August 6, 1915. THE COLLIERY GUARDIAN. 269 Influence of Moisture on Mine Ventilation.* By ARTHUR C. WHITTOME. The objects of ventilation are :—(1) To sweep all dele- terious gases out of the ventilated zone; (2) to supply to the working places such a weight of oxygen (diluted with nitrogen) as may be required by the workers therein; (3) in some cases to lower the temperature of the working places to such a degree as to make work possible in spots which would otherwise be at too high a tempera- ture. The author proposes to use the term “ oxygen stuff ’ ’ to represent the pure dry air portion of either dry or saturated air. The term, whilst somewhat clumsy, is convenient. According to Trautwinef, a grown person hard at wrork breathes from 0-5 to 1'05 cu. ft. of air per minute (pre- sumably at sea level). Pure dry air comprises, by weight, about 23-14 parts oxygen and about 76'86 parts nitrogen; 1 cu. ft. of air at sea level and 60 degs. Fahr, temperature weighs about 0-07639 lb.; therefore it follows (taking Trautwine’s figures) that each person working hard inhales from 0-0088 to 0-0178 lb. of oxygen per minute, and requires from 34 to 441b. of oxygen to be circulated in the working place in order to keep the air at the necessary standard of purity; in each case the oxygen is, of course, diluted with sufficient nitrogen to make the air breathable, this necessitates the circulation of from 14'4 to 19-5 lb. of oxygen stuff per minute per person. The author wishes to emphasise that the primary object of ventilation is the provision at each working place of such a weight of oxygen stuff as will ensure an atmosphere suitable for breathing, and not the supply of a stated volume of air. If the latter definition is accepted, the modifications of requisite volume involved by the varying density and humidity of the air are almost certain to be overlooked. The first object of this paper is to show the importance of these factors. Following upon this is the relation which the degree of humidity and the temperature of the air bear to Hie intensity of draught required to obtain a given ventilating current by either natural or artificial means. Air is either :—(1) Dry, i.e., there is no vapour of water mixed with it; or (2) saturated, i.c., it contains inversely wTith the degree of humidity of the air. To find the elastic force and the weight of water vapour present in Icu.ft. of partially saturated air, the dew point of the air must be ascertained, the two factors then corre- spond with those of saturated steam at this lower temperature, the necessary correction being made for the contraction of volume of the vapour. Water vapour can only be separated from free or com- pressed air in three manners :—(1) By chemical separa- tion, as in the chemical determination of the weight of aqueous vapour in air by passing the air through test tubes containing pumice saturated with sulphuric acid; (2) by lowering the temperature of the air whilst keep- ing its pressure constant, a portion of the water vapour then falls out of the air as dew or rain; (3) by compress- ing the air whilst keeping its temperature constant, this again resulting in the condensation of a portion of the water vapour. It therefore follows that in mine work, where the air is gradually rising in temperature as it passes through the -workings, there can be no condensa- tion of water vapour. In some respects the last column is the most interest- ing, as showing the great differences which exist as regards the volume of air required to carry a given weight of oxygen under different pressures and tempera- tures. Assume that it is necessary to ascertain that volume of free saturated air at 90 degs. Fahr, on the Rand, which would carry the same weight of oxygen stuff that is contained in 1,000 cu. ft. of free dry air at sea level at 60 degs. Fahr. From the table dry air at 14'7 lb. and 60 degs. Fahr, has a ratio of 1-0568; saturated air at 90 degs. Fahr, on the surface at the Rand (about 12'5 lb. absolute) has a ratio of 1-3919. The volume required is found by multiplying the volume at sea level by the ratio under Rand conditions, and dividing by the ratio under sea level conditions, giving about 1,317 cu. ft. in place of the 1,000. Again, say that it is necessary to find out how many cubic feet of saturated air at 131b. barometric pressure and 100 degs. Fahr, equal 1,000 cu. ft. of drv air at 12-5 lb. barometric pressure and 70 degs. Fahr. The respective ratios, from weight of oxygen