cooling power for a new age - saimm · cooling power for a new age 239 introduction ventilation,...

COOLING POWER FOR A NEW AGE 239

IntroductionVentilation, cooling and refrigeration systems will play anincreasingly significant role in future platinum miningoperations. The increasing depth of operations will result inescalating heat loads having to be handled by expandingventilation and cooling systems. Considering that theperformance of these systems is measured principally bythe air wet bulb temperature and (arguably) air coolingpower achieved, the most effective strategy to counter theincreasing demand lies in providing adequate air quantitiessupported by effective cooling strategies.

Even if, for some reason, deepening of current operationswere not to occur, the role of ventilation and coolingsystems will continue to have a bearing on the standard ofunderground conditions and the profitability of operations.The mean rock breaking depth of current operations willincrease, thereby raising the heat load to be handled andextending the magnitude of airway infrastructure to theedge of the existing lease areas. This will result in increasedair leakage, airway age and resistance. These aspects haveto be evaluated against the increasing age of the capitalequipment and infrastructure serving these operationsrequiring increased maintenance and repairs and generallydeteriorating in efficiency.

The increasing cost of powering these systems must beconsidered. Indications are that power consumption costs

will double in the next three to five years and the outlookbeyond this horizon is indicative that the age of ‘cheap’power in South Africa is over. Given the ageinginfrastructure and equipment it must be expected that,unchecked, any deterioration will result in lowerefficiencies and in an increased cost burden to theoperations.

The intended extended use of mechanized miningmethods, irrespective of whether this will occur at greaterdepths or not, is a reality that must be addressed by thedesign of future ventilation and cooling systems. Standardsregulating emissions from diesel-powered engines and,more importantly, worker exposure to diesel particulatematter present in the exhaust emissions from these vehiclesare being revised both internationally and locally. At thisstage, indications are that exposure limits will be reducedconsiderably and that therefore, if increased dilution ofthese fumes is selected as a control strategy, the capacity ofcurrent systems would have to be increased significantly.

Irrespective of the above considerations relatingspecifically to diesel-powered equipment, the introductionof low and extra-low profile electro-hydraulic machinery inthe reef horizon will introduce heat loads that until nowwere not present and that will raise significantly the heatload experienced in the stoping cycle.

The above factors are deemed to be primary drivers indetermining the capacity and effectiveness of future

BIFFI, M. and STANTON, D.J. Cooling power for a new age. Third International Platinum Conference ‘Platinum in Transformation’, The Southern AfricanInstitute of Mining and Metallurgy, 2008.

Cooling power for a new age

M. BIFFI and D.J. STANTONAnglo Platinum

Anglo Platinum is in the process of introducing a number of initiatives, varied in nature, on a scalethat will impact operations in the short, medium and long-term. The advent of mechanization on alarge scale and the introduction of new mining technologies therefore require a different approachto the design of ventilation and cooling systems. In addition, the ventilation and cooling systemsserving increasingly deeper operations will place a greater demand on electrical powerconsumption—this in an environment where power generation is limited and increasinglyexpensive.

Ventilation and cooling systems currently being employed in platinum operations have evolvedfrom practices first pioneered in deep level gold mines. However, in modern and future platinummines, deepening of operations will take place in a somewhat different environment—particularlyin terms of geothermal settings and electrical power utilization. This provides the opportunities todevise novel and energy-efficient ventilation and cooling strategies tailor-made to meet thesechallenges.

The need for refrigeration and cooling at depth will increase the capital and operational costcomponent of ventilation systems and the medium-term limitations on the availability of electricalpower will require the provision of innovative systems. The increased use of diesel-poweredvehicles underground, the viability of energy recovery and feasibility of thermal storageopportunities are discussed in this paper. Some of the challenges and the solutions currentlyproposed in terms of ventilation system design, refrigeration plant selection and chilled waterdistribution methodologies are described. Changes in strategies core to this discipline that affectthe health and safety of employees and the profitability of modern operations are deemednecessary to support the transformations that are envisaged.

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ventilation and cooling systems. However, the mostsignificant aspects on the design of these systems are thesequence and rate at which the orebody is mined that arerelated to the extraction schedule and degree ofconcentration of mining operations respectively. Theextraction sequence determines the number of stopes andgeometry of the airway system serving them. The fact thatstopes are to be ventilated simultaneously all of the time—a prerequisite in terms of Anglo Platinum standards—dictates to a large extent the air quantity and degree of re-cooling that are required to satisfy this condition. Also, ithas been shown consistently that concentrating miningoperations will decrease the size of the ventilation andcooling systems.

