LECTURE 4 THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS: 1.0 INTRODUCTION 2.0 SOURCES OF HEAT IN MINES 3.0 STRATA HEAT

Transcription

1 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 Primary Factrs Secndary Factrs REFERENCES Page 1 f 9

2 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

3 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

4 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

5 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

6 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 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 Suth African gld mines 96 Cal mines, UK Munt Isa mines, Australia 50.8 Magma cpper, Arizna 26.8 Agnew nickel, Australia Thermal Cnductivity f Rcks Page 6 f 9

7 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 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

8 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 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

9 Rate f mineral prductin 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