stuff flows when the air in the down- cast shaft is perfectly dry and that in the upcast shaft is 100 per cent, saturated; (3) increasing the degree of humidity of air in the downcast shaft (without lowering its temperature) decreases the weight of oxygen stuff flowing; (4) increasing the degree of humidity of the air in the upcast shaft (without lowering its temperature) increases the weight of oxygen stuff flowing. It is now necessary to consider the effect on the tem- perature and density of dry, or slightly humid, air, when water vapour is added to it. Again, to take the simplest condition, it can be assumed that the water is at the same temperature as the air with which it is in contact at the moment of its conversion to water vapour. There- fore, the water absorbs only latent heat to an amount equal to the latent heat of water vapour at the temper- ature of the water. The latent heat of water vapour at 60 degs. Fahr, is about 1,072 British thermal units, and at 90 degs. Fahr, about 1,051 British thermal units. It will be found that atmospheric air can never be absolutely dry, and seldom 100 per cent, saturated. Immediately upon the cessation of heavy rain the atmo- sphere can be considered as being 100 per cent, saturated. In such a case, with a barometric pressure of 12-5 lb., and an atmospheric temperature of 50 degs. Fahr., 1 cu. ft. of air would contain 0’00059 lb. of water vapour. Should the temperature be then raised to 70 degs. Fahr., without further addition of water vapour, the 1 cu. ft. will expand to 1-037 cu. ft. This volume of air, if saturated, would contain 0-00119 lb. of water vapour; therefore, as it only contains 0-00059 lb. of vapour, it is about 50 per cent, saturated. The air can, and, if water is available for conversion to vapour, will now pick up further wrater vapour. Should the air (whilst still remaining at 70 degs. Fahr.) become saturated, the volume will expand to 1-055 cu. ft., and it will then carry 0'0012 lb. of water vapour, i.e., 0-00061 lb. greater weight than the original cubic foot at 50 degs. Fahr, carried. On the temperature again fall- ing to 50 degs. Fahr., this surplus water vapour will be condensed, and the air volume will contract to 1 cu. ft. It can be safely stated that air is never 100 per cent, saturated whilst its temperature is constant or is rising, unless water is sprayed into it. If the temperature of the saturated air at 50 degs. Fahr, is lowered, to, say, 32 degs. Fahr., a portion of the water vapour will be t Water Vapour Weight, Dry in Lbs Per Cubic Foot, of Air at Various Absolute Pressures Ei-istk; Force, in’Lp.s. Absolute Per I | Square Inch, of the Oxygen Stuff in Saturated Air at Various Absolute Pressures Weight, in Lbs.,-of the Oxygen Stuff in 1 Cubic Foot of Saturated Air at Various Absolute Pressures. 1 Weight, in Lbs. Per Cubic Foot, of Saturated Aik at Various Absolute Pressures. | Ratios of the Volumes of Dry and of Saturated Air, at Various J’empekatures . ii inx z 1 1 « I IXi ; 12 I Volume at 14 7 Lb» Abs. Pressure and 32’ Fahr. Being Taken as Unity r ii iM Weight in •per cubic f< lbs 111 13 lbs. 12-5 lbs 14-7 lbs. 14 lbs. 13 lbs. 125- Ibs. 14-7 lbs. 14 lbs. 13 lbs. 12-5 lbs. 14-7 lbs. 14 lbs. 13 lbs... IS'5 - lbs. 14'7 lbs. Abs. 14 lbs. Abs. 13 lbs. Abs. 12'5 lbs. Abs. j 1; “ * il II Abs Abs Abs. Abs j Abs. Abs. Abs. Abs. Abs. Abs. Abs. Abs. Abs. Abs. Aba. Abs. Dry. Saturated. Dry. Saturated. Dry. Saturated. Dry. Saturated. | £5 | 1 1 2 I 3 4 6 6 7 8 9 10 .. 1 " 12 13 14 15 16 L.!W 18 I J9 20 21 22 23 24 25 26 | 32* •089’ •00030 ■•0807,3 •07688 •07140 •06864 14-611 13 911 12-911 12-411 •08024 •07640 •07091 06816 08054 •O7G7O 07121 •06846 ! 1 1-0061 1 0500 1 0566 1-1306 11384 1-17C1 1-1844 j 32* 35* •100 •00034 08024 •07642 •07096 1 •06823 14-600 13900 12 900 ‘12-400 •07969 ■ .•07587 •07041 48768 •09003 •07621 •07075 06602 3 1.0061 1-0130 1 0564 1 0640 11376 11465 11832 1.1928 1 35* w* ■122 •00041 07944 •07566 •07025 . -06755 14-578 13878 12-87 A 12-378' • •07878 ‘ •07500 ■06959 •06689 •07919 07544 •07000 •06730 j 1 1016.? 1-0247 1-0670 1-0764 1-1491 1-1600 11951 . 1-2069 1 40‘ w* •147 •00049 •07865 ■07491 •06955 •06688 14-553 13 853 12 853. 12-353 •07786 ■ •07412 •06876 •06609 i -07835 ■07461 •06925 •06658 | 1-0264* 1-0367 1 0777 1-0891 1-1606 1 1740 1-2071 1-2215 I 45- 50* 178 •00059 ■07788 •07417 •06888 06622 14-522 13'822 12'823 : 12'322 07694 •07323 •06793 •06528 07753 07382 •06852 . 