This paper discusses the above-mentioned aspects againstthe background of the foreseen expansion of operations inthe next ten to fifteen years and proposes a number ofdesign solutions aimed individually or as part of a co-ordinated approach at providing effective, efficient andviable ventilation and cooling systems together with theassociated refrigeration infrastructure.

New mining methodologies and new challengesFrom a functional perspective, modern ventilation andcooling systems have to be fit for purpose in terms of bothdesign and operation. These systems must support theprincipal mining methods while being flexible enough tocater for variations in mining strategies. Furthermore, theymust adapt to changing mining engineering requirementsdictated by geological structures and the need foreconomically viable extraction of the orebody.

Of immediate impact on existing and brownfield projectsis the increasing rock breaking depth and increasing age ofcurrent infrastructures. In the Western Limb, operationshave reached an advanced stage in their life. Existinginfrastructures will be used to ventilate the extraction of theUG2 reef from the original Merensky Reef network whilecurrent Merensky operations will be deepened. In somecases, this entails the introduction of tertiary extensions toexisting shaft-decline cluster combinations or the sinking ofaltogether new shafts to enable, on average, mining atdepths varying between 1 300 m and 1 800 m belowsurface. The corresponding virgin rock temperature (VRT)will range between 51°C and 62°C.

The introduction of mechanized mining methods, both inan absolute or hybrid fashion, will have a significant impacton the capacity and design of future in-stope ventilationsystems. Also, Anglo Platinum is extending theintroduction of electrically-powered blast-hole drills instopes while the use of hydro-electrical pumps will also beincreased. The intent is a reduction in the use of massiveand inefficient compressed air distribution systems in orderto diminish operational costs.

From a ventilation perspective, the introduction ofalternative electrically-powered units in stopes and inhaulages close thereto, many of which convey fresh air tothese, means that all of the energy consumed by thismachinery will be released directly in the undergroundenvironment. In contrast, the expanding compressed airused by pneumatically-powered machinery provides alimited and localized cooling effect. This aspect will bediscussed in greater detail later in this paper.

The extension of current operations will result in increasesystem resistance to airflow, will have a greater propensityto air leakage due to the larger number of sealed-off areas

and higher pressure differential across these, and will behighly susceptible to an uncontrolled deterioration of thereturn airway conditions.

Ventilation strategyVentilation systems currently operating in modern platinumoperations have evolved from strategies pioneered innarrow reef South African gold mining operations.Typically, ventilation systems make use of the mininginfrastructure consisting of a primary vertical shaft ordecline clusters developed to an economically advantageousprimary horizon.

As the mine is deepened, through the development of asub-vertical decline cluster or by deepening the existingmain decline infrastructure, the ventilation system must beextended accordingly.

The number of production, ledging and vamping stopesdetermines the air quantity drawn into each half-level.Fresh air is also provided for the on-going developmentoperations on that level. The strategy of drawing air throughthe mine to a return airway system operated by mainsurface fans is tried and tested and almost universal inSouth African mining operations. Controls of these systemsare more effective and leakage may be managed moreeffectively. Most importantly, this tactic precludes the useof trap-doors in the intake airway system and relegates theheat energy generated by the main fans outside of the mineairway circuit. It is unlikely that future mine ventilationsystems will deviate from this architecture.

Heat loadsHaving outlined briefly the role played by ventilating air, itis suggested that the effect of heat energy resulting from theanticipated expansion of mining operations in AngloPlatinum will have the greatest impact on the design offuture ventilation system design. It must be added at thisstage that the need for higher dilution levels required tocounter the increasing use of diesel-powered equipment andthe exposure limits (such as those recently legislated in theUSA) might overshadow this consideration altogether.

The use of low profile machinery on the reef horizon aspart of a hybrid or fully mechanized stoping suite ofequipment is a large contributor to the heat load. For fullymechanized operations, it is estimated that the suite ofvehicles required for a production rate of 12 000 tons permonth from a single-sided stope, exclusive of the heatenergy contributed by conveyor belts and utility vehicles,will increase the heat load from 5.7 MJ/t (contribution byelectrical winches only) to 15.4 MJ/t. This means anaverage increase in heat energy absorption rate from 33.6 kW (over a 24 hour cycle) to 90.9 kW for the samecycle. On average, the contribution seems modest.However, when peak loads that occur during the productionshift are considered, this amounts to 195 kW forconventional scraper cleaning combined with electricdrilling, against a load of 625 kW for mechanisedoperations. This contribution is not insignificant and willlead to high panel temperatures if not controlled.

