the air current should be thus divided. The most important reason is that the mine is thereby divided into separate districts, each of which has its own ventilating current, which may be increased or decreased at will. Fresh air is thus obtained at the face of the workings, and the ventilation is under more perfect control. It often happens that certain portions of a mine are more gaseous than others, and it is necessary to increase the volume of air in these portions, which can be readily accomplished when each district has its own separate circulation. Again, the gases and foul air are not conducted from one district to another, but each district is supplied with fresh air direct from the main intake. Should an explosion occur in any part of the mine, it is more apt to be confined to one locality when a mine is thus divided into separate districts. Another consideration is the reduced power necessary to accomplish the same circulation in the mine; or the increased circulation obtained by the use of the same power.

Requirements of Law in Regard to Splitting.-The Anthracite Mine Law of Pennsylvania specifies that every mine employing more than 75 persons must be divided into two or more ventilating districts, thus limiting the number that are allowed to work on one air current to 75 persons. The Bituminous Mine Law of Pennsylvania limits the number allowed to work upon one current to 65 persons, except in special cases, where this number may be increased to 100 persons at the discretion of the mine inspector.

Practical Splitting of the Air Current.-When the air current is divided into two or more branches, it is said to be split. The current may be divided one or more times; when split or divided once, the current is said to be traveling in two splits, each branch being termed a split. The number of splits in which a current is made to travel is understood as the number of separate currents in the mine, and not as the number of divisions of the current.

Primary Splits.-When the main air current is divided into two or more splits, each of these is called a primary split.

Secondary Splits.-Secondary splits are the divisions of a primary split. Tertiary Splits.-Tertiary splits result from the division of a secondary split.

Equal Splits of Air.-When a mine is spoken of as having two or more equal splits, it is understood to mean that the length and the size of the separate airways forming those splits are equal in each case. It follows, of course, from this that the ventilating current traveling in each split will be the same, inasmuch as they are all subject to the same ventilating pressure. When an equal circulation is obtained in two or more splits by the use of regulators, these splits cannot be spoken of as equal splits.

Unequal Splits of Air.-By this is meant that the airways forming the splits are of unequal size or length. Under this head we will consider (a) Natural Division of the Air Current; (b) Proportionate Division of the Air Current.

Natural Division of the Air Current.-By natural division of air is meant any division of the air that is accomplished without the use of regulators; or, in other words, such division of the air current as results from natural means. If the main air current at any given point in a mine is free to traverse two separate airways in passing to the foot of the upcast shaft, and each of these airways is free or an open split, i.e., contains no regulator, the division of the air will be a natural division. In such a case, the larger quantity of air will always traverse the shorter split of airway. In other words, an air current always seeks the shortest way out of a mine. A comparatively small current, however, will always traverse the long split or airway.

Calculation of Natural Splitting. It is always assumed, in the calculation of the splitting of air currents, that the pressure at the mouth of each split, starting from any given point, is the same. Since this is the case, in order to find the quantity of air passing in each of several splits starting from a common point, the rule given under Potential Factor of a Mine is applied. This rule may be stated as follows:

The ratio between the quantity of air passing in any split and the pressure potential of that split is the same for all splits starting from a common point. Also, the ratio between the entire quantity of air in circulation in the several splits and the sum of the pressure potentials of those splits is the same as the above ratio, and is equal to the square root of the pressure.

Expressed as a formula, indicating the sum of the pressure potentials (X1+X2+ etc.) by the expression Xp, this rule is

Q

=

ΣΧΡ

Hence,

Q2

Q3 and u= express the pressure and power, respectively, (EXp)2 (ZXp) 2 absorbed by the circulation of the splits. These are the basal formulas for splitting, from which any of the factors may be calculated by transposition. They will be found illustrated in the table at the end of this section.

We will

give here two examples only, showing the calculation of the natural division of an air current between several splits. We have, from the above formulas, X1 91= Q. EXP

EXAMPLE. In a certain mine, an air current of 60,000 cu. ft. per min. is traveling in two splits as follows: Split A, 6 ft. X8 ft., 5,000 ft. long; split B, 5 ft.X8 ft., 10,000 ft. long. It is required to find the natural divi

sion of this air current.

Calculating the relative potentials for pressure in each split, we have

EXAMPLE. In a certain mine, there is an air current of 100,000 cu. ft. per min. traveling in three splits as follows: Split A, 6 ft. X 10 ft., 8,000 ft. long; split B, 6 ft.X12 ft., 15,000 ft. long; split C, 5 ft. X 10 ft., 6,000 ft. long. Find the natural division of this current of air.

