LECTURE 9 THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS: 1.0 NEED OF CONTROLLING MINE ENVIRONMENT 2.0 REFRIGERATION 3.0 METHODS OF REFRIGERATION 3.1 Use f Freezing Mixture 3.2 Use f Dry Ice 3.3 Use f Refrigerants 4.0 DIFFERENT TYPES OF REFRIGERATION PLANTS BASED ON LOCATION 4.1 Surface Plant 4.1.1 Advantages 4.1.2 Disadvantages 4.1.3 Example f surface plant in India 4.2 Undergrund Plant 4.2.1 Advantages 4.2.2 Disadvantages Page 1 f 14
4.2.3 Example f undergrund air-cling plant 4.3 Spt Clers 4.3.1 Advantages f spt clers 4.3.2 Disadvantages f spt clers 4.3.3 Example f spt cler installatin in India REFERENCES Page 2 f 14
1.0 NEED OF CONTROLLING MINE ENVIRONMENT The climate in undergrund mines becme mre and mre adverse with increase in the depth f mine, increase in the extent f mechanizatin etc. There is an increase in the temperature as mines becme deeper. This results int adverse mining envirnment which is nt suitable fr cmfrtable wrking f miners. Let me tell yu that in undergrund mines, it is nt the high relative humidity which is mre dangerus. Actually it is the high wet bulb temperature, which is mre dangerus in mines. A relative humidity f 100 % at lwer temperature gives a sensatin f cling, while at higher temperature like 25 degree Celsius r abve, it gives a sensatin f htness. We have already learnt the effect f high wet-bulb temperature n efficiency and health f wrkers in mines. Let us have a lk at Fig. 1 (a: Prbable perfrmance levels f acclimatized wrkers, b: Estimated heat strke risks) which shws fall in wrking efficiency with wet bulb temperature and increase in risk f heat strke with increase in wet bulb temperature. Hence we require cntrlling the mine climate, s that the wrking cnditins can be made cmfrtable. As mst f the heat prduced in a wrking district is remved by air current, either we increase quantity f air r cling f mine air when it s nt pssible t increase the quantity f air beynd a certain limit. Thus air can carry away mre heat and helps in maintaining gd wrking climatic cnditins. The cling f mine air is usually dne by using refrigeratin plants. This air cnditining helps in imprving the wrking cnditins. The Statuary limit in India fr climatic cnditins is 33.5 C Wet Bulb Temperature with minimum air velcity f 1m/s. 2.0 REFRIGERATION When simple methds f cntrlling the mine climate dn t make an effect n mine envirnment then we have t g fr artificial cling f the air. This is dne by refrigeratin f mine air. We install refrigeratin plants fr this purpse. They are usually designed t prduce tlerable envirnment cnditin thrughut the year. Page 3 f 14
3.0 METHODS OF REFRIGERATION 3.1 Use f Freezing Mixture Freezing mixture is ice and a salt (NaCl, r CaCl 2, etc.). If they are made insulated, the latent heat required fr freezing can be made available frm the mixture itself. Thus, the sensible heat f the freezing mixture gets cnverted t latent heat f ice. Thus, the liquid frmed frm the mixture is much lwer than the freezing pint f ice. This liquid can be used fr cling ther substances t very lw temperature. Fig. 1 (a) Prbable perfrmance levels f acclimatized laburers (after Le Rux, 1972) Page 4 f 14
Fig. 1 (b) Estimated heat strke risk (after Le Rux, 1972) 3.2 Use f Dry Ice Dry ice is als called slid carbn dixide. We knw that dry ice sublimes at -78 C. Due t this unique feature, it rules ut the prblem f dispsing f liquid residue. Hwever, it is expensive cmpared t rdinary ice. 3.3 Use f Refrigerants All the refrigerating machines make use f vlatile liquids. These vlatile liquids when used fr the purpse f cling in a refrigerating machine, they are called Page 5 f 14
refrigerants. The biling pint f a liquid is dependent n pressure. The higher is the pressure, the higher is the biling pint. Thus, with variatin in pressure, the biling pint can be varied. Fr an ideal refrigerant, it shuld have: Lwer biling pint clse t 0 Nn- crrsive nature High efficiency f heat transfer Nn-txic nature Table 1 lists three refrigerants, mst cmmnly used in mines fr refrigeratin plants Table 1 Cmmnly used refrigerants in refrigeratin plants Name Refrigerant 717 (Anhydrus Ammnia) Refrigerant 11 (Trichlrmnflur methane) Refrigerant12 (Dichlrdiflur methane) Chemical frmula B. P. at atmspheric pressure Abslut saturatin pressure (bar) at 5 NH3-33.4 5.16 20.3 CCl3F +23.9 0.49 2.35 CCl2F2-30 3.62 12.2 Abslut saturatin pressure (bar) at 50 Limitatins Txic nature in By-prducts depletes zne layer By-prducts depletes zne layer These refrigerants can be used fr refrigeratin purpse in tw different ways: a. Absrptin system f refrigeratin b. Cmpressin system f refrigeratin The cmmnly used refrigeratin plant in mine is f vapur cmpressin type i.e. cmpressin system f refrigeratin. In this type a liquid refrigerant is used t extract the latent heat f vaprizatin frm mine air. The refrigerant is used in a cyclic system and the whle prcess can be illustrated as shwn in Fig. 2. Page 6 f 14
Cndenser -Refrigerant is cled and extracted heat is rejected. -Becmes liquid frm super heated vapur - Pressure f refrigerant is high here Cmpressr - Pressure is incereased here - refrigerant gets cnverted int super heated steam Expansin Valve - Refrigerant expands -Pressure reduces Evapratr -Lw temp, Lw Pressure refrigerant extracts heat frm mine air - Refrigent evaprates and temp. gets incereased Fig. 2 Different cmpnents f a refrigeratin plant Based n the lcatin f mine refrigeratin plants, their classificatin can be as shwn in Fig. 3. Page 7 f 14
