Quantifying the impact of water
mismanagement on latent heat in
deep-level mines
J Swanepoel
orcid.org/0000-0002-0216-5680
Dissertation submitted for the degree
Master of Engineering
in
Mechanical Engineering
at the North-West University
Supervisor:
Dr Johan Marais
Graduation:
November 2019
| Acknowledgements ii
ACKNOWLEDGEMENTS
As the author of this study I would like to show my gratitude by thanking persons that have had any influence on the completion of this research study:
• Prof Eddie Mathews and Dr Marius Kleingeld for this unique opportunity to do my post graduate studies while developing my engineering skills in the industry. Enermanage (Pty) Ltd thank you for the financial support to finish my studies.
• I would like to thank my parents, Johan and Caron Swanepoel, for giving the love and support the gave me. I am privileged to have parents that sacrifice so much for me.
• To my study leader, Johan Marais, for giving me guidance to deliver the best work possible.
• To Kristy and Diaan Nell. Thank you for the knowledge you passed on and guidance during the research study. It is highly appreciated.
• A special thanks to Juanita du Preez, my fiancée. Thank you for believing in me and supporting me through the whole process. Thank you for understanding when I had to work late and the time spent with you was little. I love you very much.
• My Heavenly Father, thank You for giving me the ability to complete my studies. This would not have been possible without Your guidance. Thank You for all Your unconditional love.
| Abstract iii
ABSTRACT
Title: Quantifying the impact of water mismanagement in deep-level mines Author: J. Swanepoel
Promoter: Dr J. Marais
Keywords: Deep-level mine, latent heat, water mismanagement, energy management, water management cooling, high temperatures
South African mines are increasing in depth and temperature. This temperature increase has a negative effect on an employee’s ability to perform their daily tasks underground which directly affects the mines’ production.
Water mismanagement leads to increased water on the footwall. This increase in water contributes to the increase in latent heat pick-up in the airstream.
The need of the study is to quantify the latent heat pick-up in deep-level mines due to water mismanagement. A universal method was developed to quantify the latent heat and translate it to the cost implications of supplying additional cooling to mitigate this latent heat.
The methodology is applied to two real-world case studies (deep-level gold mines). Case study 1 was a conventional mine, and the results show that for every 1 𝑚2 of water, R276 000 of additional cooling is required annually to counteract latent heat pick-up. Case study 2 was a mechanised mine, and the results show that for every 1 𝑚2 of water, R1.5M of additional cooling is required annually to counteract latent heat pick-up.
Water mismanagement was addressed in Case study 2 and the effect was evaluated. This resulted in a wet-bulb temperature decrease of 4.2°C which is equivalent to an additional cost of cooling of R912 500. This indicates the importance of addressing water mismanagement in deep-level mines for both costs saving as well as production benefits.
| iv
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ... II
ABSTRACT ... III
TABLE OF CONTENTS ... IV
LIST OF FIGURES... VI
LIST OF TABLES ... VIII
LIST OF EQUATIONS ... IX
LIST OF ABBREVIATIONS ... X
LIST OF SYMBOLS ... XI
GLOSSARY ... XII
1.
INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES ... 1
1.1 Background to the South African gold industry ... 1
1.2 Heat in underground environments ... 4
1.3 Importance of water in mines ... 10
1.4 Mine cooling to reduce heat loads ... 13
1.5 Previous studies ... 15
1.6 Problem statement and need for the study ... 18
1.7 Study objectives ... 18
1.8 Conclusion ... 18
2.
METHODOLOGY ... 20
2.1 Preamble ... 20
2.2 Step 1: Identify the limitations ... 20
2.3 Step 2: Gather the data ... 24
2.4 Step 3: Simulation ... 30
2.5 Step 4: Generate the graph ... 35
2.6 Handling mismanagement of water ... 37
2.7 Verification of methodology ... 37
2.8 Conclusion ... 38
| Table of contents v
3.1 Introduction ... 39
3.2 Case study 1: Conventional mine ... 39
3.3 Case study 2: Mechanised mine ... 50
3.4 Implementation and validation ... 57
3.5 Results discussion ... 63
3.6 Conclusion ... 64
4.
CONCLUSION AND RECOMMENDATIONS... 65
4.1 Study summary ... 65
4.2 Recommendations for future work ... 66
REFERENCES ... 67
APPENDIX A: EXPANSION OF THE PREVIOUS STUDIES ... 73
APPENDIX B: PSYCHOMETRIC CHART ... 86
APPENDIX C: PRACTICAL PRESENTATION OF DATA ... 87
APPENDIX D: CASE STUDY 1 DATA ... 88
APPENDIX E: CASE STUDY 2 DATA ... 92
| List of Figures vi
LIST OF FIGURES
Figure 1-1: Gold price from 2011 to 2019 (Rand per kg) [4]. ... 1
Figure 1-2: South African gold production (2008 - 2017) [3]. ... 2
Figure 1-3: Electrical users in a mine [8]. ... 3
Figure 1-4: Annual average price of electrical power supplied by Eskom [8]. ... 3
Figure 1-5: Relationship between the wet-bulb temperature and the productivity losses [14]. .... 8
Figure 1-6: Explanation of how latent heat influences the airstream [32]. ... 9
Figure 1-7: Drilling in the stopes [39]. ... 10
Figure 1-8: Cooling of rock after blasting [39]. ... 10
Figure 1-9: Water cannon used in the stopes [39]. ... 10
Figure 1-10: A picture of a pipe that burst and water is leaking out [39]. ... 12
Figure 1-11: A hose that was left open after the miners finished working [39]. ... 12
Figure 1-12: An example of a cooling car [39]. ... 13
Figure 2-1: The four methodology steps to determine the cost of cooling. ... 20
Figure 2-2: Step 1 of the methodology (including the sub-steps). ... 21
Figure 2-3: Schematic representation of a conventional mine. ... 22
Figure 2-4: Schematic representation of a mechanised mine. ... 22
Figure 2-5: Step 2 of the methodology (Including sub-steps) ... 25
Figure 2-6: Example of a boundary with A (inlet) and B (outlet). ... 26
Figure 2-7: The pathway of a Davis vane when the transverse method is followed [35]. .. 26
Figure 2-8: Wetness factor in PTB ... 32
Figure 2-10: The developed methodology with all the steps (including sub-steps) ... 38
Figure 3-1: Step 1 of the methodology - Identifying the limitations. ... 40
Figure 3-2: Layout X with the boundary inlet (A) and outlet (B) (CS1). ... 40
Figure 3-3: Step 2 of the methodology - Gather the data. ... 41
Figure 3-4: Step 3 of the methodology - PTB Simulation. ... 44
