Secondary Splits.-(1) 4 ft. X5 ft., 800 ft. long.

(2) 4 ft. X5 ft., 500 ft.

long. (3) 4 ft. X5 ft., 400 ft. long. (4) 4 ft. X 5 ft., 300 ft. long.

The calculation is often shortened, when many splits are concerned, by using the relative potential, omitting the factor k; but the final result must then be multiplied by k to obtain the pressure or power; or, these factors must be divided by k, when finding the quantity, as in formulas (49) to (51). PROPORTIONATE DIVISION

Primary Splits (only).-(1) 4 ft. X5 ft., 800 ft. long = 3,500 cu. ft. (2) 4 ft. X5 ft., 1,200 ft. long = 6,500 cu. ft.

To accomplish this division of air, the pressure in split (1) must be increased by means of a regulator to make it equal to the pressure in the free or open split (2), and, hence, the pressure due to the regulator is equal to the difference between the natural pressures in these splits.

Secondary Splits.-(1) 4 ft. X5 ft., 800 ft. - 3,500 cu. ft. 500 ft.-6,500 cu. ft. (3) 4 ft. X5 ft., 400 ft. -4,000 cu. ft. 300 ft.-2,500 cu. ft.

NOTE. When using the relative potential, multiply the obtain the pressure, or the power.

(2) 4 ft. X5 ft., (4) 4 ft. X5 ft.,

result by k, to

2

(1) .0000000217 7(2,50092) =.091546 lb.

Since the natural pressure in (3) is greater than that in (4), (3) is the free split, and its natural pressure is the pressure for the secondary splits. The pressure for the primary splits is then found by first adding the pressures in (2) and (3), and if their sum is greater than the natural pressure for (1), it becomes the pressure for the primary splits, or the mine pressure. If the natural pressure for (1) is the greater, this is made the free split, and its natural pressure becomes the primary or mine pressure. In this case, the secondary pressure must be increased by placing a regulator in split (3).

Primary or mine pressure.

Pressure due to the regulators.

.0004 X 2,500
.220934
5.2
.0004 X 3,500

=4.8514 sq.

ft.

(1)

.8654
5.2

METHODS AND APPLIANCES IN THE VENTILATION OF MINES

Ascensional Ventilation.-Every mine, as far as practicable, should be ventilated upon the plan known as ascensional ventilation. This term refers particularly to the ventilation of inclined seams. The air should enter the mine at its lowest point, as nearly as possible, and from thence be conducted through the mine to the higher points, and there escape by a separate shaft, if such an arrangement is practicable. Where the seam is dipping considerably and is mined through a vertical shaft, the upcast shaft should be located as far to the rise of the downcast shaft as possible. The intake air is then first conducted to the lowest point of the dip workings, which it traverses upon its way to the higher workings. In the case of a slope working where a pair of entries is driven to the dip, one being used as the intake and the other the return, there being cross-entries or levels driven at regular intervals along the slope, the air should be conducted at once to the inside workings, from which point it returns, ventilating each pair of crossentries from the inside, outwards. Where the development of the crossentries or levels is considerable, their circulation is considered separately, and a fresh air split is made in the intake at each pair of levels. In ali ventilation, the main point to be observed is to conduct the air current first to the inside workings, from whence it is distributed along the working face as it returns toward the upcast.

The

General Arrangement of Mine Plan.-Every mine should be planned with respect to three main requirements, viz.: (a) haulage; (b) drainage; (c) ventilation. These requirements are so closely connected with one another that the consideration of one of them necessitates a reference to all. mine should be planned so that the coal and the water will gravitate towards the opening, as far as possible. There are many reasons, in the consideration of non-gaseous mines, why the haulage should be effected upon the return airways. The haulage road is always a dusty road, caused by the traveling of men and mules, as well as by the loss of coal in transit, which becomes reduced to fine slack and powder. If the haulage is accomplished upon the intake entry or air-course, this dust is carried continually into the mine and working places, which should be avoided whenever possible. When the loaded cars move in the same direction as the return air, the ventilation of the mine is not as seriously impeded. It is often the case that fewer doors are required upon the return airway than upon the intake, which is a feature favorable to haulage roads. Again, in this arrangement, the hoisting shaft is made the upcast shaft, which prevents the formation of ice, and consequent delay in hoisting in the winter season. The arrangement, however, presupposes the use of the force fan or blower, since if a furnace or exhaust fan is employed, a door, or probably double doors, would have to be placed upon the main haulage road at the shaft bottom, which would be a great hindrance.

