MINE CLIMATE 1,0 CLIMATIC CONDITIONS IN MINES In order to carry out various mining operations underground, normal climatic conditions have to be maintained. In coal mines for example temperature of air in working faces and development areas should not exceed 26 C°if relative humidity is more than 90 %. Similarly, atmospheric pressure has to be monitored as well, L.1 Sources of heat in mine air In Mines, changes in temperature come about as a result of heat being released from various heat generating sources. ‘Temperature of mine air usually refers to dry — bulb temperature and in most cases is measured by a mercury thermometer. In order to get a correct temperature of air, the dry bulbs thermometer should be held at least 315 mm away from any surface so that the leading is not affected by heat radiated to or from the surface ‘1.1.2 Surface air as heat source During hot days when surface temperatures are high, surface air entering the mine will generally carry some heat to underground mine workings. 1.1.3 Heat from rock blasting During blasting works, a certain amount of heat, is released to the mine environment. 1.1.4 Heat due to oxidation processes In coal mines, this is a major source of heat contributing 80-85 % of heat to mine air particularly in coal seams reliable to spontaneous heating, Assuming all oxygen consumed in the mine to be utilized in oxidation of coal, [Haldane] ‘states that a fall of 0.1 in the percentage of O2 in the air produces enough heat to raise the temperature to 7 K . Apart from that, during emission of methane from coal seams, de-sorption of methane produces a certain degree of cooling by extracting 835-1025 kj of heat per m’ of methane desorbed. 1.1.5 Heat from lighting Systems Heat is produced from lighting systems such as electric bulbs and carbide lamps. Cooke estimates that a carbide lamp consuming 156g of carbide per shift produces 96.5 j of heat per second while an electric bulb produce about 40 j/s depending on the current and voltage.1.1.6 Heat Generated from Machinery In highly mechanised mines, this can be a major source of heat. For most Machineries ‘working in production areas (cutting machines, drilling machines and transport machines) most of the work is frictional except for that part of the work done against gravity. ‘Therefore most of the power input to such machinery is converted to heat partly in machine itself (ine(Ticiency), and partly through friction work. Example 1. A.2000kw internal hoist machinery running at 60% overall efficiency will add 800 Kw of heat to mine air at the site of hoist in addition to the heat produced by friction work in the shaft 2. Diesel locomotives produce more heat that electric locomotives of either trolley-wire or battery type. A diesel locomotive consuming 6.26 kg of fuel perlewh generates about 2.93 Kw of heat per Kw of locomotive power. 3. Fans during their work also add heat to the air. Almost all the work done by the fan is converted 10 heat except for the part used for increasing the kinetic energy of air undergoes adiabatic compression. There is an increase in the air temperature of 0.826 K for every KPa pressure developed by the fan. 1.1.7 Compressed air in pipes This air add a certain amount of heat to the mine air. Where improper cooling is done compressed air may be generated at high temperatures. Ait passes in the pipes, itis further gains heat due to auto- compression in the pipe range in the shaft. As the compressed air goes down the shaft, it gives up heat to the ventilating air and some of the moisture in the compressed air condenses also generating heat. ‘The total heat add to the mine air from compressed air can be given by the following, expression. QM {Cp (Ti-T2) + LAmi-m + bg 100 ‘Where Q- heat lost by compressed air in W M- Mass flow rate of compressed air in kgs-! Cp. Specific heat of compressed air =1005 jkg-’ K" T,, Tz Temperatures in K of compressed air pipe at the shaft top and shaft bottom respectively. L— Latent heat of evaporation of water in jkg-' H- depth of shaft in meters m, m;_ The moisture content in gKg-' of dry compressed air at the top and shaft bottom respectively,Question Find the amount of heat added to the mine air by a fan at a face of an opening, if 3.5 m/s of air coming from the duct of $00 mm is made to circulate at this face. Take the input of the fan to be equal to 2.5 KW. 1.1.8 Heat generated from men Heat is produced by man in the process of metabolism. A person at rest also produces heat by basal metabolism ( when food is held for specific period of time at 46.5 W/m? of body surface area The surfadt yea of a man (A) can be estimated by using the following expression. A=0.2024 Q™5 X HOS Where Q- mass of body in Kg. Hi — height of body in meters Note: An overage man has a surface area of 1.8 - 1.9 m? so that he has a basal metabolism rate of 84 - 88 W 1.1.10 Heat due to auto compression In the process of air moving down the shaft it is compressed by weight of shaft air column approximately at a rate of 1.1 KPa/100 m depth, And its potential energy is converted to heat energy. Provided the flow is friction less and non-accelerative and no heat or moisture is lost or gained by the air, the compression of air in the down cast shaft will be reversible adiabatic. T/T =( Pa/P) = (VV)? @ ‘Where