Calculating the respective relative potentials with respect to pressure, we have

60

for split A, X1=60√√2(6+10) ×8,000

72

=

.9185;

for split B, X2=72

for split C, X3=50

2(6+12) ×15,000

50

.8333.

2(5+10) × 6,000

Adding these potentials, we have Xp=.9185+.8314+.8333≈2.5832. Then, applying the foregoing rule, we have

Proportional Division of the Air Current.-It continually happens that different proportions of air are required in the several splits of a mine than would be obtained by the natural division of the air current. It is usually the case that the longer splits employ a larger number of men, and require a larger quantity of air passing through them. They, moreover, liberate a larger quantity of mine gases, for which they require a larger quantity of air than is passing in the smaller splits. The natural division of the air current would give to these longer splits less air, and to the shorter ones a larger amount of air, which is directly the reverse of what is needed. On this account, recourse must be had to some means of dividing this air proportionately, as required. This is accomplished by the use of regulators, of which there are two general types, the box regulator and the door regulator. Box Regulator. This is simply an obstruction placed in those airways that would naturally take more air than the amount required. It consists of a brattice or door placed in the entry, and having a small shutter that can be opened to a greater or less amount. The shutter is so arranged as to allow the passage of more or less air, according to the requirements. The box regulator is, as a rule, placed at the end or near the end of the return air

way of a split. It is usually placed at this point as a matter of convenience, because, in this position, it obstructs the roads to a less extent, the haulage from the back entry in this split being carried over to the main haulway, through a crosscut, before this point is reached. The difficulty, however, can be avoided, in most cases, by proper consideration in the planning of the mine with respect to haulage and ventilation. The objection to this form of regulator is that, in effect, it lengthens the airway, or increases its resistance, making the resistance of all the airways, per foot of area, the same. It is readily observed that, by thus increasing the resistance of the mine, the horsepower of the ventilation is largely increased, for the same circulation. This is an important point, as it will be found that the power required for ventilation is thus increased anywhere from 50% to 100% over the power required when the other form of regulator can be adopted.

Door Regulator.-In this form of regulator, which was first introduced by Beard, the division of the air is made at the mouth of the split. The regulator consists of a door hung from a point of the rib between two entries, and swung into the current so as to cut the air like a knife. The door is provided with a set lock, so that it may be secured in any position, to give more or less air to the one or the other of the splits, as required. The position of this regulator door, as well as the position of the shutter in the box regulator, is always ascertained practically by trial. The door is set so as to divide the area of the airway proportionate to the work absorbed in the respective splits. The pressure in any split is not increased, each split retaining its natural pressure.

Calculation of Pressure for Box Regulators.-When any required division of the air current is to be obtained by the use of box regulators, these are placed in all the splits, save one. This split is called the open, or free, split, ksq? and its pressure is calculated in the usual way by the formula p= a3 The natural pressure in this open split determines the pressure of the entire mine, since all the splits are subject to the same pressure in this form of splitting.

First, determine in which splits regulators will have to be placed, in order to accomplish the required division of the air. Calculate the natural pressure, or pressure due to the circulation of the air current, for each split, ksq2 when passing its required amount of air, using the formula p= The split showing the greatest natural pressure is taken as the free split. In each of the other splits, box regulators must be placed, to increase the pressure in those splits; or, in other words, to increase the resistance of those splits per unit of area.

EXAMPLE. The ventilation split A, 6 ft. X9 ft., split B, 5 ft. X8 ft., split C, 9 ft. X9 ft., split D, 6 ft. X8 ft.,

required in a certain mine is: 8,000 ft. long; 40,000 cu. ft. per min. 6,000 ft. long; 40,000 cu. ft. per min. 8,000 ft. long; 10,000 cu. ft. per min. 10,000 ft. long; 30,000 cu. ft. per min. In which of these splits should regulators be placed, to accomplish the required division of air, and what will be the mine pressure?

Calculating the pressure due to friction in each split when passing its required amount of air, we find,

.0000000217×2(6+9)8,000 X 40,0002

543

.0000000217X2(5+8)6,000 X 40,0002

403

.0000000217X2(9+9)8,000 X 10,0002

813

=52.92 lb. per sq. ft.;

-84.63 lb. per sq. ft.;

1.176 lb. per sq. ft.;

.0000000217X2(6+8)10,000 X 30,0002

483

49.45 lb. per sq. ft.