The use of electric-powered drills for blast-holepreparation in stopes will result in a net increase in the heatenergy absorbed by the air. Compressed air drills contributeto a net cooling load while all the electrical power suppliedto an electric drill will be liberated as heat energy in theimmediate environment. It is estimated that for eachelectrical drill there will be a net heat energy increase of

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5.25 kW that the system will have to absorb (inclusive ofthe cooling effect of the pneumatic drill of 3.5 kW).Globally this effect is of secondary value. However, duringa drilling shift the gain dry bulb air temperature in a panelwill vary between 0.3°C (at a flow rate of 20 kg/s) and0.7°C (at a flow rate of 10 kg/s).

The introduction of these novel mining systems willimprove the efficiency of the production cycle and willreduce the overall power consumption. However, the heatenergy released by this machinery, particularly in the reefhorizon, will be significant and must be countered by meansof effective ventilation and cooling strategies.

Design standardsIn terms of the thermal environment, it is required to use airso that the maximum reject temperature at a minimum airspeed of 0.4 m/s is 27.5°C. For naturally acclimatisedworkers, this relates to an air cooling power (ACP) of 280W/m2 which is considered to be an absolute ACPminimum. Air cooling power is principally a function of airwet bulb temperature and air speed (clothing and skinwetness are also parameters that contribute to ACP but forthis discussion are considered to be outside the realm ofoccupational environment control). Figure 1 shows therelationship between air speed and wet bulb temperaturesup to 29.5°C for an equivalent ACP value of 280 W/m2.This implies that environments within the envelope shownhave equivalent ACPs and for practical purposes aredeemed acceptable in planning and managing conditions instopes.

The ACP plot indicates that higher wet bulb temperaturesmay be tolerated as long as higher face air speeds areattained.

An increase in panel air speed also reduces the rate atwhich air temperature increases in stopes at depth. Figures 2, 3 and 4 show the wet bulb temperature variationexperienced as air passes through three in-line stopes.These temperatures were obtained from software models.These have been obtained for stopes located at depthsvarying between 400 m and 1800 m below surface foroperations typically located in the Western Limb. Figure 2shows the air wet bulb temperature variation for air enteringthe bottom crosscut of each four model stopes at a wet bulbtemperature of 25.0°C. The stopes are at a depth between400 m and 1 000 m below surface. The air quantity and airutilization in each case has been adjusted for an effectivepanel air speed of 0.4 m/s. The graph clearly indicates thatup to a depth of 600 m the wet bulb temperature will remainbelow 30.0°C. The decrease in wet bulb temperature shownin the upper stope is as a combination of a number ofsecondary effects:

Figure 3 shows the same results for stopes to a depth of 1800 m below surface. In these models, however, the airspeeds used for the models were differentiated as indicatedin Table I.

The results summarized in Figure 3 indicate that theincreased (differentiated) air speeds result in decreasedintermediate reject temperatures for the same inlet andoperational conditions. In order to make this comparison

Figure 2. Wet bulb temperature variation

Figure 1. Equivalent air cooling power

Air speed (m/s)

Inlet Crosscut Stope 1 Stope 2 Stope 3

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even more realistic, cooling was introduced for the modelstopes located at 800 m and deeper. The cooling wassimulated to reduce the inlet wet bulb temperature to20.0°C saturated (which is deemed realistic if not slightlyconservative). The results are acceptable for operations to 1000 m below surface as shown in Figure 4.

Other simulations (not shown here) reveal that even if theair were cooled to an unrealistic 0.0°C at the inlet to thefirst stope, the temperature at the top of stope 3 will behigher than the design limit.

This comparison is demonstrative in nature but showscertain principles that hold true in reality. These are:

• In order to delay the use of secondary cooling below adepth of 600 m, the panel design air speeds should bemaximized (an economic optimum should be sought).

• Ventilation of more than three stopes in line,particularly below 600 m, is not recommended.

• Below 800 m, secondary cooling (at stope entrance)will be necessary.

• Irrespective of the degree of secondary air cooling, theeffectiveness of the heat energy absorption process

reaches an optimum below which increased secondaryair cooling will have few overall benefits

• Following from this, below a certain critical horizon(possibly 1 000 m) tertiary cooling of the air in thestoping horizon will be necessary.

Figure 5 shows the temperature envelope for models thatinclude differentiated air speeds, secondary cooling at inletto the bottom stope and tertiary cooling in Stopes 2 and 3.

These results demonstrate the evolution of a feasiblestope air distribution and cooling strategy. It isrecommended that simulations of systems based on thegradual introduction of individual parameters be performedto verify the validity of these postulates and to maximizethe effect of any optimization.

New methodsThis section of the paper outlines strategies that areproposed generically and conceptually for futureapplications. These are based on the foregoing analyses andon considerations aimed at improving the efficiency andeffectiveness of air conditioning processes.