T;,2 — Temperature in K 1 = CYC, =1.404 , for dry air. For calculating sake y=1.4 V - Specific volume (volume of unit mass of air) P- Barometric pressure and sub scripts 1 and 2 indicate the state of air at the shaft top and bottom respectively. The rise in temperature due to auto-compression at any depth can be determined from equation (i) by finding the barometric pressure at the shaft top and at that depth or from specific volumes ( or densities, since density=1/Volume) at the two points, Alternatively, rise in temperature due to auto-compression can also be obtained by ‘equating the potential energy with enthalpy change under the assumption made dQ =0 as no heat is transferred dw=0, as no work is done and dKE=0 as the flow is non- accelerative so that dH=dPE hg-AH-C,ATWhere AT ~ rise in temperature in K, AH change in enthalpy, /kg h=depth of shaft in meters, Cp=Specific heat of air in jkg-'. K* For estimation of heat transfer in road ways, stopes and development ends [Ramsden] uses the following expression: Qu=5.57(fw 40.255) Fv — TAP)” (r/3)%1" (K/S.SX1O (Qn ~ Heat pick up in KW per 100 m length of air fw — Wetness factor fw= 0 for dry air ways, fw=l for very wet air way, fw=0.25 for wet flow dry roof. Ty Age of road ways in years K - Thermal conductivity of rock, KWm'' kt P—Perimeter of air way, m ‘T—Dry bulb Temperature of air k TW Temperance gy Ogu vente For Stopes, heat pick up Kw per metre of face length Q.0.11(1+0.05W)(Ty-T)V(Lav/10)(Ls/S0)(KpC/13) W- Stoping width, m ‘Lm ~ Average monthly face advance, m Ls - Stop span, m i.e. average distance from center gully to stop face. p- Density of rock, kg/m’ C- Specific heat rock KjKg"K" For development up to 50m behind the face Qe=La (Ty-T’)(P/12)"? (KpC/13)"* L_Daily rate of face advance in meter, QeRate of heat pick up in the vicinity of the development, KW. Heat from rock generally increases with increase in mine depth. This rate of increase in temperatutre with depth is called Geothermal/ Geothermal gradient. Because of different physical properties of rocks such as thermal conductivity, density, specific heat which governs the rate of heat transfer in the rock, this parameter, usually very in different places. In Witwaters land (SA), the geothermal gradient. Because of different physical properties of rocks such as thermal conductivity, density, specific heat which govern the rate of heat transfer in the rock, this parameter usually vary in different places, In Witwatersland (SA), the Geothermic gradient is 1k/109.8 m, while in Lanchashire coal fields, UK it is about 14/34 m. Determination of Geothermic gradient GG is determined by recording temperature values of virgin rocks at various mining depth. In order to do this boreholes of about 9-30 m are drilled in the virgin rocks followed later by placement of a thermometer on the bottom of the bole hole.d=WCp k- thermal conductivity of the rock. C- Specific heat of the rock p= Density of the rock Coefficient of thermal conductivity (thermal conductivity) is the amount of heat flow through a unit thickness of material over a unit area per unit time for a unit temperature difference and is given by the following expression: q= KALDUL 4 - amount of heat flowing Ac area, t time AT- temperature difference L- thickness of material Note: Different rocks have different rock thermal conductivity. Its high for metals and Iowest for gases. In rocks it depends mainly on the extent of pores. Enthalpy (Total heat) Is given by the following expression H=U+PV 1) Where U- Internal energy P- Absolute pressure V- Specific volume Differentiating the above equation we get dh=dU+PdV+VaP (2) From the first law of thermal dynamics Amount of heat exchanged dg=dU+PaV BI Combing 2 and 3 we get dh=dq+VdP or dq=dH-VdP for incompressible flow in an open system, as mine air there is no change of pressure on heating, Therefore, the change in enthalpy equals the amount of heat added to air. Change in enthalpy is also given by dh=CedT Note: Normally enthalpy of air at 273.15 kis assumed to be zero. Note also that the enthalpy at any particular state is given by the sum of sensible heat of dry air, latent heat of water vapour and the sensible heat of water vapour. 2.0 PHYSIOLOGICAL EFFECTS OF HEAT AND HUMIDITY ON MANFor a rapidly advancing face (USA-bureau of Mines) virgin rock temperature is found by inserting a 299m long psychrometric thermometer mounted in a groove on a 19 mm diameter wooden ROD INTO A 1.5 - 1.8 m deep drill hole for at least 10 minutes. However, accurate results may be hamphered if there is too much circulation of air in the face and if the bole hole is wet. 1.1.11 Heat transfer from rock to mine air, This is mainly by convection when rock surface is dry and by sensible and latent heat transfer. Direct heat transfer from the rock to the mine air is governed by the rate of heat transfer within the rock mass and the heat transfer within the rock wall of excavation to mine air. The rate of sensible heat transfer from the rock wal to the air is linear function of the difference between tempereature as well as the coefficient of heat transfer. qa (Ts-T.) 4 Rate of heat transfer, that is the amount of heat flow per unit area per unit time t - Coefficient of heat transfer (thermal emmisitivity), taking into a account properties of