Chapter 106

DESIGN AND SIMULATION OF ULTRA-DEEP MINE COOLING SYSTEMS

R. Ramsden	T.J. Sheer
Bluhm Burton Engineering South Africa	School of Mechanical Engineering University of Witwatersrand South Africa M.D. Butterworth CSIR Mining Technology South Africa

ABSTRACT

Several South African gold mines are planning to mine below 4 000 m. At present, these mines are mining at shallower depths and to satisfy the increased cooling requirements, existing cooling infrastructure must be extended. The cooling requirements to provide acceptable temperatures at these depths for a production of 200 ktons per month will be more than 100 MW. This paper shows that the most cost-effective way to provide this cooling will be with a combination of surface and underground cooling installations that make full use of existing infrastructure. The surface installation will provide cold water and ice which is piped underground to ice melting dams. Cold water from these dams is circulated to air coolers and the return hot water is used to melt the ice. The underground cooling installations will use refrigeration machines to cool water that will be used in air coolers and as chilled mining water. These cooling installations represent a major investment and it is necessary to operate them efficiently with minimum cooling losses and in the most effective manner.

In the first part of this paper, a model mine producing 180 ktons/month of ore from mining depths between 4 000 m and 5 000 m [ultra-deep mine] is considered. A thermodynamic ventilation simulator is used to determine the airflow distribution and cooling requirements throughout the mine. Refrigeration systems using both surface and underground plants, which will satisfy the cooling requirements are described. Based on the findings from this study, generic guidelines for ventilating and cooling ultra-deep mines are given.

In the second part of this paper, benefits of selectively providing ventilation and cooling to match mining activity rather than ventilating and cooling all-of-the-mine, all-of-the-time are described. Finally the savings due to minimising inefficiencies in both the ventilation and cooling systems are presented.

KEYWORDS

Ventilation, cooling, refrigeration, ice, deep mines, costs, optimisation

INTRODUCTION

Several South African gold mines that are presently mining ore bodies between 2 500 m and 3 500 m are examining the feasibility of extending workings to below 4 000 m. To justify mining below 4 000 m it will be necessary to make extensive use of existing infrastructure. Existing shaft systems will be extended to reach deeper ore bodies and the capacity of existing cooling systems will also be extended to meet the increased cooling requirements. Primary intakes and returns will be fixed by the existing mine. Redundant upper levels will be sealed off and ventilation air will be coursed to the deeper workings. Existing main surface fans will continue to be used and may be supplemented with underground booster fans.

For ultra-deep mines there will be a significant increase in cooling requirements. Cooling will be provided by a combination of underground refrigeration machines and surface cooling installations consisting of pre-cooling towers to cool return water that is pumped to surface, water chilling installations and ice makers. Where possible existing cooling infrastructure will be fully utilised and integrated into the enlarged cooling systems.

In spite of this strategy of making full use of existing equipment, ventilation and cooling costs at depths in excess of 4 000 m will be extremely high. For a production of 180 ktons per month the additional capital costs of ventilation and cooling equipment will be about US $125 million and power costs over the life of the mine will be about US $100 million. In view of these large costs it is essential that the most appropriate and cost-effective ventilation and cooling systems are selected. It will also be necessary to ensure that all inefficiencies and cooling losses are minimised. In order to select and optimise the most appropriate systems, the interaction between ventilation and cooling systems must be taken fully into account in a consistent manner.

At 4 000 to 5 000 m the virgin rock temperature [VRT] will be between 60°C and 70°C and the heat flow from the rock surfaces in tunnels will be twice that for current mining depths where VRTs are about 50°C. Heat pick-up within stopes will also be much higher and the use of in-stope cooling cannot be avoided.

A number of mining companies recognised that there was a need to examine ventilation and cooling strategies for this new generation of ultra-deep mines and develop general guidelines for the most appropriate methods for providing cooling. This work was carried out under the collaborative DEEPMINE programme.