| List of Figures vii
Figure 3-6: Step 4 of the methodology - Generate the graph. ... 47
Figure 3-7: Graph generated for Case study 1 - Cost of cooling vs. area of water. ... 50
Figure 3-8: Unconventional mining schematic (CS2). ... 51
Figure 3-9: PTB model of the refrigeration system (CS2) ... 51
Figure 3-10: Boundary inlet (point A) and boundary outlet (point B) (CS2) ... 52
Figure 3-11: The graph generated for Case study 2 - Cost of cooling vs. area of water (CS2). 56 Figure 3-12: Investigated Level Z airflow layout. ... 58
Figure 3-13: The graph with the point that was read off for the water on the footwall. ... 59
Figure 3-14: Layout on Level Z data loggers installed at A and B. ... 60
| List of Tables viii
LIST OF TABLES
Table 1-1: Recommended WBGT limit for workloads [22]. ... 6
Table 1-2: A summary of all the literature studies and the applicable keywords. ... 17
Table 2-1: Water classification's example and definition ... 24
Table 2-2: Example of a table used in the control room to track the water ... 28
Table 2-3: Summary of the data to be collected in step 2. ... 30
Table 2-4: PTB Simulation components, A) AirPressureBoundary, B) AirNode, C) AirTunnel, D) AirMassFlow ... 31
Table 3-1: The ventilation survey data collected at point A and point B (CS1). ... 42
Table 3-2: Data collected to track the water on Level X (CS1). ... 42
Table 3-3: Individual COPs of the refrigeration plants (CS1). ... 43
Table 3-4: Summary of all the parameters gathered in Step 2 (CS1). ... 44
Table 3-5: The wetness factors used during the different iterations (CS1). ... 46
Table 3-6: Change in enthalpy each air mass flow rate - wetness factor of 0.25 (CS1). ... 46
Table 3-7: Cooling duty for each air mass flow rate - wetness factor of 0.25 (CS1). ... 47
Table 3-8: Electrical power for each air mass flow rate - wetness factor of 0.25 (CS1). ... 48
Table 3-9: Cost of cooling for each air mass flow rate - wetness factor of 0.25 (CS1). ... 48
Table 3-10: Area corresponding to the wetness factor (CS1). ... 49
Table 3-11: Individual COP’s of the refrigeration plants (CS2). ... 53
Table 3-12: Summary of parameters gathered in Step 2 (CS2). ... 54
Table 3-13: Change in enthalpy each air mass flow rate - wetness factor of 0.25 (CS2). ... 55
Table 3-14: Impact of water on the footwall. ... 60
| List of Equations ix
LIST OF EQUATIONS
Equation 1-1: Calculating the WBGT without a solar load [25]. ... 6
Equation 1-2: Relationship between decrease in work performance and temperature [20]. ... 7
Equation 1-3: Determining the cost of cooling [16]. ... 14
Equation 1-4: Determining the COP of a refrigeration plant [35]. ... 14
Equation 2-1: Calculating the air density from psychrometric chart [35]. ... 27
Equation 2-4: Calculating the air mass flow rate [35]. ... 27
Equation 2-5: Calculating the COP of a refrigeration plant [35]. ... 28
Equation 2-6: Calculating the length of the simulation haulage. ... 31
Equation 2-7: Calculate the cooling duty from the change of enthalpy [35]. ... 35
Equation 2-8: Calculating the electrical power of the cooling system based on the cooling duty and COP [20]. ... 35
Equation 2-9: Calculation of the cost of cooling for a day [16]. ... 36
| List of Abbreviations x
LIST OF ABBREVIATIONS
SA South Africa
DMR Department of Mineral resources VRT Virgin rock temperature
WBGT Wet-bulb globe temperature
BAC Bulk air cooler
COP Coefficient of performance
OCOP Overall coefficient of performance
| List of Symbols xi
LIST OF SYMBOLS
Symbol Description Units
𝐴 Area 𝑚2
𝐶𝑝 Specific heat of water
𝑘𝐽 𝑘𝑔∙℃
∆𝐻 Change in enthalpy −
ℎ Height 𝑚
𝑚̇ Mass flow rate 𝑘𝑔
𝑠 𝑝𝑎𝑖𝑟 Air density 𝑘𝑔 𝑚3 𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 Electric power 𝑘𝑊 𝑄̇ Cooling duty 𝑘𝑊
𝑄𝑒𝑣𝑎𝑝 Evaporator heat transfer 𝑘𝑊
𝑇1 Evaporator inlet temperature °C
𝑇2 Evaporator outlet temperature °C
𝑇𝑒𝑓𝑓 Effective temperature °C
𝑇𝐺 Globe temperature °C
𝑇𝑊 Wet-bulb temperature °C
𝑇𝑤𝑎𝑡𝑒𝑟 𝑡𝑜𝑡𝑎𝑙 Total area of water on the footwall 𝑚2
𝑡 Time 𝑠
𝑉 Velocity 𝑚
𝑠
𝑣 Specific volume 𝑚3
𝑘𝑔
𝑊 Total electrical power 𝑘𝑊
𝑤 Width 𝑚
𝑊𝐴 𝑊𝑎𝑡𝑒𝑟 𝑎𝑟𝑒𝑎 𝑚2
WP Work performance %
| Glossary xii
GLOSSARY
“Blasting” Using explosives to break-up the rock formation underground and advance further with a haulage.
“Chilled water“ Cold water coming from bulk air coolers in the mine and are used as an example for cooling and drilling.
“Deep-level mine” Underground mines that extend underground more than 1.5km. “Fissure water” Water that flows into the mine from old working areas and
underground dams.
“Fridge plant” Balk air coolers that uses a gas as heat transfer agent to produce chilled water.
“Footwall “ The ground level of a haulage .
“Geothermal sources” Heat that originates from the surrounding rock. “Haulage” A underground tunnel surrounded by solid rock.
“Mining operations“ All the critical operations in a mine that is needed to operate a mine efficiently. This includes the ventilation, mining and engineering. “Narrow reef access” Access to the stopes where narrow-reef drilling takes place. “Narrow-reef drilling” Using drills to drill holes in the narrow reef that contains high grade
gold, where explosives are inserted. “Ore” Gold rich rocks that are broken into pieces.
“Reticulation system” The whole water system in the mine that includes the supplying of the water and the dewatering.
“Skid dam” A portable dam that is used to collect water from varies sources before the water is pumped to a central dam.
“Stopes” The area where narrow-reef drilling takes place.
“Trackless mining” A mining design that uses machinery to mine which does not use rail-road tracks.
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 1
1. INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL
MINES
1.1 Background to the South African gold industry
The South African gold mining industry was seen as one the wealthiest in the world [1]. In 1886, the first gold deposits were found in Johannesburg1. Johannesburg is situated in the
Witwatersrand basin, which is known for the vast amounts of gold mined in the past century. The Witwatersrand contributes to almost half of world’s gold production and still has an abundance of gold hidden in its vanes [1]. South Africa’s development is due to the rush of gold prospectors, where supporting industries started to develop.
The gold price increased over the past few years, which is beneficial for the gold mining sector as well as the economy of South Africa (SA). Figure 1-1 presents the price of gold from 2011 to 2019, in Rand per kilogram.
