Oct. 15, 1946. ‘ J, P, REES 2,409,388 CONDITIONING THE ATMOSPHERE IN SUBTERRANEAN EXCAVATIONS Filed Aug. 18, 1942 , \w I v v v v > INVENTQQ John PrIQQ, @QS , (Eng/‘4 WI; GL1 ATTTKI. Patented Oct. 15, 1946 UNITED 2,409,388 STATES PATENT OFFICE 2,409,388 CONDITIONING THE ATMOSPHERE IN SUBTERRANEAN EXCAVATIONS John Price Rees, Johannesburg, Transvaal, I Union of South Africa Application August 18, 1942, Serial No. 455,268 'In the Union of South Africa July 4, 1942 6 Claims. >(Cl. 98--50) 1 2 This invention relates to conditioning the at mosphere of subterranean excavations and par ticularly deep mines. Essential requisites of such depth of downcast travel. It is moreover inde pendent of the kind of gases of which the Ven tilating current is composed. an atmosphere are that it be maintained in re Some limitation of the temperature rise at the lower termination of the downcast system has in practice been attained by pre-cooling the ven tilating air at the surface where the extracted heat can readily be disposed of to the earth’s at mosphere; but it seems that the limit of-tem perature reduction that can economically be at tained in this way amounts to about 10 degrees spirable condition in the sense of preserving the proper pro-portion of its physiologically active constituents to its diluent and physiologically inert constituents, and ensuring the removal of deleterious substances such as phthisical dust and gaseous products of blasting; and further that its temperature, with regard to its cooling e?ect upon human beings working in it, be kept from rising to a level at which persons cannot work with a reasonable degree of, efficiency and may Fahrenheit; su?icient, when the temperature gradient is ‘of the order of l‘degree Fahrenheit per 200 feet of depth, to extend the feasible depth oimining by about 4,000 feet. ' The present invention attacks the autocom pression problem in a different manner, by tak ing account of the fact that whilst the quantity of heat developed by autocompression is virtual even be in danger of physical collapse. The maintenance of the atmosphere in the respirable condition mentioned above can ‘ be effectively carried out by ventilating currents of su?icient volume drawn. from and returned to the earth’s atmosphere. v As regards the heat‘ problemrthe ventilating air undergoes ariseof temperature due to acquir ing heat from two principal sources, viz. (a) heat inflowing from the rock, which is at atem perature higher than that of the earth’s atmos phere from which the ventilating current is de rived; and (b) the heat due to the autocompres sion of the ventilating air. In ‘the process of 20 ly~beyond control, the‘temperature rise clue to the heat of autoco'mpression can be lessened by increasing the‘ speci?c heat of the ventilating current. ~ For this purpose there is substituted ‘ for the normal air that is passed vintothe mine and forms the mine atmosphere, a respirable gas mixture having a higher speci?c heat than that of normal air; so that with a given quantity of . heat added to such substitute atmosphere‘ the‘ autocompression, the air descending the down temperature rise is less, in correspondence with cast shaft moves towards the centre of the earth 30 the greater ‘capacitay for heat of the substitute and is thereby made denser by the -correspond— atmosphere.‘ More speci?cally, since any‘ respir ing increase of the mass of air lying above it; ableatmosphere must contain oxygen and car the energy which thus appears as increased bon dioxide in invariable proportions within denseness of the air and heat being the poten small limits diluted with a physiologically inert tial energy that the air has parted with during gas, the change made by the invention consists its descent. in substituting the nitrogen of the normal at Rise of temperature of the mine atmosphere mosphere partly or completely by a similarly due to cause (a) can be and is being kept within physiologically inert gas, but one which has a control by such means as making the split ven higher speci?c heat than nitrogen. Hydrogen tilating currents ample enough to sweep away 40 for instance which has a specific heat about the heated air as it is formed; and to some‘ex fourteen times that of nitrogen, and which is tent by employing heat pumps to extract‘the heat physiologically inert, could be used under condi from small local areas of the mine where work is tions that would preclude chemical combination actually going on and conveying away the ex of the hydrogen with the necessary quota of tracted heat in a manner that avoids re-heating 45 oxygen. But under present circumstances the of the downcast air. ' most‘ suitable gas is helium, which has been The control of temperature rise due to auto V proved‘ to be physiologically suitable, and even compression is however a problem of a different bene?cial as compared with nitrogen‘; is available kind. The heat developed inthis way cannot in large quantities; is inert chemically in regard