COOLING GENERATION SYSTEMS

Before the 1970s almost all refrigeration plants on South African mines were installed underground and used to cool water which was then distributed in insulated pipes to air coolers located close to the workings. Due to high operating costs associated with underground cooling plants, there was a trend in the 1970s to locate refrigeration plants on surface where they were used to cool downcast ventilation air and provide chilled service water. For some mines, at moderate depths, adequate conditions could be achieved with cold downcast ventilation and cold service water. In deeper mines, to reduce energy costs associated with pumping, energy recovery turbines were introduced initially coupled to pumps but more successfully coupled to generators. By 1985 there was 1 000 MW of installed refrigeration capacity, split equally between surface and underground. In the last decade installation of refrigeration plant has slowed down with mainly surface plants installed, although the trend, [for a variety of reasons] is now giving underground plants more consideration.

In future, for ultra-deep mines a combination of surface and underground refrigeration plants will be used. On surface, refrigeration plants will be both water chillers and icemakers while underground refrigeration plants will be water chillers. Because of limited heat rejection capacity for underground refrigeration plant most of the cooling will be produced on surface.

The main features of the surface refrigeration plant will be:

• Surface pre-cooling towers in which warm mine water is first cooled.

• Water chilling refrigeration plants for pre-chilling water feed to ice makers and for bulk cooling of ventilation air that is sent underground.

• Ice making plant [Sheer, et. al., 2001] including vacuum slush ice with concentrators or particulate ice equipment.

• Ice conveying system including pipe feeders and pipe systems.

• Ice melting dams underground.

The DEEPMINE studies [Bluhm, et. al., 2000] have supported earlier studies [Shone & Sheer, 1988 and Bluhm, et al., 1998] that have shown that ice systems are cheaper than chilled water systems with energy recovery turbines for ultra-deep mines. The pumping costs for returning cooling water to surface is a major operating cost and as mining depths increase so do the pumping costs. The main advantage of using ice is that the quantity of return cooling water is reduced by a factor of about four and hence pumping costs to surface are significantly lower.

The main features of underground plant will be:

• Large underground excavations for refrigeration machines and heat rejection chambers [cooling towers or spray chambers].

• Refrigeration machines using acceptable non-toxic refrigerants [such as R123 or R134a] and operating with high condensing temperatures of about 50°C, due to the difficulty [and limit] of heat rejection facilities.

• Underground chilled water dams.

MODEL MINE

In order to develop guidelines for ventilating and cooling ultra-deep mines, a model mine, that is an extension of a generic existing mine, with a production of 180 kton / month was considered. Figure 1 shows the general arrangement of the model mine. There are two existing intake and return shafts [surface to 2 000 m and 2 000 m to 3 500 m] and new intake and return shafts [3 500 m to 5 000 m].

It is assumed that for the existing mine there are two refrigeration installations, one on surface and the other at a depth of 2 600 m. For the new ultra-deep mine the size of the underground refrigeration plant was maximised based on heat rejection limits. The existing surface water chillers were used, as part of the requirements for pre-cooling water required by the icemakers. Other refrigeration systems were examined; however, it was found that in all cases the use of underground refrigeration and surface icemakers was the most economical refrigeration strategy.

Various cooling and ventilating strategies were studied:

• Effects of controlled recirculation of ventilation air [primary and secondary].

• Effects of different cooling strategies, [location and types of air coolers and outlet air temperatures from air coolers].

• Effects of different stope water usage [water usage patterns and temperature].

Figure 1. Model mine layout

INTERACTION BETWEEN VENTILATION AND COOLING SYSTEMS

For ultra-deep mines, ventilation and cooling reticulation systems will extend over many kilometres with cooling water reticulation pipes installed in shafts / tunnels. Although pipes will be insulated there will be heat transfer between the ventilation air and chilled water in the cooling reticulation networks. For example, a 300 mm insulated steel pipe would transfer about 50 kW per 1 000 m from the air to the cooling water in the pipe. This heat transfer increases the temperature of the water and decreases that of the air. The `knock-on' effects due to this heat transfer include increased heat flow from the rock surfaces in the tunnel and reduced air cooler requirements. In order to fully understand the interaction between the ventilation and cooling systems for an extensive mine layout it is necessary to adopt a systematic modelling approach. The basis of the iterative approach adopted in the DEEPMINE work is illustrated in Figure 2.