Surface Plants Refrigeratin Plants Spt clers Undergrund Plants Fig. 3 Classificatin f refrigeratin plants based n lcatin 4.0 DIFFERENT TYPES OF REFRIGERATION PLANTS BASED ON LOCATION 4.1 Surface Plant In this type f plants, air is cled n the surface and then taken t undergrund via intake shaft. This cl air extracts the heat frm the envirnment. 4.1.1 Advantages Simplicity Lwer cst as cheaper refrigerants are used Cnvenience f peratin and inspectin Page 8 f 14
The dispsal f waste heat can be easily dne here Aid in prducing large natural ventilatin pressure because f the difference in the temperature f air in dwncast shaft and upcast shaft. 4.1.2 Disadvantages Pr psitinal efficiency (psitinal efficiency can be defined as the rati f effective cling units prduced at the wrking face t the ttal cling units prduced by the refrigeratr). In deep shafts, because f aut cmpressin cl air frm the surface plant picks up much heat and thus becme less efficient. 4.1.3 Example f surface plant in India SURFACE AIR CONDITIONING PLANT AT CHAMPION REEF MINE, KGF: This surface air cnditining plant was installed in Champin Reef Gld mine in Klar Gld Field in early 1940s (Fig. 4). Specificatins 3.7 MW capacity with ammnia as refrigerant Cnsisted f 3 reciprcating cmpressrs each f 300kW rating 3 cling water pumps each f 22kW rating and 6.1m 3 /min capacity Shell and tube type cndenser Plant was manufactured by Messers J&E Hall Ltd., Dartfrd, Kent, UK Wrking Ammnia is cmpressed and passed thrugh il separatr prvided n the delivery side. This il free ammnia gas is cled t ammnia liquid in the cndenser. Water is circulated in this cndenser by 3 water pumps. Then liquid ammnia is passed thrugh shell and tube type evapratr. Heat exchange takes place here between liquid ammnia and the calcium chlride slutin [Calcium chlride slutin is used here t extract heat frm mine air]. Page 9 f 14
Liquid ammnia is thus cnverted again t ammnia gas and sent t cmpressr. NOTE: With this plant we can cl 70.8 m 3 /s f air at 21.1 C Dry Bulb Temperature and 18.3 C Wet Bulb Temperature t 4.4 C saturated. Fig. 4 Surface refrigeratin plant at Klar Gld Fields (after Misra, 1986) 4.2 Undergrund Plant It is the next step in air cling in which the cling plant is entirely lcated undergrund. The air can either be cled at the centrally lcated undergrund plant itself r at chilled water spray chamber near the face. Anther methd f cling air is t use radway cler. Here the cling cils shuld be psitined such that they are at minimum distance frm the wrking place. The cling cils get chilled water frm refrigeratin plant and cl the air. 4.2.1 Advantages It has high psitinal efficiency. The heating f cled air due t aut cmpressin is avided here. Page 10 f 14
It eliminates the need f surface-cnnecting pipe ranges and the pumping csts assciated with this. It als avids any envirnmental prblems that may arise frm surface plant. 4.2.2 Disadvantages The cst f refrigerant is mre as nn-txic refrigerant has t be used here which csts fur times as that f ammnia (cmmnly used in surface plants). The dissipatin f the heat extracted frm mine air is als difficult. Because f the dust depsitin n the tubes f cling pipes, it requires frequent cleaning. The dust depsitin results int pr efficiency f heat transfer. This happens in dry and dusty mines. The capacity f these plants is usually small (up t 1.75MW). S a single plant may nt be capable f serving the whle mine. 4.2.3 Example f undergrund air-cling plant Lcatin: Champin Reef Mine, Klar Gld Fields, India. It was installed at the 80 th level t cl the part f intake air which was 2318m belw the surface (Fig. 5). It had 3 cylinder 125 h.p. reciprcating cmpressrs and Fren 12 (CCl 2F 2) as refrigerant. A 30kW pump with 51m head was used t maintain the circulatin f water. The cndensing water was cled by spraying it in an upcast shaft. It had a capacity f cling 9.44m 3 /s at 305.9K dry bulb temperature and 293.2K wet bulb temperature t 276.5K saturated. Page 11 f 14
Fig. 5 Undergrund plant at Klar Gld Fields (after Misra, 1986) 4.3 Spt Clers These are small capacity refrigerating units, applied mainly t ventilate islated ht wrkings. They are semi-prtable in nature and have refrigeratin capacities in the range 50 t 500 kw. Based n size r mechanism f refrigeratin, they can be divided int tw brad categries Page 12 f 14
a. Small spt clers These have arrangement fr direct cling f air stream n the evapratr cils using evapratin f refrigerants. b. Big spt clers These use sme intermediate clant such as water t cl the air. 4.3.1 Advantages f spt clers Maximum psitinal efficiency f all the three Helps in dust suppressin at the face Cmpactness Very effective in deep metal as well as cal mines 4.3.2 Disadvantages f spt clers Thugh it cls face, leaves the rest f mine air ht Very expensive 4.3.3 Example f spt cler installatin in India Spt cler in 102 nd level f Champin reef, KGF. Features f the installed spt cler were: Capacity 140 kw Refrigerant used R 12 Vlume f air cled- 6.37 m 3 /s Cling range Dry bulb temperature frm 41.6 t 33.3 Wet bulb temperature frm 25.8 t 22.2 Page 13 f 14
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 14 f 14