LECTURE 4 THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS: 1.0 INTRODUCTION 2.0 SOURCES OF HEAT IN MINES 3.0 STRATA HEAT 3.1 Gethermal Step and Gethermal Gradient 3.2 Thermal Cnductivity f Rcks 3.3 Heat Flux 3.4 Virgin Rck Temperature 3.5 Furier s Law 3.6 Factrs determining Strata Heat Flw in Mines 3.6.1 Primary Factrs 3.6.2 Secndary Factrs REFERENCES Page 1 f 9
1.0 INTRODUCTION Human beings are capable f wrking efficiently within a certain range f temperatures. Temperature is cncerned with degree f htness. Temperature f a substance r bdy is liable t change when we have a surce f heat. Heat always flw frm high temperature t lw temperature bdy. This flw can take place in any ne f the three ways f transferring heat- cnductin, cnvectin and radiatin (Fig.1). These mdes f transferring heat mainly cause flw f sensible heat. Thrugh evapratin, transfer f latent heat takes place. In this case, we d nt sense the rise in temperature, but heat cntent f the bdy is increased. In undergrund mines, there are many surces f heat which cause rise f temperature f air during its travel in mine airways. The cnditin may wrsen if the temperature f the air increases beynd a certain limit. The situatin becmes mre critical especially when humidity f air als increases simultaneusly. In such situatin, we require air cnditining f mine air. We have t set up refrigeratin plants, spt clers, etc. s as t ensure cmfrt, safety and high wrking efficiency f wrkers in mines. On the ther hand, in areas with extreme cld climatic cnditins, we need t supply heat t the air n the surface befre sending it t undergrund mines. 2.0 SOURCES OF HEAT IN MINES There are varius surces f heat in undergrund mines. Sme are majr cntributrs f heat additin t air and sme are minr. Sme f them are unavidable, while sme f them require utilizatin f prper techniques fr their reduced effect. Fig.2 shws the varius surces f heat in mines. The majr surces f heat in undergrund mines are: Strata heat Aut-cmpressin Machinery and lights Undergrund water Page 2 f 9
Fig.1 Types f heat transfer in mines (after Vutukuri and Lama, 1986) The minr surces f heat in mines include Human metablism Oxidatin Blasting Rck mvement Pipelines Energy lsses in airflw Page 3 f 9
Energy lsses in air flw Strata Heat Autcmpr essin Pipelines Machinery and lights Surces f heat Rck mvement Blasting Human metablis m 0xidatin Fig.2 Surces f heat in mines 3.0 STRATA HEAT Strata heat is a majr surce f heat undergrund mines. In sme cases especially in deep mines, it becmes intlerable and we require installatin f refrigeratin systems s as t prvide safe and cmfrtable wrking cnditins. The term strata heat means the heat emitted frm the surrunding rcks and getting added t the mine atmsphere. Subsurface rcks mainly have their heat reservirs in the cre prtin f the earth, which emits heat and that heat gets transferred t the upper Page 4 f 9
part f the earth like mantle and crust. In crust part f the earth, mining is practiced. Thus, whenever we g fr subsurface mining, strata heat is a majr factr that has t be cnsidered while planning ventilatin system s as t create cmfrtable wrking cnditins. 3.1 Gethermal Step and Gethermal Gradient The term ge means earth, thermal means heat and gradient means change in a particular quantity with respect t distance. Thus, we can define gethermal gradient as the change f heat/temperature f subsurface rcks with respect t distance. In general, temperature f the subsurface rcks increases with depth. The increasing trend with depth is because f the cre part f the earth which gets nearer and nearer as we g dwn, Fig. 3. Fig. 3 Different layers f earth with depth In mining, we generally make use f the term gethermal step, which is defined as the depth per degree centigrade rise in temperature. If we lk carefully, we can easily make ut that, gethermal step is inverse f gethermal gradient. Let us have a lk at their units. gethermal gradient = T /m and Page 5 f 9
gethermal step = T m/ T = Change in temperature ( ) = Change in depth (m) Bth gethermal step as well as gethermal gradient may vary frm place t place depending upn the types f the rcks fund in the area, thermal prperties f the rck, presence f undergrund water reservirs, etc. It is als greatly influenced by the age f the rck, and igneus activities ging in the regin. At arund 15 m depth frm the earth surface, temperature is cnstant, as at this depth, there will be n significant variatin f temperature with the change in the climatic cnditins bserved n the surface f the earth. After apprximately 15 m, it starts shwing a unifrm increase in the temperature with depth at a particular place. Table 1 gives a brief idea abut the variatin f gethermic gradient in different mine districts f the wrld Table 1 Values f gethermic gradient in varius mines (after Banerjee, 2003 & Hartman et al, 1982) Lcatin Gethermic gradient (m/ ) Jharia calfield, India, cal measure rcks 17.2-39 Raniganj calfields, India, cal measure rcks 38.4 Singareni calfields, India, cal measure rcks 30 Klar Gld Fields, India 91.1 Msabani cpper mine, India 50-54.8 Suth African gld mines 96 Cal mines, UK 25-55 Munt Isa mines, Australia 50.8 Magma cpper, Arizna 26.8 Agnew nickel, Australia 76.8 3.2 Thermal Cnductivity f Rcks Page 6 f 9