Figure 1-1: Gold price from 2011 to 2019 (Rand per kg) [4]. 1.1.1 Gold mine production
SA was previously one of the largest gold producers in the world. However, production started to decrease, and SA was ranked 8th in the world in 2018. Presently, China is the largest gold
producer in the world with 426 tonnes of gold per year and Australia is ranked second with
1 “Mining in SA: Then, now, and into the future“, Politicsweb, 15 February 2017
267 363 403 350 391 461 448 453 651 0 100 200 300 400 500 600 700 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Pr ic e ( R /kg) Year
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 2
295 tonnes in 20182 [2]. During the last ten years, there has been a definite decline in gold
production in SA, which can be seen in Figure 1-2.
Figure 1-2: South African gold production (2008 - 2017) [3].
In 2016, the average cost of gold production worldwide was $649/oz which increased to $672/oz in 2017. The reason for this includes an increase in fuel prices, lower gold grade, and fluctuations in currencies. SA’s cost of production was already higher than the average world production cost, but in 2017 it increased even higher. In 2016, SA’s production cost was $857/oz. In 2017 the cost increased to one of the highest costs of production per country of $1010/oz, which is an increase of $153/oz over a year [3]. This production cost increase is due to the decrease in the value of the Rand, increase in electricity and fuel prices, labour disruptions, as well as an increase in the depth of the mines.
1.1.2 Mining electrical expenses
The mining sector consumes approximately 15% of the electrical power generated by Eskom, of which 47% of this is consumed by the gold mining sector specifically [4]. This is due to the depth of the mines as well as the number of gold mines in SA. Figure 1-3 represents the different electrical consumers in a typical deep-level gold mine. These consumers include material handling, processing, compressed air, pumping, ventilation, industrial cooling, lighting, and other.
2 “These are biggest gold producing countries in the world”, Businesstech, 16 July 2018
233.8 219.5 199.9 190.5 163.5 168.9 159.2 151 145.7 139.9 0 50 100 150 200 250 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Go ld p ro d u ction g o ld ( to n n e s) Year
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 3 Figure 1-3: Electrical users in a mine [8].
Cooling and mine ventilation systems are increasing electricity consumers within the mines, with operating costs also increasing significantly during the last decade [5],[6]. The cost of electricity supplied by Eskom also increased over the last 12 years. This increase has a negative impact on an electrical power-intensive sector like the gold mining sector. Figure 1-4 shows this increase in electricity cost.
Figure 1-4: Annual average price of electrical power supplied by Eskom [8]. 23% 19% 17% 14% 7% 5% 5% 10%
Material handling Processing Compressed Air Pumping Ventilation Industrial cooling Lighting Other
17.99 23.12 30.25 39.78 50.11 55.74 64.66 69.52 78.01 84.8 0 10 20 30 40 50 60 70 80 90 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Pr ic e ( c/ kWh ) Year
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 4
1.2 Heat in underground environments
Mines are increasing in depth and in temperature [7],[8]. Emphasis is placed on optimising the refrigeration underground to decrease the heat flow in deep-level mines [9]; therefore, high priority should be given to reducing heat generation in a mine [10]. Heat is the transfer of energy without exchanging matter or work [11], and can be transferred to or from a medium by using convection and radiation. Heat can be transferred through open pipes where water evaporates into the passing air [12].
High temperatures and humid conditions are a danger to mine workers’ health [7],[13] , and temperatures exceeding 33°C wet-bulb negatively affects productivity of the workers [14]. The Department of Mineral Resources (DMR) is tasked to ensure that mines adhere to the legal temperatures to prevent heat stroke or fatalities [15]. Over the last century, a significant amount of research has been done to establish heat indicators in mines. This can be used to detect how certain heat conditions impact the physical well-being of the workers’ bodies and find the ideal conditions for underground work [16]. One of these indictors is the wet-bulb temperature.
1.2.1 Heat sources
In an underground mine, the heat discharges into the environment. However, the heat stays in the system and must be handled in a controlled manner. Auto-compression, diesel machinery, mechanised equipment, and explosives are some of the sources of heat in underground mines, of which auto-compression, diesel machinery, and strata heat are the largest contributors [11], [13].
As air descends vertically into a mine, the air compresses which increases its temperature. This source is called heat generation through auto-compression. In trackless mining, diesel machinery is one of the main contributors to heat. Diesel equipment adds sensible as well as latent heat to the environment. Studies reveal that electrical equipment has an efficiency of approximately three times higher than that of diesel equipment and is, therefore, the better option to use [9]. Strata heat is the increase of the virgin rock temperature (VRT) as the mine gets deeper [16]. Water is also seen as a source of heat due to its ability to adapt to the temperature of the surrounding rock. Water adds latent heat more easily to the passing air than to the adjacent rock [11]. 1.2.2 Influence of heat on mine workers
The underground environment has a significant impact on the mine workers’ health and performance [17]. Since every human body is different, a specific temperature cannot be given where the whole workforce is comfortable [18]. One of the ways the human body regulates its temperature is by the evaporation of sweat from the skin [16]. In an environment with high relative humidity conditions, heat exchange between the air and one’s skin is difficult because the air is
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 5
almost saturated [18]. The sweat cannot evaporate into the passing air since the air is already full of moisture; and as a result, the body will start to overheat [10].
Fatigue is also one of the consequences of working in hot and humid conditions. A fatigued worker has reduced alertness and lower coordination [19]. A study done by Wagner [20] revealed that the most effective temperature to work at is 28°C. If the temperature exceeds 28°C, the efficiency of the miners’ physical work decreases. No work can safely be performed at 36°C [20].
1.2.3 Heat stress
Heat stress occurs when the body’s temperature is increasing and cannot transfer excess heat to the environment. The body cannot regulate its core temperature and this causes physical and mental stress. The internal temperature of the human body should not exceed 38°C for long periods when working in high-temperature areas. The human body uses two methods to regulate its internal temperature; evaporation and convection. Evaporation occurs when the sweat on the skin evaporates, while convection is the transfer of heat to a liquid such as water and air [19]. Heat stress effects can range from heat rashes to heat cramps and heat exhaustion. Severe effects include heat strokes, which is when the internal body temperature exceeds 40°C, which may lead to fatality. Heat stress can be reduced when the miners avoid or decrease their exposure to high-temperature areas [16]. The comfort of humans is measured by the wet-bulb globe temperature (WBGT), skin temperature, the body’s heat storage, wetness, and skin temperature [21].
Heat stress indicators
The heat stress index is a measurement that quantifies the heat stress in a thermal environment by integrating different factors affecting the body’s heat transfer to the environment [22]. The ISO 7933:2004 recommends limits by using parameters of humidity, airflow velocity, radiation, air temperature, clothing insulation, and metabolic rate to predict physiological conditions. High-temperature areas can be improved and accidents reduced by recognising a heat stress environment with the use of heat stress indices [19].These parameters evaluate the thermal environment and not the miner’s thermal status [23],[24].
The dry-bulb temperature was one of the first heat stress indices, although it is not suitable to be used on its own in hot and humid conditions. By using the dry-bulb temperature in conjunction with the wet-bulb temperature, the underground environment’s cooling power as well as the humidity of the air can be calculated which provides the ventilation air’s ability to absorb water [20]. The airflow velocity plays an important role in thermal comfort improvement with limits determined and mandated [10].