for instance be swept out by increasing the ven to oxygenand carbon dioxide; and is obtainable tilating current since the heat is developed in ‘at a cost that renders its use an economic pos the ventilating air itself and each unit mass of‘ sibility; Its speci?c heat is favourable; the air that descends the shaft develops its own specific heat of helium air—viz. air ‘in which quota of heat; the only variable factor that in nitrogen has been wholly replaced by helium-be ?uences the ‘amount of heat being the vertical 55 ing 0.57 as compared with 0.24 for normal nitro 2,409,388 3 gen air, at the same pressure. The temperature of helium air would therefore rise about 2.2 de grees Fahrenheit per 1,000 foot depth; apart from the cooling effect of evaporation from Wet sur faces. Such evaporation would cool helium air rather more than it cools an equal volume of ordinary air and the rise in wet bulb tempera ture would be less than with ordinary air. For 4 is preferred in the present case to ensure its ab sence by using an innocuous explosive such as liquid oxygen for the purpose of breaking rock. Further plant that may advantageously be in cluded in the duct I5 is that indicated by 2!, the function of which is to effect the known surface cooling of the ingoing ventilating current with the object of lowering the temperature of the atmosphere in the mine workings. Without the instance at 12,000 feet the wet bulb temperature would be about 60° F. the surface wet bulb tem 10 reduction of temperature by means of the present invention it is not-as mentioned above-feasible perature being 50° F. . M . to effect much reduction of underground temper Experience gained in connection with the Wit atures by surface cooling without the great ex watersrand mines suggests the addition of about pense involved in providing a cooling plant using 10° F. for uncontrolled heating in the shaft 1. e., brine as the cooling medium; but the cooling a little from the rockwalls and the rock being brought about by the invention, added to the sur hoisted, from hoists, from water being pumped face cooling which can be cheaply effected when out etc., giving an estimated temperature of 70° the cooling medium is kept above the freezing F. wet bulb on the shaft station on the average temperature of water, enables a substantial cool for the year. ing effect to be produced underground at moder The above calculation indicates that in order ate expense. to extend the depth of mining to about 12,000 In the operation of the ventilating system, us feet vertically below the surface, it would not be ing helium air in which all the nitrogen is re necessary to replace all the nitrogen in the air placed by helium, the helium air would reach by ‘helium. In practice the amount of helium the surface at a higher temperature than ordi may be adjusted to give comfortable conditions nary upcast air due to the reversal in the upcast for mining with minimum outlay on helium. For shaft l2 of the autocompression occurring in example, gas mixture consisting of approximate shaft H. For example, for a certain mine depth ly 40% helium by volume, 40% nitrogen, 20% at which ordinary upcast air would be expected oxgen', together with a small proportion of car bon ‘dioxide would have a speci?c heat of ‘0.32. The wet bulb temperature would be, therefore, about 80° F. at 12,000 feet on the shaft station. 'In view of the monetary value of the helium it is important that the mine ventilating system be constructed as a closed circuit that retains the v to arrive with a temperature of 70 to 75° F., the helium air would arrive with a temperature of about 80° F. This excess over about 70° is how ever easily dealt with at low cost by sprays and cooling towers and without refrigeration. If the cooler 2! is arranged to cool the current from about 70° saturated to about 50° saturated, the resulting gas temperature at 12,000 feet would helium with a minimum of loss whilst providing for there-conditioning of the gas mixture coming as a consequence be reduced by about 30° F. in from the'upcast to render it suitable for return the case of helium air as compared with about to the mine. In particular, it is necessary to dis charge from the exhaust current the heat it has 40 10° F. in the case of ordinary air. The conversion of an ordinary mine atmos acquired from the mine workings: to make up phere to the atmosphere of the invention offers oxygen consumed and helium lost; and to remove no outstanding di?iculty since it involves no deleterious substances. Such a circuit is shown change in the respirability of the atmosphere. in the accompanying drawing which is a diagram matic section through the mine. Therein ID in 45 It wouldbe necessary to continue the normal procedure of discharging the upcast ventilating dicates workings in which persons are engaged in current to the open air while the helium is being mining. II is a shaft which gives access to said introduced into the downcast current. Loss of