In-stope ventilation controlsAn important system attribute that will maximize bothefficiency and effectiveness is the proper utilization of airin the stoping horizon. Whereas the primary distributionsystem is designed to deliver on adequate quantity of coolair to the entrance of the stope, effective ventilation wallsand gully brattices will direct the fresh air to the face andreduce leakage in gullies.

Currently pilot projects are being evaluated in order toselect viable combinations of methods and materials thatcan be used in the construction of these control structures.

Stope depth (m BS) Air speed (m/s)

400 0.40600 0.40800 0.601000 0;601400 0;801800 0.80

Figure 3. Wet bulb temperature variation with differentiated panel air speeds

Figure 4. Effect of secondary cooling on wet bulb temperature variation

Table IDifferentiated air speeds


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Preliminary results from these projects indicate theeffectiveness of solid cement ventilation walling orpaddocks cast in containment bags, suspended along gulliesto improve stope air utilization.

Further testing at the deeper shafts in Rustenburg is alsoaimed at determining whether the postulated reduction inheat energy flux, expected from the effective isolation ofworked-out areas by these paddocks from the ventilatedface, will be achieved.

In addition, technologically advanced air curtain systemsare also being tested and developed at these sites. The aimof this tactic is to enable aerodynamic systems to provideair barriers in gullies while not interfering with the transportof rock and materials.

Air distribution strategies

For deeper operations, observations from projects indicatethat, in general, working places located at depths in excessof 600 m below surface and further than 1 500 m from asurface shaft or decline cluster (at the top of which a bulkair cooler is operational) require secondary cooling.Alternatively, the injection of an ultra-cold air stream fromsurface at strategic positions close to the advancingproduction front may be a viable alternative to the use ofchilled water in secondary heat exchangers. This alternativeis of value particularly for operations that will be replacedby new shaft infrastructures in future expansions.

Another alternative being evaluated is controlledrecirculation of air coupled to secondary cooling of the re-circulated air stream. This requires the provision of chilledservice water to air cooling stations located at strategic sitesunderground that favour the connection between the returnairway and section intake infrastructure. Up to 40% of theair quantity being returned to surface from a ventilationdistrict is drawn into the air cooling station, reconditionedand reintroduced into the fresh air stream serving thatdistrict. The balance of the air is returned to surface.

On a large scale, there is a potential for decreasing thedimensions of the intake and return airways. This obviousadvantage must be offset by the cost of introducing asecondary cooling infrastructure and the need to scrub allcontaminants from the recirculated portion of the air beforereintroduction into the intake airway system. This may beproblematic where carbon monoxide liberated inmechanized sections buildsup as the result of the re-circulation.

For safety reasons, the recirculation loop is monitoredcontinuously by means of gas sensors that will stop the re-circulating fans should noxious or flammable gases bedetected in the return air stream. Also, PLC logiccontrolling the system will interrupt the recirculating loopat blasting time until the blast fumes are cleared.

Cooling and refrigeration strategiesAir cooling is achieved by generating cold water in arefrigeration plant and circulating this to direct or indirectcontact heat exchangers where it is made to absorb heatenergy from the air stream. Primary cooling occurs insurface bulk air coolers that handle up to 90% of the down-cast air quantity in a shaft. Typically these units are of thedirect contact, evaporative type and are used to lower thewet bulb temperature of the air stream to temperaturesbetween 6°C and 9°C. The principal function of these aircoolers is to ensure that the air temperature is maintainedwithin a narrow, constant and comfortable range

irrespective of surface cyclical weather conditions. Inaddition, primary cooling systems are designed to counterthe effect of auto-compression and heat absorbed from thesurrounding rock mass as the air stream descends throughthe shaft and declines. Ideally, a primary air cooling systemwill provide this constant air temperature envelope to a pre-determined depth below surface and distance along strike.

As the mines advance beyond this critical boundary,additional cooling will be required. This may be achieved inone of two ways. If the mine is relatively shallow andconditions and environmental considerations allow it,‘injection’ of extremely cold air from additional surfacebulk air cooling plants may be considered. This requires thegeneration of air at a temperature of not more than 5°C bymeans of ammonia refrigeration plants or melting ice damsgenerated from glycol or ammonia plants. Alternatively,and more conventionally, secondary air coolers can belocated at strategic underground sites fed through dedicatedpipelines or sharing chilled service water off the maindistribution network. Typically the air coolers can beconveniently positioned at the top of sub-vertical declineclusters so as to condition the air serving the productionareas below this critical horizon.