Box

Split B has the greatest pressure, and is therefore the free split. regulators are placed in each of the other splits to increase their respective pressures to the pressure of the free split or the mine pressure. Therefore, the mine pressure in this circulation is 84.63 lb. per sq. ft.

The size of opening in a box regulator is calculated by the formula for determining the flow of air through an orifice in a thin plate under a certain head or pressure. The difference in pressure between the two sides of a box regulator is the pressure establishing the flow through the opening, which

corresponds to the head h in the formula =√2gh. This regulator is usually placed at the end of a split or airway, and since the regulator increases the pressure in the lesser splits so as to make it equal to the pressure in the other split, the pressure due to the regulator will be equal to the ventilating pressure at the mouth of the split, less the natural pressure or the pressure due to friction in this split. Hence, when the position of the regulator is at the end of the split, the pressure due to friction in the split is ksq2 first calculated by the formula p and this pressure is deducted from a3 the ventilating pressure of the free or open split, which gives the pressure due to the regulator. This is then reduced to inches of water gauge, and substituted for i in the formula A= The value of A thus obtained is

=

.0004g
Vi

the area (square feet) of the opening in the regulator.

EXAMPLE. 50,000 cu. ft. of air is passing per min. in a certain mine, in two equal splits, under a pressure equal to 2 in. of water gauge, and it is required to reduce the quantity of air passing in one of these splits, by a box regulator placed at the end of the split, so as to pass but 15,000 cu. ft. per min. in this split. Find the area of the opening in the regulator, assuming that the ventilating power is decreased to maintain the pressure constant at the mouth of the splits after placing the regulator. The size and length of each split is 6 ft. X 10 ft. and 10,000 ft. long.

The natural pressure for the split in which the regulator is placed will be ksq2 .0000000217X2(6+10) 10,000 X 15,0002 =7.233 lb. per sq. ft.

a3

(6X10) 3

7.233 Then, 1.4 in. of water gauge (nearly), due to friction of the air 5.2 current in this split. And, 2-1.4.6 in. water gauge due to regulator. .0004q .0004 X 15,000 =7.746 sq. ft., area of opening.

Finally, A

=

V.6

V.6

นา

Size of Opening for a Door Regulator.-The sectional area at the regulator is divided proportionately to the work to be performed in the respective splits according to the proportion A1: A2: : u1: 2. Or since A1+A2-a, we have A1a u1: u1+u2, and A1 = Xa. This furnishes a method of proportionate splitting in which each split is ventilated under its own natural pressure. The same result would be obtained by the placing of the box regulator at the intake of any split, thereby regulating the amount of air passing into that split, but the door regulator presents less resistance to the flow of the air current. The practical difference between these two forms of regulators is that in the use of the box regulator each split is ventilated under a pressure equal to the natural pressure of the open or free split, which very largely increases the horsepower required for ventilation of the mine; while in the use of the door regulator each split is ventilated under its own natural pressure, and the proportionate division of the air is accomplished without any increase of horsepower. This is more clearly explained in the following two paragraphs, and the table showing the comparative horsepowers of the two methods.

This

Calculation of Horsepower for Box Regulators. By the use of the box regulator, the pressure in all the splits is made equal to the greatest natural pressure in any one. This split is made the open or free split, and its natural pressure becomes the pressure for all the splits, or the mine pressure. mine pressure, multiplied by the total quantity of air in circulation (the sum of the quantities passing in the several splits), and divided by 33,000, gives the horsepower upon the air, or the horsepower of the circulation. Thus, in the first example given on page 920, in which for split B the pressure p=84.63 lb. per sq. ft. and the total quantity of air passing per minute is 12,000 cu. ft., we have

Calculation of Horsepower for Door Regulators.-In the use of the door regulator, each split is ventilated under its own natural pressure, and, hence, in the calculation of the horsepower of such a circulation, the power of each split must be calculated separately, and the sum of these several

powers will be the entire power of the circulation. For the purpose of comparison, we tabulate below the results obtained in the application of these two methods of dividing the air in the above example

The following table of formulas will serve to illustrate the methods of calculation in splitting. The example assumes the same airway as that given on page 912 and used to illustrate the table of formulas, pages 913, 914 and 915 but the air current is divided, as specified in the table:

NATURAL DIVISION

Primary Splits.—Split (1) = =4 ft. X5 ft., 800 ft. long. Split (2)=4 ft. X5 ft., 1,200 ft. long.

Or the natural division may be calculated from the pressure at the mouth of the several splits by using Formula (23); thus,

4,131 (2)