rock surface, moisture contents, temperature and velocity of air as well as size of air ways. ‘Ts T. rock wall and air Temperatures and air respectively. ‘Value of alpha can be predicted from the following expression [Colburn] SuPr? =f/8 St- Stranton number St-a/C,G=a/C,p.V G— Mass velocity =p,V. Pa-Density of air V — Average velocity of flow Pr- prandt! number Pr=uG, 1K, K,_ Thermal conductivity of air. 11- Viscosity of air J- Darcy — weisbach resistance coefficient of the air way. Taking Pr. =0.72 for prevalent mine air temperature Then the first equation reduces to O=fCy p.V/6.4 The Thermal diffusivity (d) is given by the following expressionMan generates most of heat through the process of metabolism. Most of this heat has to be got rid of in order to maintain the normal body temperature. This heat is dissipated from the surface of the skin by radiation, convection, evaporation and a little part through exhaled air. ‘However below 298 k or less heat transfer from the skin to air is mainly by radiation and convection. As the tempersture rises above 298 k, the heat transfer to the skin becomes faster as the blood vessels become dilated ensuring larger blood circulation to the skin. A temperatures above 3002 k, the sweat grands sigrts functioning and heat transfer from the skin is mainly by evaporation 2.1 Effects of heat factors greatly influencing the rate of heat transfer in man 1 High wet bulb temperature. High wet bulb temp. leads to reduction in the rate cooling, This reduction in the rate of cooling result in temperature rise. 2.Air velocity. Higher velocity aids cooling but may create un comfortable condition in relation to dust generation. 3.Changes in cardial-vasicular system. Under hot and humid conditions, more blood is supplied to the skin and henoe the cardial rate and out put increase so much. This may lead to heart failure resulting in decrease in blood pressure. The final stage is un consciousness 2.2 Physiology of heat stress Heat exposure often exceeds that encountered in the hottest natural climate. The burden of metabolic heat production may exceed the worker's physiologic capacity to regulate his body teraperature, leading to impaired performance or clinical signs of heat illness. By reason of physical fitness . work capacity, age, health status, living habits, and level of acclimatization, men vary in ability to tolerate heat stress. 2.3 Body temperature regulation Man, and other homeotherms, regulate internal body temperature within narrow limits by physiologic control of blood flow from sites of heat production in muscles and deep tissues to the cooler body surface where heat is dissipated through physical chanels of radiation, convection and evaporation to the environment. When heat loss is in balance with heat production, internal temperature is maintained at the regulated level, 2.4 Normal range ‘The figure below indicates, the usual range in body temperature (rectal, oral) in normal persons as well as extreme upper and lower limits of normal.2.43 Mine Ventilation Practice ©) Air velocity aids cooling but may create comfortable condition in relation to dust generation 4) Heat stroke — results from the break down in temperature regulating mechanism, ) Changes in cardio-vascular system. Under hot and humidity conditions more blood is supplied to the skin and hence the cardiac rate and out put increase so much. This may read increase in blood pressure resulting in heart failure. Temperature reduction in mines Despite the fact that in modem mines distances from ventilation shafts to stoping areas is measured in Kms and that the temperature in working faces is close to temperatures of rock, the seasonal fluctuation of temperature of outer air, all the ‘same affects temperature of air in working faces, which in cold countries in winter could be 3-4°C lower than in summer. Even day temperature fluctuations can be felt in underground mine openings and at distance of about 2.9 k from rim of ventilation shaft can reach about 2-23°C and at 3.5 km 0.8-0.9°C. Therefore, in designing deep mines, it is very necessary to cary out heat calculations in order to determine expected temperature of air in underground circuit of mine openings more especially in exploitation and development openings, and also in order to establish the necessity of artificial cooling of air and magnitude of this cooling, In order to reduce temperature of air in deep mines, mining technical measures or artificial cooling of air with help of refrigeration machines are done. Mining Technical Measures include: Reducing intensity of heating ventilation currents by increasing its speed in mine openings Refusal of wide application of wooden support, because its decomposition Process contributes to temperature increase Using heat/insulating methods or materials against wall in mine openings and water canals/gutters Removal of heat from heating generating mechanism and dumping it immediately in return air way. However, the most efficient and reliable method of reducing temperature of mine air is by using refrigeration equipment. This equipment can be installed centrally,Mine Ventilation Practice i.e. serving the entire mine; grouply serving only certain parts or section of mines or locally for one or two exploitation and development faces.