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 6
The WBGT is one of the most used heat stress indices, and was developed by the military to prevent heat-related casualties during training [25]. The WBGT without a solar load is given in Equation 1-1 with a bias of 70 weighed factor to the wet-bulb temperature [26].
Equation 1-1: Calculating the WBGT without a solar load [25]. 𝑊𝐵𝐺𝑇 = 0.7𝑇𝑊+ 0.3𝑇𝐺
Where:
WBGT = Wet bulb globe temperature (°C) 𝑇𝑊 = Wet-bulb temperature (°C)
𝑇𝐺 = Globe temperature (°C)
ISO 7243 standard suggests using the WBGT to asses hot environments [22].
Lower impact of heat on workers
Heat stress management plans need to be in place to deal with heat stress situations and future problems that may arise [19]. One of the plans to manage heat stress is the implementation of heat stress avoidance measurements by reducing heat stress contact time. Implementation of this plan is inexpensive but has a great impact on production [16]. Table 1-1 identifies recommended limits in terms of the WBGT to perform certain workloads safely at two different air velocity ranges [22].
Table 1-1: Recommended WBGT limit for workloads [22].
Workload WBGT Velocity ˂ 1.5 𝒎/𝒔 Velocity ˃ 1.5 𝒎/𝒔 Light 30.0°C 32.2°C Moderate 27.8°C 30.6°C Heavy 26.1°C 28.9°C 1.2.4 Wet-bulb temperature
The wet-bulb temperature is based on the humidity of the air and heat content to give the suitability of the mine conditions [27]. It provides accuracy and practicality as a heat stress component [22]. The legal wet-bulb temperature limit to work underground is 32.5°C [28],[29]. A 1°C wet-bulb temperature increase has the same physiological impact as a rise of 5°C in the dry-bulb temperature [30]. There is a decline in production and a significant increase in the rate of accidents if there is an increase from 25°C to above 32.5°C in bulb temperature. The
wet-Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 7 bulb temperature is also used to determine the air-cooling power that gives the air’s worker-cooling-capacity [29].
A relative humidity above 80% makes heat transfer on the skin by evaporation moisture ineffective as a cooling method, as the air is almost saturated [10]. The wet-bulb temperature can be lowered by removing the moisture from the air in a process called dehumidification. During this process, the relative humidity decreases as the wet-bulb and dry-bulb temperature gap increases [31],[32]. Research done on Crown mine stated that when the water in the stopes was controlled, the wet-bulb temperature was reduced [33].
1.2.5 The relationship between production and heat
The work performance in a mine decreases and the number of accidents increases in a heat stress environment [13]. Physical work performance is optimal up until a temperature of 28°C. Above this, the decrease in physical work performance can be described by Equation 1-2. Equation 1-2: Relationship between decrease in work performance and temperature [20].
𝑊𝑃 = 100 − (𝑇𝑒𝑓𝑓− 28)2.25 Where:
𝑊𝑃 = Work performance (%)
𝑇𝑒𝑓𝑓 = Effective temperature (°C)
According to Equation 1-2, at a temperature of 36°C, work is no longer possible [20].
The relationship between the percentage productivity loss and the wet-bulb temperature in a gold mine is shown in Figure 1-5. The relationship was established when Whillier [14] saw that the hot mines’ environment negatively influenced productivity.
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 8 Figure 1-5: Relationship between the wet-bulb temperature and the productivity losses
[14].
Other studies done on the influence of the wet-bulb temperature stated that a change of 1°C could negatively affect productivity by 3%. The percentage should increase significantly by a wet-bulb temperature above 30°C. Financial prospect is a priority in mines, but there must be a trade-off between higher productivity and the cost of additional refrigeration. If the temperatures in the working areas improve, the productivity should rise. [34]
1.2.6 Enthalpy
Enthalpy (H) is the energy of the dry air and has the unit of kJ/kg dry air [16]. Change in enthalpy is the energy lost to or gained from the environment. During a constant pressure process, the heat loss to the environment is the change in enthalpy. The total energy added to the airflows between the inlet and outlet can be calculated from the change in enthalpy [35].
1.2.7 Heat components
If the moisture content of air stays the same and the dry-bulb temperature increases, only sensible heat is added to the air. If the dry-bulb temperature stays constant and the wet-bulb temperature increases, only latent heat is added to the air [35]. In a study by Hemp [36], the heat transfer rate to the air increased by 14.7% when the footwall was wet compared to when the footwall was dry.
1.2.8 Latent heat
Heat is transferred from the water to the air through evaporation and increases the air’s latent heat [11]. Energy is removed with the water molecules when it leaves a wet surface, which transfers “latent heat” to the air [35]. The water temperature increases due to geothermal sources.
0 10 20 30 40 50 60 70 80 90 100 22 24 26 28 30 32 34 Pr o d u ction lo ss (% ) Wet-bulb temperature (°C)
𝑦 = 0.0383𝑥
3-1.8787𝑥
2+23.67𝑥
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 9
Mine water, as well as service water, can increase in temperature because of interactions with the surrounding rock [11].
In this example the air enters the haulage with a higher dry-bulb temperature as the temperature of the water. Sensible heat transfer happens from the air to the water; this will result in a decrease in the dry-bulb temperature. The water will transfer latent heat to the air due to the molecules that escape from the water surface [37]. The process will continue until the dry- and wet-bulb temperatures becomes equal, and saturation is met. This concept is represented in Figure 1-6.
Figure 1-6: Explanation of how latent heat influences the airstream [32].
The heat transfer is balanced by the exchange of the sensible and latent heat; thus, there is no heat added or lost. The mass flow increases by the molecules that are added to the air which results in the change in enthalpy [35]. Conversely this is also true; when the water temperature is higher than the air temperature, latent heat, as well as sensible heat, is transferred to the air. The dry-bulb and wet-bulb temperature becomes equal at a higher temperature; the same temperature as the water. To calculate the energy pick-up of the air flowing over a water surface, the change in enthalpy must be calculated [16].
The heat transfer rate and rise in the wet-bulb temperature depend on the temperature difference between the wet-bulb temperature and the rock or water [38]. The air flowing over the exposed water surface area governs the rate of evaporation and not the quantity of water [27]. The human body uses evaporation (through sweat) to cool down in high-humidity and high-temperature conditions; thus, evaporation is limited because when the air is saturated such cooling cannot take place [22].
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 10
1.3 Importance of water in mines
One of the most important processes to extract ore is water distribution in gold mining. Water consumption is directly proportional to the amount of gold extracted [39]. The environment can be controlled by strict managing of the evaporation of water moisture into the airstream or installing refrigeration plants to cool the air and produce chilled service water [27],[40]. In this section, the users and the impact of water in mines will be discussed.
1.3.1 Water users
Cold water from cooling plants are used all over the mine and is called the service water. In the stopes where most of the mining occurs, service water is used for drilling (shown in Figure 1-7).
Figure 1-7: Drilling in the stopes [39].
The cooling water is also used to cool the rock after blasting, seen in Figure 1-8 [39]. A water cannon is used in the stope to move fine particles, with some gold particles, to the loading box. The water cannon in use can be seen in Figure 1-9 [39].
Figure 1-8: Cooling of rock after blasting [39].