06587 1-0366 10591 1 0884 11022 1 1721 11884 1-2191 1-2366 jj SO- W '214 •00070 •07713 •07345 •06821 ■06558 14-486 13 786 12786 12 28'6 . dOlAOl •07234 ■06709 •06446 07671 . 07304 •06779 •06516 1 0467 1 0621 1 0991 11158 1-1836 1-2033 1 2310 1 2524 | 55’ 60* •253 •00083 •07639 •07275 •06755 •06195 14’444 13-744 12744. 12-244 ■07505 07141 •06621 •06361 •07588 •07224 •06704 •06444 1 0568 1-0756 11097 11301 11951 1-2189 1-2429 1 2691 | 1'2863 j 60* 35? •305 ; 00098 ■07566 •07206 •06691 : -06434. 1 .14-395 13 695 12'695 12-195 ■07409 •07048 •06534 •06276 •07507 07146 •06632 •06374 1-0670 1-0896 11203 11452 1-2065 1-2353 1'2548 65* TO’ •363 ;oons 1 07495 •07138 •06628' 06373 14-337 13-637 12 637 12137 -07310 •06953 ■06443 •06188 •07425 •07068 •06558 •06303 1 0771 11044 11310 1-1611 . 1-2180 . 1-2527 1-2667 1 3046 S 70’ 75' •429 •00135 ■07425 •07071 •06566- : -OOSIS 1 14.271 13 571 12'571 12071 •0720.3 •Q6855 •06350 •06097 ■07543 •06990 ■06485 •06232 1-0872 11200 11417 11776 U995 1-2712 1-2787 ' 1-3240 | 75’ 80’ •505 •00157 •07356 •07006 •06506 •06256 L 14195 13'495 13:495 11 995 07104 •06753 •06253 •06003 j 07261 •06010 •06410 •06160 1 0974 . 1 1364 ’ 11'523 11964 I-24O9 1 2910 1 2906 1'3448 E SO- 85’ •594 00183 •07289 06942 06446 •06198 14'106 13 406 12'406 11-906 •06994 •06647 •06151’ •05903 •07177 ■06830 •063S4 •06086 11075 11542 11629 1-2145 1-2524 1 3124 I 3025 1-3674 | 85* 90*- ■696 •00213 07223 •06879 •06388 •06142 14004 13-304 12 304 11-804 | •06381 . 06537 06046 •05800 .07094 ■06750 ’ •06259 •06013 11176 11732 1-1735 1-2349 1-2638 1-3352 1-3144 1’919 | 90* 95* ■813- •00247 07158 •06817 ■06330 ■06087 13-887 13 187 12187 11-687 •06762 •06’421 •05934 •05691 ■07D09 •06668 •06181 •05938 1 1278 1-1938 14842 1 2574 1-2753 1-3604 j 1 3263 1'4185 J 95* 100” •946 •00285 •07094 ■06756 06273 ■06032 13-754 ' 13.054 12 054 11-554 •06638 •06300 •05317 •05576 •06923 J •06583 •06103 •05861 11380 1-2161 11949 1 2615 1-2868 j |-3878 1-3383 1-4478 ’ ICO* ' 1 098,. ■00328 070.31 | 06696 •06218 05979 13 602 12902 1 11 902 11-402 { •06506 •06172 •05693 ■05454 •0C834 j 06500 •05782 ■11482 1-2408 1-2056 1 3080 1-2983 j 1 4180 1-3502 1 4801 _ _ 1 105* the maximum amount of water vapour which it can hold in suspense; or (3) partially saturated, in which case the “ degree of humidity ” represents that percentage of the maximum amount of water vapour that the air can con- tain which is present in it. Air (he., oxygen stuff) and water vapour possess that property of diffusion which is common to all gases. By Dalton’s law the pressure exerted by the mixture will be the sum of the pressures of the gases and vapours present. Therefore, with air partially or completely saturated with water vapour, or mixed with other gases, the absolute pressure of the mixture is the sum of the elastic forces of its consti- tuents. So that if saturated air is at barometric pressure, it follows, naturally, that the elastic force of the oxygen stuff equals the barometric pressure, minus the elastic force of the water vapour. The elastic force of water vapour at normal atmospheric temperatures is quite low. At 60 degs. Fahr, it is only 0-256 lb., and at 90 degs. Fahr. 0-696 lb. per sq. in. When calculating the weight of a cubic foot of saturated air, under any given conditions of pressure and temperature, it is necessary first to separate the absolute pressure of air into the elastic tensions of water vapour and oxygen stuff, and then to find the weight of a cubic foot of each of these constituents under the given tem- peratures and tensions. The sum of these weights is the weight of the cubic foot of saturated air. Speaking broadly, water vapour can be considered as being saturated steam when the air is 100 per cent, saturated, and superheated steam when the degree of humidity is less than 100 per cent. The elastic tension, the total, latent and specific heat, and the density of the water vapour in saturated air, would be the same as those for saturated steam at the same temperature as the air. If the air is not 100 per cent, humid, the cubic foot of partially saturated air still contains 1 cu. ft. of water vapour at the same temperature as the air, but, to provide this cubic foot of vapour, a fraction of a cubic foot at a lower temperature has been superheated to the air temperature, the amount of superheat varying * Abstract of a paper read before the South African Insti- tute of Engineers. t 1904 edition, p. 320. table or curve, are 1-3878 and 1'2667, giving 1,096 cu. ft. as the requisite volume. From the table it is seen that, under