Presently the use of secondary bulk air cooling is limitedto the deeper operational shafts but this strategy willbecome increasingly widespread in the near future as meanrock breaking depths increase. Cooling systems servingcontrolled air recirculation sites are defined as secondary aircoolers.

Cooling water required for secondary air cooling may besourced from refrigeration plants located on surface orunderground.

Surface refrigeration plants are easily accessible formaintenance and repairs. More importantly, they reject theabsorbed heat energy directly to the atmosphere. Thismeans that the coefficient of performance is hig, implyingthat for each unit of input power to the plant, it can transferup to six units of heat energy. The penalty for this is areduced ‘positional efficiency’. Intuitively this means thatthe effectiveness of the cooling process diminishes asmining operations move away from the surface air coolingplant. In the extreme, the heat energy removed from the airin the bulk air cooler will eventually be re-absorbed as theair moves along the intake airways thus providing no reliefto the working places.

Chilled water generated on surface is conveyedunderground for use in secondary air coolers throughthermally insulated pipelines to ensure that the watertemperature remains as low as possible in order tomaximize the heat absorption process at the air cooling site.

In open-ended pipe networks, the return water is pumpedto surface, usually mixed with the mine’s service water.This results in contamination of the chilled water whichwill now require treatment to protect the chiller evaporatorsfrom fouling and corrosion.

As an alternative to ‘open’ systems, the use of highpressure closed-circuit water chilling networks will reducethe pumping power demand considerably but will result in ahigher initial capital expenditure. In addition, as the resultof the higher levels of complexity and sophistication arisingfrom very high hydrodynamic and hydrostatic systempressures, the system’s risk profile will be elevated,therefore requiring high quality safety measures, andadequate levels of skilled maintenance for the network,machinery and equipment.



Underground refrigeration plants enjoy greater positionalefficiencies and require considerably less energy for thecirculation of chilled water. However, the coefficient ofperformance of these plants is considerably lower (aroundthree as opposed to six for surface plants). This implieshigher power consumption for the same plant coolingcapacity. Larger plants require large excavations for theplants and for the underground heat rejection arrangements.Maintenance of and accessibility to the underground plantsare more onerous. Furthermore, these plants use exhaust airfor heat energy rejection, therefore limiting the coolingcapacity generated underground and precluding the use ofthe air used for heat rejection further downstream.

If it is intended to implement controlled air recirculationas a strategy, with underground chillers providing thenecessary cooling water, the capacity of the recirculatingsystem must be optimized to ensure that enough reject air isavailable for the underground plant’s condensers. It is clearthat trade-off studies must be performed to establish themost cost-effective configuration for the provision ofsecondary cooling.

As discussed previously, mines operating below the 1 300 m horizon will require in-stope tertiary cooling toensure that the reject air temperature objectives areachieved. Similar considerations to those presented aboveare required to establish the most energy- and cost-efficientrefrigeration plant configuration.

The need to provide chilled water for secondary andtertiary cooling in quantities comparable to the servicewater requirements may favour the bulk cooling of all thewater circulated through the mine. As an example, a mineproducing 120 ktpm will consume 45l/s of service waterdaily with a peak demand of 125l/s. At these flow rates, theair cooling capacity of the chilled service water stream willbe equivalent to 1 800 kW and 5 000 kW respectively.These relate to a cooling capacity of 15 kW/ktpm and 41.7 kW/ktpm respectively. Considering that the heat loadfor mines in the Western Limb at depths between 1 300 mand 1 800 m below surface varies between 100 kW/ktpm to200 kW/ktpm, it is apparent that chilling of the bulk servicewater stream will not be sufficient to meet this load and thatadditional chilled water must be conveyed underground forsecondary and particularly tertiary air cooling.

Refrigeration plant requirementsThe planned expansion and deepening of mining operationswithin Anglo Platinum in the medium- to long-term

requires the introduction of considerably more refrigerationand cooling power than has been the case up to now. Moresignificantly, the electrical power demand for theseoperations is set to increase along a similar trend.

Chilled water distribution strategiesThe introduction of chilled service water or bulk servicewater cooling is set to become the norm as operationsextend beyond the 1 300 m horizon. As a result of this, it isexpected that electrical power consumption will increaseand therefore power-efficient water distribution strategiesmust be considered.

The use of high pressure piping for the distribution ofchilled cooling water underground has been mentionedpreviously and possibly offers the most power-efficientsolution. Retrofitting these high pressure pipes in workingshafts is not deemed feasible.

