would be over 3,500 gal. per day. This going on for months makes a mine more and more dry. No ordinary sprinkler system will entirely overcome this for the air will only absorb moisture. As it becomes heated it expands and dries. On the other hand, moisture in saturated air entering a mine at a higher temperature than that within will condense over the sides and roof of the haulageways and working places, thereby depositing water instead of withdrawing it. This principle seems to be the solution for the coal-dust problem from a humidifying standpoint. In this connection, the Colorado Fuel and Iron Co. installed radiators and steam pipes in the intake of its coal mines in southern Colorado, the radiators to raise the temperature of ingoing air, and the steam pipes to inject the necessary moisture in the form of steam.

The percentage of saturation obtained will depend upon the volume of air entering the mine and its temperature, the heating surface of the radiator and the amount of moisture supplied. That is, the larger the volume and the lower the outside temperature, the greater the heating surface and amount of moisture will have to be to give the same results.

Another method of supplying a mine with a preheated and humidified ventilating air is suggested by the operation and tests of an evaporative condenser installed at the central power plant of a group of mines near Pittsburg, to handle the exhaust steam from turbo generators. This suggestion is made in contradistinction to the steam jet and steam coil heating method.

The condenser referred to for the purpose of humidifying air, consists of a nest of 900 vertical, 1-in. diameter copper tubes, 19 ft. long, fixed top and bottom in suitable headers. The tubes are housed in on two sides, as

shown in Fig. 1, one side being left open for the admission of air, which is drawn in and around the tubes, by a fan placed opposite the open side. The vapor generated by the evaporation of water, with which the tubes are mechanically wetted, is picked up by the air as it passes around the tubes. Where the arrangement of a mine's power equipment will permit, it is suggested that the usual mine fan be made to perform the double service of

drawing the air around condenser tubes, where it takes on heat and moisture in proportion to the work of condensation, as well as through the mine By test performance, the amount of water evaporated per pound of steam. condensed is approximately 1 lb. Figures 2 and 3 will indicate the arrangement of the connections to a condenser, fan, and mine, when blowing or exhausting.

Many variations of the condenser as described could be used for the suggested purpose, but the evaporative kind seems especially adapted, inasmuch as the air required to maintain its efficiency obtains its heat and humidity in a single operation.

It is conceded that cold air must be heated and have a sufficient amount of moisture given to it, to prevent it from absorbing moisture from the mine, as it gradually becomes heated during its passage through the air-courses, thereby increasing its moisture-carrying capacity. The assumption is made that, if the air supplied to a mine be heated to the mine's normal temperature, and that be also given a high relative humidity, it will issue from that mine having practically the same temperature and humidity. It is further assumed that, should it be possible to sufficiently heat and humidify the required amount of ventilating air, somewhat in excess of the mine's normal temperature, and give it a proportional burden of humidity, an amount of moisture would be given off by the air, as its temperature is adjusting itself to that of the mine. The basis for the last assumption lies in the nil effect that any quantity of heat given off by the ventilating air, so treated, would have towards raising the normal temperature of a mine.

However, any interchange of heat that might take place, from air to the walls of a mine, would tend to diminish the moisture-carrying capacity of the air, and would result in the deposit of a certain amount of moisture. The possibility of conditioning sufficient ventilating air, and the amount of exhaust steam required to perform the work, can be judged from the result of a series of problems, the results of which can be displayed graphically by means of curves.

By virtue of a suitable condenser, the air used as a vehicle to carry off the vapor, equal to the amount of water placed on the tubes necessary to effect condensation within them, produces in one operation a preheated and humidified atmosphere. It seems possible to adjust the degree of heat and humidity imparted to air passing through such a condenser to such a degree that the comfort of the miner would in no way be affected. By so doing, however, it might be necessary to forego some inches of vacuum which might otherwise be available at the engine in order to maintain the adjustment. Where a sufficient horsepower of exhaust steam is not at hand, the amount of coal required under a boiler, working at 60% efficiency, to produce lowpressure steam used in coils to heat air, is approximately 6 T. per 24 hr., for a unit of 100,000 cu. ft. of ventilating air per min. In such an arrangement, it must be understood that the air is humidified by a second operation and does not come into direct contact with the steam formed in the boiler. From a hygienic standpoint, mine ventilating air treated in the manner described would, to a considerable extent, bring about the same results that are claimed for devices now being used to condition air used to ventilate public buildings, assembly halls, and many up-to-date residences.

Considering the many factors entering into the successful operation of such an air-tempering device, when adapted to the general mine proposition, it is quite difficult to draw definite conclusions; however, the foregoing matter possesses sufficient merit to warrant consideration of mine operators.