The approach adopted is highly iterative and the broad outlines of the approach are summarised below.

STEP 1: A thermodynamic computer simulator [Environ] was used to determine airflow and temperature distributions throughout the mine. A total of 500 branches were used to describe the model mine. To ensure acceptable temperatures `heat sinks' [air coolers] were located as necessary.

Figure 2. Modelling procedure for ventilation and cooling design

STEP 2: A spreadsheet program for determining water flow requirements for all the identified air coolers in Step 1 was used. Based on the water flow requirements, the pipe reticulation system was sized and the water temperature increases [due to pipe friction and heat transfer between ventilation air and chilled water] determined. The updated inlet water temperatures to the air coolers were used to update the water flow requirements for all air coolers.

Heat transfer between ventilation air and water provides some useful cooling and Step 1 was repeated with the useful cooling included. The above steps were repeated until the updated and previous values for air cooler duties, and water temperatures were similar.

STEP 3: A refrigeration simulator [Bluhm, et al., 2000] that was developed for DEEPMINE studies was used to determine the size of the underground refrigeration plant. Both return ventilation air and water that must be pumped out of the mine were used for heat rejection purposes. It was assumed that the balance of the refrigeration requirements was provided by surface installations using icemakers.

STEP 4: The capital and operating costs for ventilation system, refrigeration plant and chilled water reticulation system were determined.

STRATEGIES FOR VENTILATING AND COOLING ULTRA-DEEP MINES

Controlled Recirculation of Ventilation Air

For the model mine, various controlled recirculation systems were considered. The most appropriate system was a primary controlled recirculation system in which air was recirculated around the tertiary shaft. A fraction of the return air from the workings is mixed with the fresh air from surface at the top of the tertiary shaft [3 500 m below surface]. The major savings associated with this system arose from:

• Less fan power requirements since airflow in the main [surface to 2 000 m] and secondary [2 000 m to 3 500 m] shafts is reduced.

• Less cooling requirements since airflow from surface is reduced and hence the autocompression heat load is also reduced.

For these depths, the recirculation fraction should be as high as possible [at least 30%] and the most appropriate recirculation fraction will be determined by practical considerations such as sufficient air for removal of blast contaminants rather than cooling costs.

Surface Bulk Air Cooling

A surface bulk air cooler [BAC] should be used to cool all air that is sent underground. In spite of increased heat pick-up from rock surfaces and leakage of cooled air to return, it is recommended that air is cooled to, as low a temperature as practical. A temperature of 10°C is a conservative practical mixed minimum temperature for cooling air on surface.

Underground Bulk Air Cooling

As air flows underground the wet-bulb temperature increases by about 4°C per 1 000 m of depth due to autocompression. Thus for a mining depth of 4 000 m the temperature increase due to autocompression is about 16°C. Hence in ultra-deep mines it will be necessary to re-cool air as it travels underground. For the model mine a convenient location for re-cooling the air would be at the top of the tertiary shaft, [3 500 m below surface].

For the model mine it will be necessary to install two BAC's at the top of tertiary shaft; one to cool recirculated air and one to cool downcast air. It was shown that about 20°C is the optimum temperature that air should be cooled to. The air coolers should be multi-stage high efficiency spray chambers. If air is cooled to lower temperatures then cooling losses due to cooled air leakage and heat flow from rock surfaces in the tunnels will increase. It should be noted however that the cost of multi-stage spray chambers is much less than airway coolers [see below] and more airway coolers would be needed if the duty of the multi-stage spray chambers were reduced.

Airway Coolers

Although air is cooled at 3 500 m it will be necessary to re-cool air in the intake airways as it flows towards the workings. In order to minimise the heat flow from rock surfaces in intake airways, the outlet air temperature from airway coolers should be relatively high, typically about 24°C. These air coolers could be open-circuit air coolers in which cooling water is allowed to flow over standard cooling tower `fill' and ventilation air is blown through the fill. Alternatively closed-circuit air coolers could be used in which cooling water flows through a bundle of finned tubes and air to be cooled is blown across the tube bundle.