The use of direct contact heat exchangers inevitablyprecludes the pressurized return of the cooling water tosurface. This means that the water has to be pumped-outtogether with the service water. Again, retrofitting thistechnology in an existing plant may only be viable if sparepumping capacity and shaft infrastructure are available. Theadvantage of direct contact heat exchangers is that they aremore efficient, less sophisticated, more robust, portable andrelatively inexpensive. Logistically they are the obviouschoice for tertiary cooling strategies where constant re-positioning of these units is required as, for example, in astope.

Indirect contact heat exchangers, typically consisting ofmetal-finned tube assemblies are more delicate, heavier,more difficult to handle in limited spaces (particularly highpressure units) and susceptible to fouling, which willdiminish the efficiency of the heat transfer processes andreduce the air cooling rating while increasing the overallpower consumption for the installation.

Figure 6 outlines schematically the evolution of a genericair cooling system. Typically, the primary bulk air coolerwill be the first component that will be commissioned andused for shaft sinking and initial development of the shaft.The unit is self-contained, with heat energy beingtransferred to the atmosphere through the condenser coolingtower (CCT). This plant will generate water at 5.0°C.

As the mine is extended and production increases thesecond phase will be introduced to generate water at 1.0°C.This will be used as chilled service or as cooling waterdistributed to secondary and tertiary air coolers. A pre-cooling tower (PCT) is used to handle the primary feed

Figure 5. Effect of tertiary cooling and differentiated air speeds on wet bulb temperature variation


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(usually treated water returned from underground) in orderto maximize the free cooling effect available from theatmosphere. The PCT reduces the water temperature from28.0°C to about 24.0°C on a hot summer day.

As indicated in Figure 6, the water enters an undergroundsection along an insulated pipe and is coursed directly intoa secondary air cooler. The three-way valve downstream ofthe cooler will drive the demand for cooling through thisheat exchanger. This means that during the working shift,the chilled water will be circulated to the secondary aircooler first and if any is required by mining operations inthe stopes or development ends served by this line, then thethree-way valve will allow through passage of the coolerreturn water to the face. When the service waterconsumption in the section downstream of the cooler isreduced or stopped, the excess water is released into a(dedicated) return line. If the return water is kept separatefrom the drain water, it may report directly to the clearwater dam by means of a separate drain-hole system. At theend of the shift, the system may be shut down totally tolimit power usage.

At the start of the following shift, the system is re-activated. The process may be automated to allow pre-cooling of the section prior to re-entry. Figure 6 alsoindicates the possibility of connecting tertiary coolerslocated in the stopes, effectively utilizing the service waterrejected from the secondary cooler. If water is produced at1.0°C on surface it is conceivable (see below) that it willreach the stope at about 18°C to 20°C, provided that pipesare insulated and the system is operated optimally. Water atthis temperature may be used effectively for tertiary coolingeither directly through a jet spray or indirectly in blast-holedrilling or water jetting (where this takes place).

Observations and considerationsAn important aspect of this analysis is the transient natureof thermodynamic conditions encountered at depth. Coolingsystems are designed to provide adequate conditions atsteady state with surface BACs buffering the effect ofvarying surface atmospheric conditions. At the same time,underground cooling systems are usually designed tohandle heat loads that have been ‘smoothed’ over thediurnal cycle.

In reality, particularly where rock temperatures exceed40°C, re-entry into a blasted panel at the start of thecleaning shift can subject workers to high temperatures dueto the lower ventilation regime (due to the reduced cross-sectional area) and the presence of a sizeable, fragmentedand hot rock mass. The best line of intervention to correctthese conditions rapidly is to ‘boost’ cooling by means ofchilled water sprays cooling the air stream and that can bemoved as the workers proceed with their activities.Alternatively, the use of water jets operated with highpressure chilled service water has shown to be effective tocounter these conditions. It is therefore important that lowtemperature water reaches the face to provide this coolingboost capacity.

The optimal operation of such a system thereforerequires:

• Good insulation of chilled water supply lines• Low residence times in pipes (reduce the number of

dams on the supply side of the system, maximizeservice water through-put in the off-shift period)

• Enhance the through-flow of chilled service water afteroff-shift periods for air cooling purposes (for example,automate the three-way valve to start-up the secondarycooler one hour prior to the start of the shift).

In general terms, as conditions become more severe withincreasing depth, the demand for cooling should not bedictated by service water utilization but rather by the needfor reducing the ambient temperature. The two are notalways concurrent and the occurrence of ‘temporal hotspots’ requires that the cooling system is able to provide thenecessary boosts when necessary.

In the foregoing sections it was stated that chilled watershould be used optimally and that the chilled water shouldreach the working face at the lowest possible temperature.As a last comment it was said that the cooling plant shouldbe able to provide ‘boosting’ capacity to manage hot spots.These performance requirements may be achieved bydesigning and operating these systems with the assistanceof a number of components that have been shown in Figure 6.