Hygrometers.-The use of the hygrometer is in its infancy for observations in coal mines and while the complete rotary sling hygrometer or psychrometer is undoubtedly the most accurate for obtaining humidity readings, it is too delicate an instrument to carry around underground. The hygrometer shown in Fig. 4 is inclosed in a carrying case which converts it into a pocket instrument that is not liable to become broken when carried about in the mine. The wet and dry thermometers are inserted in each side of a split cylindrical case which is readily closed or opened by a handle. It is easy to swing but it is not so quick as the sling hygrometer. The thermometers are mounted on springs to lessen the danger of breakage, and this, with the case, makes a handy arrangement for underground observations.

Recording hygrometers, giving a single record of the relative humidity have been in use for a long time. Engineers have appreciated the distinct advantage of having a record of both the dry-bulb temperature and the wet-bulb temperature independently but simultaneously on the same chart.

Such an instrument has the added advantage in the ease with which its accuracy can be checked with a standard thermometer at all times. The importance of proper conditions of temperature and humidity is being more and more appreciated in its effect on coal dust.

The recording hygrometer illustrated in Fig. 6 consists of two sensitive bulbs mounted in tandem back of the case, the wet bulb being jacketed and kept moist by maintaining water at a constant level in a trough beneath the bulb. The pen arms are attached directly to shafts concentric with the

FIG. 5 helical tube bulbs. The case is mounted on a swivel bracket enabling the swinging of the instrument at right angles to the wall or support, and giving easy access to the inverted glass bottle serving as a water reservoir. It is made to cover ranges between the freezing and boiling points of water (32° to 212°F. or 0-100°C.).

The principle of the construction of the recording hygrometers is shown in Fig. 5. In the cut A represents the reservoir, the tube B, the dry bulb which records the atmospheric temperature, and C the wet bulb which is covered with a special jacket leading down into trough D, containing water. The bulb C is always cooler than B due to the evaporation of the water. The evaporation increases or diminishes according to the amount of moisture in the air. Taking the difference between the two thermometer readings and consulting the table that is sent with the instrument the relative humidity is quickly ascertained.

Pressure as Affecting Explosive Conditions.-Gaseous mixtures that are not explosive in the ordinary condition of a mine, often become explosive under the momentary pressure to which they are subjected by heavy blasting, and, in some instances, this may occur from the concussion of the air caused by the quick shutting of a door. In the latter case, however, the

explosive condition of the air would necessarily have to be close to the limit, in order for such a slight occurrence to precipitate an explosion. The factor of pressure as increasing the explosiveness of gaseous mixtures should be considered and constantly borne in mind.

Rapid Succession of Shots in Close Workings. It constantly happens that two, three, or more shots are fired by means of fuse or touch squibs in a single chamber or heading, where the circulation of air is not always the best. The practical effect is that a considerable quantity of carbonic-oxide gas, CO, is produced by the firing of the first shot, and this gas does not have time to diffuse or become diluted by the air current before it is fired by the flame of the following shots. An explosion may often be precipitated by such an occurrence, if the workings are at all dusty. Two shots at the most are all that should be fired at one time in a close chamber or heading.

Mine explosions are commonly the result of the ignition of firedamp with an open lamp, or coal dust exploding after being set in motion by an explosion of gas, blown out shot, fall of roof or a rush of air and an electric arc. Numerous other cases of explosions are recorded but are not in the class commonly referred to as mine explosions.

Before the coal-dust theory was advanced and proven, it was believed that wherever the greatest damage was done, was the point where the explosion originated. This is by no means always the case.

QUANTITY OF AIR REQUIRED FOR VENTILATION

The quantity of air required for the adequate ventilation of a mine cannot be stated as a rule applicable in all cases. Regulations that would supply a proper amount of air for ventilation of a thick seam would be found to cause great inconvenience if applied without modification to the workings in a thin seam. Likewise, the ventilation of an old mine with extended workings, a large area of which has been abandoned, and in many cases not properly sealed off, will require, naturally, a larger quantity of air per capita than a newly opened mine or shaft. The natural conditions existing in rise and dip workings, with respect to the gases that may be liberated or generated in those workings, call for the modification of the quantity of air required in each case. For example, dip workings, where much blackdamp is generated, will require a larger quantity of air, or higher velocity at the working face, to carry off such damps; and rise workings, liberating a large amount of marsh gas, will likewise require a higher velocity at the working face. the other hand, a reversal of these conditions, such as a large quantity of marsh gas being liberated in dip workings, or a similar amount of blackdamp being generated in rise workings, will require a comparatively low velocity of the air at each respective working place.

On

Quantity Required by State Laws.-The quantity of air required by the laws of the several states is generally specified as 100 cu. ft. per man per min., and in many cases an additional amount of 500 cu. ft. per animal per min. is stated. This quantity is in no case stated as the actual amount of air required for the use of each man or animal, but is only the result of experience, as showing the quantity of air required for the proper ventilation of the average mine, based on the number of men and animals employed. The number of men employed in a mine is an indication of the extent of the working face, while the number of animals employed is an indication likewise of the extent of the haulage roads, or the development of the mine. These amounts refer particularly to non-gaseous seams.