Thermal cnductivity f a rck (k) is defined as the heat flw acrss a surface per unit area per unit time when a particular temperature difference exists in a unit length perpendicular t the surface. It depends n fllwing factrs:- a. Chemical cmpsitin f the rcks (rcks are aggregates f minerals) b. Water cntent f the rck c. Temperature d. Pressure e. Radiactive decay (if any) etc. Thermal cnductivity has units f W/(m ) 3.3 Heat Flux It is defined as the heat rate (J/s) per unit area, the directin f heat flux is in the directin f negative temperature gradient. The earth s heat flux ranges frm 0.04-0.06 W/m 2. We take average value as 0.05 W/m 2 fr calculatin purpse. 3.4 Virgin Rck Temperature It is als called as the natural rck temperature. The temperature f a rck at a particular depth belw earth s surface because f the gethermal energy stred in it is called virgin rck temperature. It is determined by brehle technique with an accuracy f ±1.5. It is a very imprtant factr in determining the heat flw frm the strata t a mine airways. 3.5 FOURIER S Law It is als called Furier s law f cnductin f heat. The law states: The rate f heat transfer thrugh a material medium is prprtinal t the negative f temperature gradient and area f the medium. Mathematically, it can be explained as Heat transfer rate (q) T Page 7 f 9
A Therefre, we can write q A T Where, T = ka q = heat transfer rate (W r J/s) k = thermal cnductivity f the material (W/(m )) A = area f the material (m 2 ) T = temperature/thermal gradient ( /m) -ve sign indicates reductin f temperature in the directin f heat flw. Using Furier s law, if we knw the thermal cnductivity f rck and heat flux at a particular place, we can easily calculate gethermal gradient at that place. It is imprtant t mentin that it is a very difficult task t exactly determine the thermal cnductivity f rcks. The reasn being, the cnductivity f a rck btained in the labratry test are likely t differ frm the actual cnductivity f rcks in the field. This is because, cnductivity varies with pressure at a particular depth, directinal nature f mineral cntents, presence f any fluid, etc. Further, determinatin f heat flux f strata in the mine is als a cumbersme task as it invlves a large number f variables (discussed in detail latter). 3.6 Factrs Determining Strata Heat Flw in Mines Heat flw int mine airways frm the strata is very cmplex in nature. It invlves a large number f variables/factrs that significantly influences the rate f heat flw. These variables may be classified int tw brad categries i.e., primary factrs and secndary factrs. 3.6.1 Primary factrs Gethermic gradient Thermal prperties f rck like thermal cnductivity, thermal capacity Difference between virgin rck temperature (VRT) and dry bulb temperature Length f the mine airways Humidity and pressure Page 8 f 9
Rate f mineral prductin 3.6.2 Secndary factrs Rughness f rck surface Shape and size f airway Air Vlume flw Nature f air-rck interface Inclinatin f airway Age f the airway There are many ther factrs like design and layut f air cnditining, cling plant, etc. which t play a vital rle in strata heat flw t the undergrund mine envirnment. Besides a large number f factrs invlved, the determinatin f heat flw frm strata is dne empirically. Analytical apprach invlves cncept f three dimensinal, time transient heat cnductin (McPhersn, 1993). REFERENCES Banerjee S.P. (2003); Mine Ventilatin ; Lvely Prakashan, Dhanbad, India. Hartman, H. L., Mutmansky, J. M. & Wang, Y. J. (1982); Mine Ventilatin and Air Cnditining ; Jhn Wiley & Sns, New Yrk. Le Rux, W. L. (1972); Mine Ventilatin Ntes fr Beginners ; The Mine Ventilatin Sciety f Suth Africa. McPhersn, M. J. (1993); Subsurface Ventilatin and Envirnmental Engineering ; Chapman & Hall, Lndn. Misra G.B. Calcutta, India. (1986); Mine Envirnment and Ventilatin ; Oxfrd University Press, Vutukuri, V. S. & Lama, R. D. (1986); Envirnmental Engineering in Mines ; Cambridge University Press, Cambridge. Page 9 f 9