Figure 1-9: Water cannon used in the stopes [39].
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 11 Other users of the service water include haulage sprays, stope atomising sprays, as well as development water blasts that are used as a dust prevention system. The service water is also used to clean the shaft area as a dust-prevention strategy. [41]
1.3.2 Dewatering of a mine
Dewatering is the transporting of water underground out of the mine; some of the methods used are sump pumps, boreholes, drainage galleries, stormwater control, and audits [42],[43] . The cost to pump water to the surface for cooling can be high in deep-level mines [20]. Dewatering is one side of the water reticulation system. The reticulation system circulates the service water for cooling purposes and mining activities; it also collects the water to be used again for cooling [39]. One of the important aspects of dewatering a mine is to minimise the contact time of the air with the water that was heated during use [44].
1.3.3 Water used as cooling
Chilled water is provided by a refrigeration plant that is situated either on the surface or underground [13]. The chilled water travels to the work areas where it flows through a heat exchanger or comes directly in contact with the air. Water can return to the plant through pipes, but most of the time the used cooling water is dropped onto the ground, returning to the refrigeration plants via the dewatering network. The chilled water temperature is below that of the wet-bulb temperature. A cooling effect is provided to the airflow when the condensate absorbs the latent and sensible heat [35]. Although it is economically the best way to counter heat by piping chilled water to energy transfer units [45], the difficulties lie in the pipelines to the units [44]. 1.3.4 The impact of water on the environmental conditions
The temperature difference of the air and the water determine the rate of heat transfer as well as the water transport method; piped or open channel. Contact between air and water must be avoided, because when the wet-bulb temperature of the air is lower than that of the water, heat transfer will take place via evaporation [35]. In 1952, in the Witwatersrand basin, a test at Crown mine indicated that the reduction of water in the stopes lowered the wet-bulb temperature [33]. Some mines controlled the evaporation of underground water in a bid to manage the high temperature of the rock, while neighbouring mines had expensive refrigeration plants to do the same [20]. The effect of a refrigeration plant can be naturalised at the working areas if care is not taken to prevent the evaporation of water into the air stream [27].
Fissure water can have the same temperature as the VRT or in some cases, even 6°C higher. This can make fissure water one of the main heat loads in a mine [46], [47]. Sharp temperature increases and increased humidity can be the results of fissure water [31].
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 12 1.3.5 Water wastage and water contact with air
Water wastages happen most often through leakages, when water is not able to fulfil its purpose. A piping network is used to transport water to all the users in the mine. The main causes of the leakages in the network are due to rust. Maintenance on the pipes prevents cracks from forming and ensure they get fixed before water starts to leak. Figure 1-10 is an example of a leaking pipe [39]. Another reason for water wastage can be due to the bad habits of miners, which include leaving the chilled water tap open. Figure 1-11 shows hoses left open after the miners have finished working [39].
Figure 1-10: A picture of a pipe that burst and water is leaking out [39].
Figure 1-11: A hose that was left open after the miners finished working [39]. Poor water practices are one of the main concerns when it comes to water that has contact with air. Water is not channelled in an ordinary manner to the dewater system by means of a pump or enclosed audits. The used water increases rate of heat transfer rate to the inlet air because of the higher temperature of the water. [35]
When insufficient management of water distributed to the working areas happen, closed-off and abandoned areas may receive water. Although the pipes can blanked-off leakages can occur unnoticed. [48]
Measurements must be taken to restrict water contact with airflow and to dewater the water in closed pipes, restrict water flowing in the intake air, and where there are drainage channels, cover the channels with material to restrict contact between the air and water [35].
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 13
1.4 Mine cooling to reduce heat loads
At a depth below 1 600 m, health and safety requirements are difficult to meet due to ventilation alone not being able to give the necessary cooling to maintain safe temperatures [49]. The cooling system is required to remove a portion of the heat load in the mine [50]. The cooling power of the refrigeration plants is based on the dry-bulb temperature together with the wet-bulb temperature and the air velocity which has a substantial influence on heat stress. With the increase of the temperature due to auto-compression and the use of chilled water, refrigeration plants are installed underground [20]. Underground refrigeration plants are limited by the return air’s heat rejection capacity [35]. The heat loads of current and future-dated projects need to be calculated to determine the maximum cooling capacity when the plants are designed [51]. The importance and the design of mine cooling systems will be discussed (Section 1.4.1). Furthermore, information regarding the distribution of the cooling by way of ventilation will be provided.
1.4.1 Cooling methods
There are a number of different cooling methods, each with its own advantages and disadvantages that will work best for certain scenarios. A common method of cooling is the use of cooling water [7]. Some of the cooling technologies include ice slurry systems, cooling towers, bulk air coolers (BAC), and spot coolers [16]. Figure 1-12 shows an example of a closed-circuit cooling car.
Figure 1-12: An example of a cooling car [39].
To design an adequate cooling system, a critical parameter to consider is the wet-bulb temperature [14]. The heat loads and cooling in the mine need to be predicted to ensure the cooling system will deliver working conditions for maximum production at reasonable cost [52]. Strategies can be followed to reduce heat flow by using the current refrigeration system while improving its efficiency and decreasing operational costs [9],[8]. The efficiency of the cooling
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 14 systems can be improved by piping chilled service water into insulated pipes. The cooling power losses need to decrease as well as minimising air losses by reducing the contact of air with water [20]. To determine a satisfactory method to help improve the cooling system further will depend on the amount of heat flow, the working areas’ issues with heat, and the economic cost [7]. A heat management strategy has a low implementation cost and helps to lower the miners’ risk of heat stress by limiting the contact with high heat and humidity areas [14].
1.4.2 Cost of cooling
Cooling is an expensive part of mining operations and can contribute up to 25% of the total electrical power consumption of a mine [53],[6]. The decision to determine whether a cooling project needs to be implemented is evaluated by the cost of the system and the ability to give acceptable environmental conditions [22]. South African mines use the Eskom Mega Flex tariff structure, where during peak hours of the day, electricity is more expensive than during low peak periods [48]. The operations’ costs and the kWh-usage of the plant determines the cost of operations. Equation 1-3 can be used to determine the cost of the cooling plant [16].
Equation 1-3: Determining the cost of cooling [16].
𝐶𝑜𝑠𝑡 (𝑅) = 𝐸𝑛𝑒𝑟𝑔𝑦 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡𝑠 (𝑘𝑊
𝑠 ) × 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑠𝑡 ( 𝑅
𝑘𝑊) × 𝑟𝑢𝑛 𝑡𝑖𝑚𝑒 (𝑠) 1.4.3 COP (coefficient of performance)
The coefficient of performance (COP) is a value that can be used to assess the performance of a refrigeration plant [16]. The COP determines the cooling that was delivered by the plants [14]. It is calculated by dividing the discharge, the evaporator heat transfer, by the electrical power of the refrigeration unit, as seen in Equation 1-4.
Equation 1-4: Determining the COP of a refrigeration plant [35].