Pre-cooling of Air at Stope Entrances

A major problem associated with providing cooling in ultra-deep mines will be to provide sufficient cooling within stoping areas. The practical problems and costs associated with providing cooling within stopes will be much higher than for airways. Therefore to reduce in-stope cooling requirements, air should be cooled in stope crosscuts to as low a temperature as practical. A temperature of 20°C is regarded as the minimum practical temperature.

In-Stope Cooling

For ultra-deep mines it is unlikely that all the required in-stope cooling can be provided by chilled service water [Funnell and Sheer, 2001]. For a service water consumption of 1 ton of water per ton of rock mined, chilled service water will provide about 25% of the required in-stope cooling. To provide additional in-stope cooling, chilled water must be used either as part of the mining operation or in formal air coolers. It has been shown [Funnell and Sheer, 2001] that additional chilled water should be used in formal in-stope air coolers and not as extra service water.

The recommendations are that mining water should be supplied to stopes as cold as practical [about 12°C] and that any additional cooling should be supplied by in-stope air coolers. [The development of in-stope air coolers is being undertaken as part of the DEEPMINE research programme.]

SUMMARY OF MINE HEAT LOADS
AND COSTS

Using the above guidelines, ventilation and cooling requirements have been determined and costed for the model mine.

The overall mine heat load for the model mine is 105 MW of which over two thirds of the heat load is in the intake airway system [shafts, stations and tunnels] whereas stope heat loads are less than 20 % of the total. For current mining depths, stope heat loads are about 35 % of the total mine heat loads. The refrigeration requirements to satisfy these mining heat loads are shown in Figure 4

Figure 3. Breakdown of mine heat loads
for model mine

Figure 4. Mine cooling requirements

The difference between the refrigeration requirements and mine heat load is due to `cooling losses'. Cooling losses are heat pick-up in the cooling reticulation system that does not provide useful cooling [pipe friction, decreases in potential energy, heat pick-up in dams, etc.]. It is important that these losses are kept to a minimum. The capital and operating costs for the ventilation and cooling are given in Figures 5 & 6.

The major capital costs comprise refrigeration plants [icemakers, surface & underground refrigeration plants] whereas the major operating costs are power for pumps and fans.

The operating costs are given as a Present Value costs over the life-of-mine.

INEFFICIENCIES IN VENTILATION AND COOLING SYSTEMS

For the model mine, 80 potential air leakage paths, where intake air could leak to return were identified and the effects of changes in air leakage on costs were determined. For each potential leakage path three classes of air leakage were defined:

• Low leakage where considerable effort is made to minimise all air leakages.

• Average leakage [used in the analysis for the model mine] which is typical for many existing deep mines.

• High leakage where there is poor ventilation control.

The ventilation and cooling costs were calculated for all three leakage classes. If effort is made to ensure that all air leakages are minimised then the saving in total costs [capital and operating] would be about 5% [US $10 million]. On the other hand if ventilation controls are not maintained and there is no dedicated effort to minimise air leakage then total costs could increase by about 15% [US $30 million]. This example shows that for ultra-deep mines it will be essential to maintain good ventilation controls and a policy to minimise all air leakages must be adopted.

Figure 5. Capital costs for model mine

Figure 6. Operating costs for model mine

The efficiency of cooling distribution systems by chilled water is dependant on keeping the water as cold as possible for use at pre-determined strategic sites such as air coolers and for in-stope mining use. To minimise heat gains from external sources, pipes are insulated and the design of cold water dams must be carefully considered.

In the present analysis it was assumed that all pipes were `optimally' sized and that insulation was typical of a 'good' system. If insulation is damaged or missing then heat gains in the reticulation system can increase by an order of magnitude. Therefore it will be essential for ultra deep mines to adopt a maintenance programme that ensures that all damaged pipe insulation is replaced as soon as possible.