The Bituminous Mine Law of Pennsylvania specifies that there shall be not less than 150 cu. ft. per min. per person in any mine, while 200 cu. ft. are required in a mine where firedamp has been detected.

The Anthracite Mine Law of Pennsylvania specifies a minimum quantity of 200 cu. ft. per min. per person. Each of these laws contains modifying clauses, which specify that the amount of air in circulation shall be sufficient to "dilute, render harmless, and sweep away" smoke and noxious or dangerous gases. Some mining companies specify the amount of air that must pass the last breakthrough and that the breakthrough shall not be more than a certain distance from the face of room or heading. One such company operating several mines in a region where mine explosions are fairly frequent has never had an explosion. Its rule is to have 12,000 cu. ft. of air per min. passing the last breakthrough which must not be over 100 ft. from the working face.

Quantity of Air Required for Dilution of Mine Gases.-To determine this requires a knowledge of the quantity of gas generated or liberated in the

workings. The quantity of air for dilution should be ample, and should be such as not to permit the condition of the current to approach the explosive point. The ventilation should be ample at the face.

Quantity of Air Required to Produce the Necessary Velocity of Current at the Face. This consideration modifies considerably the quantity of air required for the ventilation of thick and thin seams. The velocity of the current is dependent not only on the quantity of air in circulation, but on the area of the air passage. This area is quite small in thin seams, and often very large in thick seams. As a result, the velocity is often low at the face of thick seams, and insufficient for the proper ventilation of the face, although the quantity of air passing into such a mine may be very large. A certain velocity of the current is always required in order to sweep away the gases. This velocity depends on the character of the gases and the position of the workings. Heavy damps are hard to move from dip workings where they have accumulated; and, likewise, lighter damps accumulating at the face of steep pitches are hard to brush away, and the velocity of the current in these cases must be equal to the task of driving out these gases.

ELEMENTS IN VENTILATION

The elements in any circulation of air are (a) horsepower, or power applied; (b) resistance of the airways, or mine resistar ce, which gives rise to the total pressure in the airway; (c) velocity generated by the power applied against the mine resistance.

Horsepower or Power of the Current.-The power applied is often spoken of as the power upon the air. It is the effective power of the ventilating motor, whatever this may be, including all the ventilating agencies, whether natural or otherwise. The power upon the air may be the power exerted by a motive column due to natural causes, or to a furnace, or may be the power of a mechanical motor. The power upon the air is always measured in foot-pounds per minute, which expresses the units of work accomplished in the circulation.

Mine Resistance.-The resistance offered by a mine to the passage of an air current, or the mine resistance, is due to the friction of the air rubbing along the sides, top, and bottom of the air passages. This friction causes the total ventilating pressure in the airway, and is equal to it. Calling the resistance R, the unit of ventilating pressure (pressure per square foot) p. and the sectional area of the airway a, we have, R = pa; that is to say, the total pressure is equal to the mine resistance.

Velocity of the Air Current.—Whenever a given power is applied against a given resistance, a certain velocity results. For example, if the power u (foot-pounds per minute) is applied against the resistance pa, a velocity (feet per minute) is the result; and since the total pressure pa moves at the velocity v, the work performed each minute by the power applied is the product of the total pressure by the space through which it moves per minute, or the velocity. Thus, u = (pa)v.

Relation of Power, Pressure, and Velocity.-The relation of these elements of ventilation is not a simple relation. For example, a given power applied to move air through an airway establishes a certain resistance and velocity in the airway. The resistance of the airway is not an independent factor; that is to say, it does not exist as a factor of the airway independent of the velocity, but bears a certain relation to the velocity. Power always produces resistance and velocity, and these two factors always sustain a fixed relation.

This relation is expressed as follows: The total pressure or resistance varies as the square of the velocity; i.e., if the power is sufficient to double the velocity, the pressure will be increased 4 times; if the power is sufficient to multiply the velocity 3 times, the pressure will be increased 9 times. Thus, we observe that a change of power applied to any airway means both a change of pressure and a change of velocity.

Again, since the power is expressed by the equation u = (pa)v, and since pa, or the total pressure, varies as v2, the work varies as v3. From this it follows that, if the velocity is multiplied by 2, and, consequently, the total pressure by 4, the work performed (pa)v will be multiplied by 23 = 8. thus learn that the power applied varies as the cube of the velocity.

MEASUREMENT OF VENTILATING CURRENTS

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The measurement and calculation of any circulaton in a mine airway includes the measurement of (a) the velocity of the air current, (b) of pres