𝐶𝑂𝑃 = 𝑄𝑒𝑣𝑎𝑝 𝑊 Where:
𝐶𝑂𝑃 = Coefficient of performance (−)
𝑄𝑒𝑣𝑎𝑝 = Evaporator heat transfer (𝑘𝑊)
𝑊 = Total electrical power (𝑘𝑊)
The overall coefficient of performance (OCOP) is the COP of the entire system, which includes all the plants’ COPs for an overall refrigeration plant COP. The OCOP of a refrigeration plant underground can be around 2 [44].
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 15 1.4.4 Ventilation
The purpose of ventilation is to supply the mine with air that can create an environment where people can work productively without heat stress conditions. It is also needed to control the heat flow from different heat sources in the mine, like the heat transfer from the rock [20]. The air temperature and velocity are the two parts of ventilation which the ventilation officer can control in a ventilation system to manipulate the environmental conditions [54], . The demand for cooling and ventilation increases with the depth in a deep-level mine, as well as with the increase in mechanisation [5]. Temperatures at the workplace are dependent on the heat flow in tunnels from the high VRT to the air flowing across it. The heat transfer in a tunnel is controlled by the tunnel's condition; whether it is wet, dry or insulated [20]. Steed [27] stated in the early years of mining that temperature in a mine can only be controlled by one of two methods, by controlling the moisture evaporating into the air, or by using refrigeration plants.
1.5 Previous studies
After a sufficient discussion in Sections 1.1 to 1.4, previous studies have been consulted to gather the necessary insights into where there is research work in this study field. This will help in quantifying the impact of heat on the production and the cost that is necessary to combat the influence of the heat by using a cooling system. During the research review, the following focus areas, or keywords, were identified: production, latent heat, human indicators, cooling, and water usage. Focus has also been placed on whether the study applied to underground mines. Herewith the keywords’ descriptions and their relationship to this study.
Production
The main reason for a mine’s existence is to remain profitable, which is achieved by the actual ore being mined, or production. Anything that influences the production has financial implications and affects the mine’s profit. Observing heat in mines and how it influences the workers, a connection can be made to production. Literature has been researched to understand the correlation between heat and production. There is research done in other industries that shows how heat influences the efficiency of workers. The researchers allocate a percentage values to the specific environments explaining how this increases or decreases the production.
Latent heat/humidity
Latent heat is a source of heat that is not fully understood in the mining industry. The biggest contributor to latent heat is water. Information regarding latent heat and humidity’s abilities and how they are influenced by the different factors in the mining environment is important for this study to reduce the build-up of latent heat pick-up into the ventilation system.
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 16
Human indicators
People are vitally important in the mining industry. Therefore, the influence of heat on the workers is part of the research study. This will motivate the reason behind the importance of the study. There are many indicators against which the comfort levels of the miners are measured. These indicators will guide the study in determining the important measures that will need to be considered when a solution is made.
Cooling
Cooling in mines is very expensive and plays a fundamental role in making the environment in a mine workable. The cooling can be used to cool working areas, but if the system is set up the wrong way, it will be unable to use for its sole purpose of cooling. To understand the different cooling system and audit the system, information is necessary to see how the system varies from cooling a mine.
Water usage
Water has different functions in a mine, such as cooling, drilling, and cleaning of workshops. The excess water can become a problem in the working areas, and misuse of the water will have expensive consequences. During the literature study, more information surrounding the users and control of the water was researched to help determine the processes and the importance of water. Dewatering is critical to the study since water lying on the footwall has a substantial impact on ventilation.
Underground mining
The main theme in this study is underground mining; this is what distinguishes this paper from open-cast mining. Although both are mining techniques, there are differences in the environment and the problems that arise. Literature not related to underground mining was also investigated in order to better understand the problem and gain insight into how other industries solve similar problems.
Table 1-2 provides a summary of the 47 studies analysed in terms of the focus areas, followed by a discussion of each of the identified focus areas.
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 17 Table 1-2: A summary of all the literature studies and the applicable keywords.
Sources of references P ro d u ctio n L aten t heat /hu mid it y Hu man ind ica to rs Co o ling W ate r usag e Un d er g ro u n d min ing [29],[55],[56] X [9],[34] X X X [7],[31],[39],[44],[50],[57],[46] X X X [27],[45] X X X X [58],[48] X X [14],[33],[49],[59] X X [16],[54],[60] X X X [61],[62] X X X [11],[12],[36],[38],[52],[63],[64] X X [17],[18],[21],[26],[25],[23],[65] X X [10],[19],[22] X X X [20] X X X X [15] X X [66] X X [67] X X X [68] X X X X
Table 1-2 shows that most of the studies analysed is research regarding underground mining, with only two studies that do not fit into this category. Water in mining that corresponds to one of the other keywords is rare. A combination where the influence of latent heat on production is studied is not an evaluation that was conducted by many in the mining industry, but work undertaken in other studies can help bridge this gap.
There is not much research done that combines the idea that latent heat is added to the airstream by water on the footwall which can have an influence of the employee’s ability to do their daily work (influences the production). Many heat stress indicators have been setup over the years with wet-bulb temperature being the main indicator into the environmental conditions. However, the reason for the high wet-bulb temperature’s link to water on the footwall as an indirect main culprit has not been researched. A cooling system is designed by using the wet-bulb temperature to determine the amount of cooling a mine need. By decreasing the latent heat pick-up in a mine, the wet-bulb temperature would decrease and less cooling is required.
Table 1-2 shows that there is a need to bind all these factors together and see how the one is influenced by the other. The cooling is influenced by a heat stress indicator, wet-bulb temperature, that in turn is on the latent heat pick-up in the airstream. The heat rate transfer coefficient is
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 18 dependent on the water on the footwall that can have an impact on the production in the system. The overview of each study used in Table 1-2 can be found in APPENDIX A.
1.6 Problem statement and need for the study
Problem statement: There is a considerable amount of water mismanagement in deep-level
mines. Such mismanagement leads to an increase in latent heat pick-up. An increase in latent heat pick-up leads to hot and humid conditions underground which requires additional cooling to be supplied.
Need for study: There is a need to quantify the impact water mismanagement has on the latent
heat of deep-level mines.
1.7 Study objectives
To quantify the latent heat due to water mismanagement in deep-level mines, this study will aim to accomplish the following objectives:
• Develop a universal method to quantify the impact water mismanagement has on latent heat
• Translate this impact of latent heat to the cost implication of supplying additional cooling to mitigate such heat
• Validate the impact of addressing water mismanagement on environmental conditions
1.8 Conclusion
The increase in depth of mines emphasises the control of heat loads. Heat stress accidents increase with the increase of high temperatures in working areas. An increase of 1°C of wet-bulb temperature has the same physiological impact as an increase of 5°C of the dry-bulb temperature, and impacted productivity negatively by 3%. Latent heat increases through heat transfer from water to the airstream. The heat flow rate increased drastically when rocks were covered with a thin film of water; which may be due to evaporation.
Chilled service water produced by cooling plants is used all over the mine. The amount of gold extracted is directly proportional to the water consumption of a mine. Heat transfer through evaporation must be limited by avoiding contact between air and water on the footwall. The main cause of water wastages is leakages in the piping network. The dry-bulb temperature, together with the wet-bulb temperature and the air velocity, is used to base the cooling power of the refrigeration plants on because of their substantial influence on heat stress. The critical design parameter for an adequate cooling system is the wet-bulb temperature.