working and is also the downcast shaft of the helium in such exhaust would be vminimized by ventilating system. I2 is the upcast shaft of the having large quantities of helium stored atthe system; and I3 is a ventilating fan which may be surface and introducing it as rapidly as possible, positioned at the foot of the upcast shaft with the so that current-from the upcast shaft can be di object of setting up a plenum in the latter and verted as soon as possible into the conduit I5 thereby preventing leakage of normal air into and the discharge of such current to atmosphere the ventilating atmosphere. I4 indicates the surface of the mine, where 55 simultaneously terminated. I claim: there isv erected an enclosing structure [5 forming 1. A closed circuit mine ventilating systemin a duct through which the gas discharge from cluding mine workings, downcast and upcast the upcast shaft I2 is passed to the downcast shaft shafts connected with said workings, means dis II. Said structure l5 encloses the headgear I6 and is provided with an airlock I‘! through which 60 tant from the workings for discharging heat from gas in the circuit, and means for injecting into tra?ic to and from shaft H can pass with mini the circuit a respirable inert gas which com mum leakage of gas into or from the closed cir prises helium in a proportion of at least about cuit. . 40% by volume. Said structure l5 also includes means indicated 2. A process of conditioning a mine atmos generally by [8 by which rock dust, smoke and 65 phere so as to minimize heating of the downcast soluble gases given off in the mine are removed ventilating current due to its autocompression, by water sprays l9, which may also cool the air to some extent. 20 indicates further means through which the gas current passes for the which process comprises the step of passing into the _mine to provide the atmosphere therein a addition thereto of make up oxygen and helium, 70 non-explosive respirable gas mixture containing the substantially normal atmospheric proportion and for the removal of excess carbon dioxide. of oxygen and in which the normal nitrogen con Carbon monoxide is a constitutent of the exhaust tent of atmospheric air is replaced by a diluent current from a mine in which nitroglycerin and gaseous component selected from the group con similar explosives, are used; but as that gas is dif?cult to remove from a ventilating current, it 75 sisting of hydrogen, helium, and the nitrogen 5 6 helium mixtures containing at least 50% by vol ume of helium under ‘conditions which preclude interaction between the components of said re spirable miXture. ‘ ' 3. A process of conditioning a mine atmos phere so as to minimize heating of the down cast ventilating current due to its autocompres a subterranean excavation having a depth of at’ least about 10,000 feet, which process comprises the step of passing into the excavation to provide the atmosphere therein at substantially atmos pheric pressure at earth surface level a respirable gas mixture containing the substantially normal atmospheric proportion of oxygen and in which the normal nitrogen content of atmospheric air is sion, which process comprises the step of passing replaced to at least 50% of its volume by helium. into the mine to provide the atmosphere therein 6. A process of conditioning the atmosphere of a non-explosive respirable gas mixture contain 10 a subterranean excavation having a depth of at , ing helium to at least about 40% of its volume. 4. A process of conditioning the atmosphere of least 10,000 feet, which process comprises pass ing into the excavation to provide the atmos a subterranean excavation having a depth of at phere therein at substantially atmospheric pres least about 10,000 feet, which process comprises the step of passing into the excavation to provide 15 sure at earth surface level a respirable gas mix ture containing the substantially atmospheric the atmosphere therein at substantially atmos proportion of oxygen and in which the normal pheric pressure at earth surface level a respirable nitrogen content of atmospheric air is replaced gas mixture containing the substantially normal atmospheric proportion of oxygen and in which to at least about 50% of its volume by helium, the normal nitrogen content of atmospheric air 20 circulating said gas mixture through a closed cir is replaced by a diluent gaseous component se cuit, cooling said mixture at at least one point of said closed circuit and removing from said mix lected from the group consisting of hydrogen, helium, and the nitrogen-helium mixture con taining at least 50% by volume of helium whilst preserving in said excavation conditions which preclude chemical interaction between the com ponents of said respirable mixture. . 5. A process of conditioning the atmosphere of ture at at least one point of said closed circuit such carbon dioxide as may be in excess of the substantially normal atmospheric proportion of carbon dioxide. JOHN PRICE REES.