the entire energy consumption of underground
machinery in the air conditioned zone of the mine is assumed to
contribute to the sensible-heat gain, often a very sizeable fraction
of the mine cooling load, especially in highly mechanised
operations.

Calculations of the sensible-heat gain from electrical machinery
proceeds from a knowledge of the total power connected in the
mine and the load/power factor.

If a single workplace or portion of the mine is under consideration,
then only the heat contributed by machinery locally need be
considered.

Fans may be handled separately, depending on their location.

Diesel powered equipment, commonly assumes that 90% of the
heat value of the fuel provides heat gain to the air.

Example:
Calculate the heat liberated by machinery
     a) In an entire mine having a connected underground
           electrical load of 2400kW and a load factor of 70%
     b) In a section of the mine continuously using the
           following equipment:
                 8 compressed-air rock drills rated at 7.5kW ea.
                 1 electric loader rated at 110kW
                 1 electric hoist rated at 60kW
                 1 diesel locomotive rated at 100kW[E], fuel
                 consumption rate 30L/hr, heat value of the fuel
                 40000kJ/L




Mine Vent. Notes                  18                                  BVC
Air Conditioning                                                   13/07/10
University of South Australia                                  School of Engineering



Solution:
     a) for the mine
              q = 2400*0.7 = 1680 kW

        b)         for the section the operating load is electrical:
                         8*7.5 + 110 + 60 = 230 kW
                         plus diesel load = 30*40000/3600
                                          = 333kW
                         Thus the total heat load is 230 + 333 = 563kW

                   If we had substituted the diesel loco for an electric loco,
                   assuming say 33% fuel efficiency. Then the electrical
                   equivalent would be 100kW and the total heat load
                   reduced by  230 kW, a substantial reduction of  40%

Other Heat Sources
The five remaining heat sources in mines are grouped summarily
because they are less important quantitatively and can only be
approximated numerically. Sometimes the most valid approach is
to conduct a heat balance of the mine, subtracting all the known
heat sources from the measured heat gain in order to isolate the
unknown fraction.

Human metabolism
The body's waste heat effect on the latent & sensible heat-gain is
small.

Oxidation
At present there is no effective way to calculate the amount of heat
produced by oxidation processes. In some high-sulphide ore or
coal mines, this can be significant, and in such cases, the mine
cooling load has to be increased by an appropriate estimated
amount.


Mine Vent. Notes                         19                                   BVC
Air Conditioning                                                           13/07/10
University of South Australia                         School of Engineering




Blasting
Blasting off-shift or during meal breaks is an effective way of
reducing or eliminating this factor from consideration in exposure
of miners to heat gains in workplaces.

Rock movement
It is doubtful that even 1% of the heat generated by rock movement
actually enters the airstream, since much is absorbed in the rock
mass itself, as in blasting; hence it is usually negligible.

Pipelines
If lines carrying drainage water that is hotter than the mine air,
there will be some heat gain experienced by the air. Service water
and sand fill slurry are usually at or slightly below the air
temperature.
Chilled water lines will be below the air temperature of the
workings and will remove heat from the air, this is considered a
waste of cooling and should be minimised by insulating the
pipeline.

Energy Losses in Airflow
The flow of air through mine workings, experiences energy loss,
but it does not generate heat. Only entropy increases as the flow
potential decreases. Thus the airflow head losses are not reflected
in any addition of heat to the mine air.




Mine Vent. Notes                 20                                  BVC
Air Conditioning                                                  13/07/10

								
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