In the ventilation of gaseous mines, however, other and more important considerations demand attention. The gaseous character of the return current prevents making the return airway a haulageway. In such mines, the haulage should always be accomplished upon the intake air, as any other system would often result in serious consequences. In such gaseous mines, men and animals must be kept off the return airways as far as this is possible.

As far as practicable, ventilation should be accomplished in sections or districts, each district having its own split of air from the main intake, and its own return connecting with the main return of the mine. Reference has been made to this under Distribution of the Air in Mine Ventilation. This splitting of the air current is accomplished preferably by means of an air bridge, either an under crossing or an over crossing. There are, in general, three systems of ventilation, with respect to the ventilating motor employed: (a) natural ventilation: (b) furnace ventilation; (c) mechanical ventilation.

Natural ventilation means such ventilation as is secured by natural means, or without the intervention of artificial appliances. such as the furnace, or any mechanical appliances by which the circulation of air is maintained. In natural ventilation, the ventilating motor or air motor is an air column that exists in the downcast shaft by virtue of the greater weight of the downcast air. This air column acts to force the air through the airways

of the mine. An air column always exists where the intake and return currents of air pass through a certain vertical height, and have different temperatures. This is the case whether the opening is a shaft or a slope; since, in either case, there is a vertical height, which in part determines the height of air column. The other factor determining the height of air column is the difference of temperature between the intake and return. The calculation of the ventilating pressure in natural ventilation is identical with that of furnace ventilation, which is described later.

Ventilation of Rise and Dip Workings. We have referred to the air column existing either in vertical shafts or slopes as the motive column or ventilating motor. Such an air column will be readily seen to exist in any rise or dip workings within the mine, and may assist or retard the circulation of the air current through the mine. It is this air column that renders the ventilation of dip workings easy, and that of rise workings correspondingly difficult, depending, however, on the relative temperature of the intake and return currents; the latter usually is the warmer of the two, which gives rise to the air column. The influence of such air columns must always be taken into account in the calculation of any ventilation. This is often neglected.

The influence of air columns in rise or dip workings, within the mine, becomes very manifest where, from any reason, the main intake current is increased or decreased. For example, a mine is ventilated in two splits, a rise and a dip split; a current of 50,000 cu. ft. of air is passing in the main airway, 30,000 cu. ft. passing into the dip workings, and 20,000 into the rise workings. A fall of roof in the main intake airway, or other cause, reduces the main current from 50,000 to 35,000 cu. ft. Instead, now, of 21,000 cu. ft. going to the dip workings and 14,000 to the rise workings, we find that this proportion no longer exists, but that the dip workings are taking more than their proportion of air, and the rise workings less. Thus, the circulation being decreased to 35,000 cu. ft., the dip workings will probably take 25,000 cu. ft., and the rise workings 10,000 cu. ft. On the other hand, had the intake current been increased instead of decreased, the rise workings would then take more than their proportion, while the dip workings would take less. The reason for this distribution is evident; suppose, for example, the intake or mine pressure is 3 in. of water gauge, and in the dip workings there is in. of water gauge acting to assist ventilation, while a like water gauge of in. in the rise workings acts to retard ventilation. The effective water gauge in the dip workings is therefore 3 in., while the effective water gauge in the rise workings is 24 in., or they are to each other as 7:5. If, now, the mine pressure is decreased to, say, 2 in., the effective rise and dip pressure will be, respectively, 24 in. and 14 in., or as 5: 3. We observe, before the decrease, the dip pressure was 7, or 1.4, times the rise pressure, while after the decrease took place in the mine pressure, the dip pressure became §, or 1.66, times the rise pressure. The relative quantities passing in the dip split before and after the decrease took place, as compared with the quantities passing in the rise split, will be as the √1.4: √1.66, showing an increase of proportion. Now, instead of a decrease taking place in the mine pressure, let us suppose it is increased, say, from 3 in. to 4 in. The effective pressures in the dip and rise workings will then be, respectively, 4 in. and 3 in., or they will be to each other as 9: 7, instead of 7: 5. Here we observe that the dip pressure is 14, or 1.15, times the rise pressure, instead of 1.4. The relative quantities, therefore, passing in the dip split, before and after the increase of the mine pressure, as compared with the quantities passing in the rise split, will be in the ratio of √1.4: √1.15, showing a decrease of proportion. We observe that any alteration of the mine pressure by which it is increased or decreased does not affect the inside dip or rise columns, and hence the disproportion obtains. In case of a decrease of the mine pressure, the dip workings receive more than their proportion of air, and in case of an increase of the mine pressure, they receive less than their proportion of air.