Energy recoveryEnergy recovery devices may reduce the electrical powernecessary to pump service, cooling and fissure water to

Figure 6. Schematic representation of a modern cooling system



surface. Energy recovery devices may reduce the pumpingpower by up to 80% in the case of highly efficient systemssuch as the three chamber pump feeder (3CPF) system orhydro-transformer, in the absence of fissure water.

Devices such as the 3CPF and hydro-transformer arerelatively simple units to operate. Essential to theiroperation is the timing of the valves that open and close thevarious water streams. The principal drawback of boththese systems is that water must be sent underground so asto pump (in some cases even more) water out of the mine.This also implies that fully operational stand-by pumpstations will be required. Considering that the pumping loadis out of phase with the de-watering demand, thesestrategies require the use of large underground storagedams to buffer the two diverging demands.

The use of energy recovery turbines coupled to electricgenerators is also a tried and tested alternative for thegeneration of electrical power. This method is not asefficient as the ‘u-tube’ methods mentioned above and alsowill not cater for fissure pumping. Turbines prefer a steadybase-load and therefore coupling these to bulk air coolers asshown in Figure 6 will be advantageous.

The primary turbine in Figure 6 is an impulse turbine(Pelton wheel) located at a strategic depth below surface.The unit will recover about 55% to 60% of the powerrequired to return the same water quantity to surface. Inorder to maximize flexibility, it might be desirable to have anumber of smaller-capacity turbines in parallel. Thisconfiguration will enable the handling of a range of flowsover the daily production cycle. This implies that during theoff-peak period, only one turbine will be operating whilethe others will be on stand-by. The turbine station must belocated high-up in the shaft system to provide enoughresidual head to enable the use of the service water furtherdownstream. In addition, the dam downstream of the stationshould not be too large to limit (chilled) water residencetime and heat pick-up.

Modern operations make use of pressure-reducingstations to regulate the water pressure on different levels.Figure 6 shows that small reaction (Francis) turbines maybe utilized to recover electrical energy while performing thesame function and providing adequate residual linepressure.

Whereas Pelton wheel turbines are readily available andhave been used within Anglo Platinum, an initial search ofFrancis turbines suitable for this application has so farfailed to identify units feasible for the required operationalregimes. Of importance here is the fact that where energy isrecovered, as a bonus, the rise in water temperature due tothe Joule-Thompson effect is reduced. It is stressed,however, that the reason behind choosing any of theseenergy recovery devices must remain the ability to recoverenergy available in the distribution network in a cost-effective manner.

Thermal storage

Positioning refrigeration plants on surface presents uniqueopportunities for the generation of large quantities ofchilled water or ice during low demand periods (usuallyafter 22:00) for use during high demand phases of the cycle(after 07:00). Thermal storage is a tactic that reduces powerconsumption during peak tariff periods and makes chilledwater quantity available during the peak consumptionperiod. This strategy is advantageous considering thecurrent power tariff structure.

Considering that the peak demand for service waterduring the day coincides with the higher tariff bands, thereis scope, if the refrigeration plants and associated thermalstorage systems are large enough, to enable the stoppage ofthe refrigeration plants during the day and utilize a thermalstorage strategy that will ‘clip’ the electrical power demandduring tariff peaks.

Another advantage of this strategy is that the refrigerationcycle efficiency is higher at night as a result of the loweratmospheric temperatures.

Ice thermal storage is a refinement of this concept. Theadvantage of using ice in a thermal storage system lies inthe use of the latent heat of fusion of the ice which reducesthe physical size of the storage tanks. This advantage mustbe tempered against the higher capital costs of ice plantsand lower coefficient of performance of these plants.

The use of thermal storage is feasible and must beselected based on individual performance, infrastructuralmerits and overall ownership costs.

Alternative power sourcesThe affinity between solar power and absorptionrefrigeration plants is well documented. Initialinvestigations indicate that this synergy is feasible incertain applications and at certain sites. This involvespowering adsorption refrigerators by means of hot fluids(oil or metal salts) generated in solar collectors. Thesetechnologies are capital-intensive and have a lowcoefficient of performance (close to 1). The benefit is thatsolar power is freely available. In designing such systems itmust be considered that stand-by capacity must be availableto counter spells of poor solar power availability. Inaddition, plants must be over-sized and storage thermalcapacity must be provided to enable energy or thermalstorage during the day for utilization at night. Thefeasibility of using solar power will be determined by costconsiderations and availability (shortage) of electricalpower.