CYCLICAL OPERATION OF VENTILATION
AND COOLING SYSTEMS

The general approach on South African gold mines is to ventilate and cool, all-of-the-mine, all-of-the-time. Since mining operations are cyclical there could be significant benefits in reducing ventilation and cooling during non-working periods of the day.

For the model mine it was assumed that there was a six-hour period each day in which there were no workers in the stopes. For this six-hour period it was assumed that:

• Downcast air from surface was reduced by 30%

• [two of the four main surface fans were assumed to be switched off].

• Underground controlled recirculation system was switched off.

• Cooling duties of the surface and underground bulk air coolers were reduced in proportion to the reduced airflow.

• All airway coolers were turned off.

In order to analyse heat flow from rock surfaces due to the cyclical operation of the ventilation and cooling systems a finite element computer program was used. To simulate the heating and cooling of the rock surface skin it was necessary to model the cyclical operation over a ten-year period. Since cooling was switched off for a period of six hours each day it was necessary to install slightly larger air coolers [10 % larger than for continuous operation] to achieve similar average temperatures within the stopes.

By adopting a cyclical ventilation and cooling strategy, there is a potential saving in capital cost of 10 % and in operating cost of 12% [overall potential saving in total cost is 11%]. Thus the total savings with this strategy are more than US$ 20 million.

The major savings in capital costs are due to smaller refrigeration machines [water chillers and icemakers] and smaller pump stations. Although the cooling is reduced for six hours daily the refrigeration machines operate continuously and cooling is stored in surface and underground dams.

The major savings in operating costs are due to less electrical power requirements for fans, refrigeration machines, [water chillers and icemakers] and pumps for returning cooling water to surface.

CONCLUSIONS

The ventilation and cooling costs to provide acceptable environmental conditions for ultra-deep mines will be extremely high even when the most cost-effective cooling strategies are adopted.

The most cost-effective systems will involve:

• Controlled recirculation of ventilation air.

• Use of both surface and underground refrigeration installations.

• Surface refrigeration installations with pre-cooling towers to initially cool the water from underground, water chillers and icemakers.

• Bulk cooling of the air both on surface and at a suitable underground location [for example, top of tertiary shaft].

• All shortfalls in cooling requirements for stopes must be provided by air coolers that are installed within the stopes. Normal chilled mining water usage will not be able to provide sufficient cooling to meet the in-stope cooling requirements.

• Minimise all cooling losses.

• Examine cyclical operations of ventilation and cooling systems. The savings will depend on mining activity but could be of the order of 10% of ventilation and cooling costs.

ACKNOWLEDGEMENTS

This paper has included results from the DEEPMINE collaborative research programme, this is acknowledged and thanks are recorded.

REFERENCES

Sheer, T.J., Butterworth, M.D. and Ramsden. R., 2001 “Ice as a coolant for deep mines” Proceedings, Seventh International Mine Ventilation Congress, Cracow, Poland

Bluhm, S.J, Hattingh R, Funnell R, Butterworth MD, Sheer TJ and Hemp R 2000 “Generation and distribution of refrigeration for ultra -deep mining: new challenges and insights” Proceedings, FRIGAIR 2000 Congress, SAIRAC, Midrand, March

Shone, R.D.C. and Sheer, T.J., 1988, “An overview of research into the use of ice for cooling deep mines” Proceedings, Fourth International Mine Ventilation Congress, Brisbane, Australia

Bluhm, S.J., Biffi M., and Wilson R., 1998, “Optimised cooling systems for mining at extreme depths” Proceedings of CIM/CMMI/MIGA conference, Canadian Institute of Mining and Metallurgy, Montreal, Canada

Funnell, R.C. and Sheer, T.J., 2001 “Optimisation of cooling resources in deep stopes” Proceedings, Seventh International Mine Ventilation Congress, Cracow, Poland

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PROCEEDINGS OF THE 7^TH INTERNATIONAL MINE VENTILATION CONGRESS

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DESIGN AND SIMULATION OF ULTRA-DEEP MINE COOLING SYSTEMS

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