Influence of Seasons. In any ventilation, air columns are always established in slopes and shafts, owing to the relative temperatures of the outside and inside air. The temperature of the upcast, or return column, may always be assumed to be the same as that of the inside air. The temperature of the downcast, or intake column, generally approximates the temperature of the outside air, although, in deep shafts or long slopes, this temperature may be changed considerably before the bottom of the shaft or slope is reached, and

consequently the average temperature of the downcast, or intake, is often different from that of the outside air. The difference of temperatures will also vary with the seasons of the year. In winter the outside temperature is below that of the mine, and the circulation in shafts and slopes is assisted, since the return columns are warmer and lighter than the intake columns for the same circulation. In the summer season, however, the reverse of this is the case. The course of the air current will thus often be changed. When the outside temperature approaches the average temperature of the mine, there will be no ventilation at all in such mines, except such as is caused by accidental wind pressure.

In furnace ventilation the temperature of the upcast column is increased above that of the downcast column by means of a furnace. The chief points to be considered in furnace ventilation are in regard to the arrangement and size of the furnace. Furnace ventilation should not be applied to gaseous seams, and in some cases is prohibited by law. It is, however, in use in may mines liberating gas. In such cases the furnace fire is fed by a current of air taken directly from the air-course, sufficient to maintain the fire, and the return current from the mine is conducted by means of a dumb drift, or an inclined passageway, into the shaft, at a point from 50 to 100 ft. above the seam. At this point, the heat of the furnace gases is not sufficient for the ignition of the mine gases. The presence of carbonic-acid gas in the furnace gases also renders the mine gases inexplosive. In other cases where the dumb drift is not used, a sufficient amount of fresh air is allowed to pass into the return current to insure its dilution below the explosive point before it reaches the furnace.

The

Construction of a Mine Furnace.-In the construction of a mine furnace, a sufficient area of passage must be maintained over the fire and around the furnace to allow the passage of the air current circulating in the mine. velocity of the current at the furnace should be estimated not to exceed 20 ft. per sec. and the entire area of passage calculated from this velocity. Thus, for a current of 50,000 cu. ft. of air per min., the area of passage through and around the furnace should be not less than

50,000 60 X 20

-4133 sq. ft.

This is a safe method of calculation, notwithstanding the fact that the velocity of the air is often much more than 20 ft. per sec., yet the volume of the air is largely increased owing to the increase of temperature.

The length of the furnace bars is limited to the distance in which good firing can be accomplished, and should not exceed 5 ft. The width of the grate will therefore determine the grate area. The grate area must, in every case, be sufficient for the heating of the air of the current to a temperature such as to maintain the average temperature of the furnace shaft high enough to produce the required air column, or ventilating pressure, in the mine. The area A of the grate of the furnace is best determined by the formula 34 = XH. P., in which A- grate area in square feet; H. P. VD power of the circulation; and D = depth of shaft in feet. The horsepower for any proposed circulation may always be determined by dividing the quantity of air (cubic feet per minute) by the mine potential Xu, and cubing and dividing the result by 33,000; thus

A

= horse

The furnace should have proper cooling spaces above and at each side; upon one side, at least, should be a passageway or manway. The furnace should be located at a point from 10 to 15 yd. back from the foot of the shaft, at a place in the airway where the roof is strong. This is well secured by railroad iron immediately over the furnace. A good foundation is obtained in the floor, and the walls of the furnace carried up above the level of the grate bars, when the furnace arch is sprung. If possible, a full semicircle should be used in preference to a flat arch. The sides and arch of the furnace should be carried backwards to the shaft; this is necessary in order to prevent ignition of the coal. The walls and arch are constructed of firebrick a sufficient distance from the furnace, and afterwards of a good quality of hard brick; the shaft is also lined with brick or protected by sheet iron a sufficient height to prevent the ignition of the curbing.

Air Columns in Furnace Ventilation. As previously stated, natural ventilation and furnace ventilation are identical, in so far as in each the venti