a given baro- metric pressure, oxygen stuff decreases, and water vapour increases in weight per cubic foot as the temper- ature rises. Further than this, as the temperature of the saturated air rises, the elastic force of the water vapour increases and, therefore, with saturated air at a constant pressure, the elastic force of the oxygen stuff will gradually decrease with increasing temperature, showing a second cause for the decreased weight of oxygen stuff in saturated air. The increase in the density of water vapour is not at so rapid a rate as the decrease in that of the oxygen stuff, the weight of saturated air, therefore, is lower per cubic foot than that of dry air at the same temperature and pressure, and, with constant pressure and increasing temperature, saturated air decreases in density at a more rapid rate than dry air does. To illustrate the effects that moisture has on mine ventilation, the author makes the following assump- tions as giving the simplest possible case. Other sets of assumptions could be made, but these serve to indi- cate the points just as well as more complicated ones would do :—(a) The collars of the upcast and downcast shafts are located on the same horizon, the shafts being sunk to a vertical depth of 3,000 ft., and connected by a single drive at the 3,000 ft. level, (b) In each shaft the mean barometric pressure is 13 lb. (c) Mean tempera- tures in upcast shaft 90 degs. Fahr., and in downcast shaft 60 degs. Fahr, (d) The following alternative con- ditions of humidity are assumed as being possible :— (Condition 1) : The air remains dry during its passage through the mine workings. (Condition 2) : The air is kept constantly 100 per cent, saturated at all points in its passage. (Condition 3) : The air is dry during its passage through the downcast and saturated during its passage through the upcast shaft. With a constant mean temperature in the downcast shaft, and a second constant mean temperature in the upcast shaft, the following laws are established :—(1) The least weight of oxygen stuff flows when the air throughout both shafts is perfectly dry; (2) the greatest condensed, but the air will be saturated at the lower temperature, and each cubic foot will contain 0'0003 lb. of water vapour. On the temperature being raised without the addition of further water vapour to the air, the degree of humidity would fall, becoming about 50 per cent, at 50 degs. Fahr., 23 per cent, at 70 degs. Fahr., and 9 per cent, at 100 degs. Fahr. “Dry” atmospheric air is therefore unknown; 50 to 66j per cent, saturation can well be assumed as the normal humidity on the Rand. The oxygen stuff in a cubic foot of saturated air at 60 degs. Fahr, and 131b. absolute pressure weighs 0-06621 lb., and, as air at constant pressure has a specific heat of about 0-2375, this weight would require about 0'0157 British thermal units to raise its temperature 1 deg. Fahr. The 0-00083 lb. of water vapour required to saturate Icu.ft. of air at 60 degs. Fahr, during its conversion from water at 60 degs. Fahr, would absorb about 0'89 British thermal units of latent heat, so that, if all the cooling effect due to its evaporation was expended on the air, the air temperature would have fallen about 56-7 degs. Fahr., i.c., from about 116-7 degs. to 60 degs. Fahr. In practice a great proportion of the heat absorbed by the vapour is derived from the body of water from which the vapour is formed, and from the solids (such as country rock, etc.) with which the water is in contact during evaporation, thereby reducing their temperatures instead of that of the air. Allowing that the air would have the normal degree of humidity, and that a portion only of the heat required to evaporate the water used to raise the humidity to 100 per cent, came from the air, it would be fair to assume that the cooling effect on the air would be sufficient to lower the air temperature about one-sixth of the 56'7 degs. Fahr., or, say, 10 degs. Fahr. Therefore, if the air had remained in its normal condition instead of becoming saturated and having its temperature reduced, it would have had a mean temperature of about 70 degs. Fahr., instead of 60 degs. Fahr., and the mean saturation would have been 50 per cent, instead of 100 per cent., in which case it would have weighed about 0-06575 lb. instead of 0'06704 lb. per mean cubic foot. With the lower weight per mean cubic foot in the downcast shaft, the draught would have been reduced, so that, though the greater