Operation on demandWhereas the foregoing technologies are technically feasibleand many have been proven in mining applications, theoperation of ventilation and cooling ‘on demand’ does notenjoy the same degree of support in South Africanoperations. The feasibility of this strategy is based on theneed to reduce overall power consumption and to determinewhether it is necessary to ventilate and cool the entire mineall of the time. If implemented properly, the ability toactivate air movement in a section of the mine or cooling atwill is deemed feasible for certain applications and mightoffer cost savings without compromising environmentalconditions.

The reluctance of switching-off fans, particularly inmines where flammable gas may be liberated is wellunderstood and it is advocated that continuous ventilationof such operations is essential.

The strategy of cooling on demand was investigated indetail for deep level gold mines where it was found that the‘thermodynamic inertia’ of the rock mass would precludethe successful utilization of this strategy as the heat build-up over ‘off’ week-ends could not be off-set by the coolingcapacity of existing systems. The reason for this was simplythat the existing plant was sized to provide cooling capacitybased on the ‘average’ demand and was not able to ‘boost’its performance over a short period of time to offset the‘thermodynamic inertia’ of the system.



It is postulated that the thermodynamic properties ofplatinum-bearing rocks are such that their combined‘thermodynamic inertia’ will be lower than that of goldmines and that the effect can be more easily overcome byadequately-sized systems. The issue is to be able toestablish the required ‘boost’ capacity, to ensure that it canbe activated timely and whether it is reasonable to expectthat the (higher) cost of the ‘over-sized system’ required forthis purpose will justify the power savings. This is astrategy that could be considered even where sections ofexisting operations are being extended.

ConclusionsThis paper illustrates concepts of mine ventilation andcooling that are deemed feasible for systems being plannedin the medium- to long-term. These concepts are consideredto be executable under given circumstances and, except forthe Francis turbine application, have been tried and testedin the field.

In addition, the strategies illustrated in this paper offer anumber of alternatives and synergies that can be exploitedin order to provide fit-for-purpose systems against thebackdrop of increasing capital and operational costs.

These new challenges can be met successfully as long asthese systems are designed and operated adequately. The‘secret’ to this is in the ability to adapt operational regimesto run at optimum efficiency over the life of the projectswhile taking into account short cyclical demands. Modernsystems have to be adaptable and flexible to adjust quicklyand accurately to changing conditions. This is a veryimportant shift in emphasis, driven by the need to beenergy-wise without compromising the output—in theinterest of health and safety.

The most important aspect is ability to manage thesesystems so that they operate continuously in the intendedmanner and respond correctly to changing conditions. Thelevel of sophistication of control systems, instrumentationand components governing these is relatively high. Inaddition, the environmentally-trying conditions encounteredunderground are well-known and must be countered withtough and reliable components and machinery.

Accurate assessment of conditions and the ability tochange operational parameters in line with the informationreceived is essential in achieving real-time control ofsystems.

The real challenge is in finding the right level of technicalskills needed to install and operate these systems. Thecurrent skills shortage is affecting the operation of existingsystems. The levels of technical complexity inherent inthese systems are higher than is currently the norm but arewell within the capability of locally available skills—ifonly these were to be found.

References1. DEL CASTILLO, O., BURN, A., PIETERS, A., and

VOLK, B. The design and implementation of a 17 MW thermal storage cooling system on a SouthAfrican mine. Proceedings International Congress ofRefrigeration, Washington D.C., 2003.

2. WILSON, R., BLUHM, S., FUNNEL, R., and SMIT,H. Surface bulk air cooler concepts producing ultra-cold air and utilizing ice thermal storage. J. of theMine Vent. Soc. Of S.A. Oct/Dec 2003, pp. 146–151

3. UDD, J.E. Deep mining in Canada—the challengesand opportunities. Proceedings Core Project on DeepMining, June 2006.

4. PAYNE, T. and MITRA, R. A review of heat issuesin underground metalliferous mines. Proceedings 12thU.S./ North American Mine Ventilation SymposiumJune 2008, Reno Nevada, Wallace, K.G. Jr (ed.), pp. 197–201.

5. WILSON, R.W. and PIETERS, A. Design andconstruction of a surface air cooling and refrigerationinstallation. Proceedings 12th U.S./ North AmericanMine Ventilation Symposium June 2008, RenoNevada, Wallace, K.G. Jr (ed.), pp. 191–195.

6. BLUHM, S.J. Bluhm Burton Engineering, personalcommunication.

Marco BiffiRefrigeration Engineer, Anglo Platinum

Over 22 years experience in mine ventilation and cooling in Narrow reef and massiveore-bodies including gold, platinum, diamonds, copper, nickel, zinc and coal operations


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