Chapter | INTRODUCTION TO LATENT HEAT IN DEEP-LEVEL MINES 19 This section provided a range of studies where different aspects have been researched surrounding human indicators, but there is insufficient research on the relationships of the water on environmental conditions, as well as the environmental conditions’ relationship to production. Despite a lot of research being done on water’s impact on the conditions in a mine, research from other industries can be used in the mining industry. From the findings of all 47 studies evaluated, there is a clear need for quantifying the impact on latent heat due to water mismanagement in deep-level mines.
Water mismanagement is responsible for latent heat pick-up into the airstream and additional cooling is necessary to mitigate the additional heat. There is a need to quantify the additional latent heat due to water mismanagement in deep-level mines.
A universal method needs to be developed to quantify the latent heat as well as translate the additional cost of cooling. In the following section, the methodology will be developed to quantify the latent heat.
Chapter | METHODOLOGY 20
2. METHODOLOGY
2.1 Preamble
Water mismanagement is a problem in mines, due to its ability to increase the latent heat of the air in the mine, and consequently result in additional cooling requirements. Chapter 1 provided the necessary background on this issue and highlighted the need for the study. This chapter will be developed based on the knowledge gathered from Chapter 1 to provide a methodology. This methodology can be applied to quantify the additional latent heat due to water mismanagement. The methodology developed consists of four steps and aims to quantify the latent heat in deep-level mines. This methodology will indicate the cost of cooling to counteract the influence of the increased latent heat due to water mismanagement. These steps include identifying the mine limitations, gathering data, simulating the scenario, and generating a graph. These four steps are shown in Figure 2-1.
Figure 2-1: The four methodology steps to determine the cost of cooling.
The methodology steps will be a guideline to establish the latent heat added to the environment by water on the footwall and is applicable to a specific site. A discussion of every step will follow in the subsequent sections.
2.2 Step 1: Identify the limitations
The first step of the methodology is to determine the limitations of the mine involved. This entails the setup of the boundary layers and categorisation of the water. If the limitations are met, then the methodology process will adhere to the assumptions made during the set-up of the methodology. Step 1 consists out of two sub-steps, as seen in Figure 2-2.
Chapter | METHODOLOGY 21 Figure 2-2: Step 1 of the methodology (including the sub-steps).
The first sub-step, determining the boundaries of the specified area, will be discussed in the next sub-section (2.2.1). The second sub-step, water categorisation, will be discussed in Section 2.2.2. 2.2.1 Inlet and outlet boundaries
Choosing the boundaries is a crucial step and will outline the main area where this methodology will be applied. The boundary is the border of the study; the water that falls inside the boundary will be acceptable to the study. The collection of the required data in the following steps will occur at the inlet and outlet of the boundaries. At the inlet and outlet of the boundary, the mass flow rate must remain more or less the same. The following concept regarding the mining classification must be explained. The mine can be classified as a conventional mine or mechanised mine by looking at the mining method.
Figure 2-3 is a representation of a typical conventional mine. A description of such a typical conventional mine, as well as a description of the figure, will follow.
Chapter | METHODOLOGY 22 Figure 2-3: Schematic representation of a conventional mine.
A conventional mine is a mine that uses the stoping method to mine; a stope is created between two levels. The mining occurs in the stopes where the workers mine for ore [69]. In Figure 2-3, the inlet, location A, and outlet, location B, to each level are on the same horizontal level underground with the levels mined directly below one another. The shaft at location A is used to downcast fresh air from the surface and location B is the return airway to the surface.
Figure 2-4 is a representation of a typical mechanised mine. A description of such a typical mechanised mine, as well as a description of the figure, will follow.
Chapter | METHODOLOGY 23 In mechanised mining, backfill stope mining is used, and the orebodies can be mined in any direction or geometry. The mining method is either mechanical mining or drilling and blasting [69]. The levels in Figure 2-4 will start on a horizontal plane and end on an entirely different plane. The Z indicates the distance underground from the surface. With mechanised mining, the inlet air also flows down the shaft as with conventional mining. The difference lies in that there is one connection (location A) to the shaft which further on splits into three or four different levels. Locations B, C, and D are the inlets to separate levels. These levels aren’t necessary horizontal; the level can be an incline or a decline, travelling to a different depth below the surface. Location E is the outlet to location B and is Z1 m below the surface. Location F’s inlet is location C, and
both the inlet and the outlet are on the same depth. Location G is again Z3 m below the surface;
which is lower than the inlet to the specific level, at location D. The outlet of the levels are at different depths underground.
Choosing the boundary for a conventional mine is relatively easy; there is only one inlet and one outlet, and both are situated at the same depth underground. Therefore, the mass flow rate of the inlet and outlet will be equal. The mine can be divided into boundaries with every level representing a separate boundary. A mechanised mine is more complex with all the levels at different depths and the connection between the levels. The boundaries for the mechanised mine will include the whole mine and will not divide the mine into different levels. The inlet of the boundary will be a location that is before all the levels divide and the outlet will be after all the levels have been connected together into one haulage. At these two points, the mass flow rate should be mostly similar.
2.2.2 Water categorisation
Water can be found in different areas in a mine. The reason for the water accumulation on the footwall varies. This study needs to determine the area of water that influences the environment. To determine what area of water influences the environment, recording all the water in the mine is necessary. The recorded water classification will be according to where the water is located, the reason for the water at the specific point ,and whether the water is actively being pumped out. The water will be classified as stagnant water and addressed water. Table 2-1 shows a representation as well as the definition of the classifications of the water.
Chapter | METHODOLOGY 24 Table 2-1: Water classification's example and definition
A – Stagnant water
Definition: Water on the footwall that is situated in the main airflow with no pump. B - Addressed water
Definition: Water that is being addressed.
The water is classified into two groups; the water that falls into the classification of stagnant water is part of the focus of this study. The water classified as stagnant water influences the latent heat pick-up of the air flowing over the water. The study aims to quantify the latent heat of water mismanagement.
In Section 1.2.8, Steed [27] concluded that the water surface area is the factor that determines the rate of evaporation and that the water quantity is unimportant. Thus, the assumption can be made that the water on the footwall is a depth of 1cm. The assumption made makes it unnecessary to measure the depth of the water, and the only measurement necessary is the area of the water.
In Step 1, the limitations of the methodology have been determined. These limitations entail the area in which the assumptions are true. The water classifications have been made that will be used in the next steps to filter out the data. In Step 2, the measurement will be taken at the inlets and outlets to the boundaries, indicate water on the footwall and data collected from the refrigeration plants. In Step 1, all the foundation has been completed, and in Step 2, the collection of the data will occur.
2.3 Step 2: Gather the data
The second step of the methodology is to collect all the data. In Step 1, the limitations of the study have been identified, and in this step the data will be collected at the boundary borders as well as inside the boundaries. Step 2 consists of two sub-steps, mining and refrigeration. These two
Chapter | METHODOLOGY 25 sub-steps represent the sections where the data for Step 2 needs to be collected. The expansion of the flow diagram for Step 2 is presented in Figure 2-5.
Figure 2-5: Step 2 of the methodology (Including sub-steps)
The mining sub-step includes all the data that needs to be collected in the area of mining. This includes the measuring ventilation quantities at the inlets and outlet of the boundaries and quantifying the area of water on the footwall. The refrigeration sub-step entails collecting the data to determine the coefficient of performance of the mine’s cooling systems as well as the cost to produce that cooling.
2.3.1 Mining data
The data collected in this sub-section is all the data that can be found underground in the mining section. In Step 1, the location of the boundaries with the inlets and outlets have been identified. In Step 2, the necessary data needed at the inlet and outlet of the boundary as well as the data inside the boundaries will be discussed.
In Figure 2-6 is an example of a boundary chosen on a level with the boundary in yellow. The inlet of the boundary is located at location A with the outlet of the boundary located at location B. At location A and location B, a ventilation survey will be done to gather the data of the air flowing in and out of the boundary. The water that is located inside the boundary lines, between location A and location B, will be classified and recorded.
Chapter | METHODOLOGY 26 Figure 2-6: Example of a boundary with A (inlet) and B (outlet).
The ventilation survey consists of doing an air velocity measurement at the point of measurement. At the same point, the area of the haulage needs to be measured, including the dry-bulb temperature, wet-bulb temperature and the barometric pressure. The air velocity is measured using a Davis vane that is transverse for a minimum of 60 seconds, perpendicular to the airflow. The transverse method is the pathway the Davis vane must move across the airway as seen in Figure 2-7 [35]. The width and height of the haulage must be measured using a distance meter. A barometer measures the barometric pressure. A whirling hygrometer spinning at 100 revolutions per minute for 30 seconds measures the dry-bulb temperature and wet-bulb temperature in the flow of air.
Chapter | METHODOLOGY 27 When the ventilation survey data is collected, the air mass flow rate must be determined. The air mass flow rate is calculated by using the following equations. Firstly, convert the Davis vane value to a velocity value and determine the area of the haulage opening.
The air density can then be determined by using the psychrometric chart (refer to APPENDIX B). The specific volume of the air can be read off the vertical axis by using the air’s properties. The specific volume is the inverse of the air density. Equation 2-1 is used to determine the air density.
Equation 2-1: Calculating the air density from psychrometric chart [35]. 𝑝𝑎𝑖𝑟 = 1 𝑣 ( 𝑘𝑔 𝑚3) Where: 𝑣 = specific volume (𝑚3 𝑘𝑔) 𝑝𝑎𝑖𝑟 = air density ( 𝑘𝑔 𝑚3)
Determining the air mass flow rate at the measuring point by using Equation 2-2. Equation 2-2: Calculating the air mass flow rate [35].
𝑚̇ = 𝑉 × 𝐴 × 𝑝𝑎𝑖𝑟 ( 𝑘𝑔
𝑠 ) Where:
𝑚̇ = mass flow rate (𝑘𝑔
𝑠) 𝑉 = velocity (𝑚𝑠) 𝐴 = area (𝑚2) 𝑝𝑎𝑖𝑟 = air density ( 𝑘𝑔 𝑚3)
The relative humidity (%) can be calculated by also using the psychrometric charts (refer to APPENDIX B). The vertical lines on the chart are the dry-bulb temperature with the wet-bulb temperature being the diagonals. The relative humidity is the curved lines, with 100% relative humidity at the edge of the graph. The relative humidity can be read off the graph, at the intersection point of the dry-bulb and the wet-bulb temperature.
These equations will convert the ventilation survey into four important values that will be used at a later stage in this methodology. These values include the barometric pressure (𝑃𝑏), the air mass flow of air (𝑚̇), relative humidity, and the air density (𝜌).
The second part of collecting data in the sub-step is to quantify the amount of water inside the boundaries and can be classified according to the classifications discussed in Step 1. The
Chapter | METHODOLOGY 28 process can be simplified and sped up by incorporating the mining personnel of the specific boundary. Table 2-2 is an example of a table located at the control room of the mine. Personnel can inform the control room of water within a specific boundary.
Table 2-2: Example of a table used in the control room to track the water
1 2 3 4 5 6 Date identified Location Water surface area [𝑚2] Temperature [°C] Reason Responsible person
The reported date of the water located in the mine is in the first column with the location of the water in the second column. The position will help to determine within which boundary the water is located and whether the water is inside the boundary lines. Entered into column 3 and 4 is the water surface area that is essential to know as well as the temperature of the water. The reason why the water is laying at that point will help to classify the water, and the responsible person is the person that is responsible for removing the water from the footwall.
The data collected from Table 2-2 can easily be classified according to Table 2-1. Table 2-1 outlines the two categories into which the water on a mine can fall. The water classified as stagnant water is the only water used for this study as it influences the temperature.
In this sub-step, the data at the inlet and the outlet of the boundary have been collected by doing a ventilation survey. Four equations were used to calculate the air mass flow rate from the data measured at the boundary’s inlet and outlet. The water that was present in the mine was recorded by placing a table such as Figure 2-2 in the control room. The reason column, column 5 in Figure 2-2, will be used to classify the water according to the classifications in Step 1.
2.3.2 Refrigerating plant
The data necessary from the refrigeration plant is to determine the coefficient of performance (COP) as well as the cost of cooling. Fridge plants are usually instrumented and collecting data can happen without measuring all the necessary data. During this step, all the COPs of the plants need to be calculated separately, and eventually, using an average of the plants as a combined COP.
The data required from the refrigeration plant must be collected on the evaporator side. The inlet and outlet temperatures are necessary as well as the mass flow rate of the water. The electric power also needs to be measured to determine the COP. When this data is collected, the COP of the plants can be determined by using Equation 2-3.
Equation 2-3: Calculating the COP of a refrigeration plant [35]. 𝐶𝑂𝑃 = 𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝐷𝑢𝑡𝑦
𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑝𝑜𝑤𝑒𝑟=
𝑚̇𝐶𝑝(𝑇2− 𝑇1) 𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
Chapter | METHODOLOGY 29 Where
𝑚̇ = Evaporator mass flowrate (𝑘𝑔
𝑠) 𝐶𝑝 = Specific heat of water (4.186
𝐽
𝑔∙℃ ) ( 𝑘𝐽 𝑘𝑔.℃) 𝑇1 = Evaporator inlet temperature (°𝐶) 𝑇2 = Evaporator outlet temperature (°𝐶) 𝑃𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 = Electric power (𝑘𝑊)
The COP of the plants is calculated in this sub-step as part of the data necessary in Step 4 to determine the power usage of the plants.
All mines in South Africa are billed according to Eskom’s Mega Flex tariff structure. This means that it cost R15.82 for 1 kW per day (Eskom’s Mega Flex tariff structure 2017-2018).
All the data that is necessary for the steps to follow have been collected in step 2. This includes the inlet and outlet boundaries ventilation survey, the water that is on the footwall, as well as all the data to determine the COP of the refrigeration plants. Table 2-3 is a recap of all the measurements and data that you need at the end of step 2.
