Assuming, as before, a constant power on the air at the mouth of the mine, since the quantity has been increased by splitting, both the velocity and resistance have been increased in the main airway, which absorbs more power thus decreasing the power on the splits.

Effect of Splitting on Velocity. In order to show the general effect of splitting the air current, at any point in a mine, on the velocity (vo) in the shaft or main airway and the velocity (v1) in the splits, it is necessary to know the ratio (m) of the rubbing surface (s1) in the splits, to that of (so) in the shaft or main airway; also, the ratio (n) of the total area (A1) of the splits, to that of (Ao) in the shaft or main airway.

Then,

S1 = mso; and A1 = nAo;

(1)

and, since for a given quantity the velocity varies inversely as the area,

But, the power on the air (u) at the mouth of the mine is equal to the power (uo) absorbed in the shaft or main airway, or both, plus the power (u1) absorbed in the splits.

u = uo + U1

or, expressed in terms of the mine, since uksv3,

3

(3)

(4)

Substituting for s1 and v13 the values given in Equations 1 and 2, gives after simplifying

Equation 5 shows clearly that, for a constant power on the air at the mouth of a mine, in splitting,

It appears from the last two equations that as the ratio of the split area to the shaft or main-intake area, represented by n, is increased the main-intake velocity (vo) is increased, while the split velocity (v1) is decreased, the increase and decrease of velocity, however, being less rapid than the change in the area ratio.

Effect of Splitting on Quantity. The quantity of air in circulation varies directly as the intake velocity vo; or, for a constant power (u) on the air,

n

Q varies as

3 n3 + m

(8)

Effect of Splitting on the Mine Resistance.—The total mine resistance is the sum of the main-intake and split resistances.

Example.—ḍ current of 25,000 cu. ft. per min. is passing in a shaft mine. The shafts are 8 X 12 ft. in section and 250 ft. deep. The airways are 6 X 10 ft. and 15,000 ft. long, including the return. (a) What is the water gage due to this circulation? (b) Assuming the power applied to the fan shaft remains unchanged and the current is divided into two equal splits, at a point 1500 ft. inby from the foot of the shaft, what volume of air may be expected to be passing? (c) What will be the water-gage reading on the fan drift and at the bottom of the shaft, after splitting?

Solution. The rubbing surface and sectional area of the shafts and airways are, respectively, as follows:

(b) For a constant power on the air, the quantity varies directly as the mine power potential; but, for a constant power applied to the fan shaft, owing to the efficiency of the fan varying inversely as the 3/5 power of the potential Xu the quantity varies as the 4/5 power of that potential.

The mine potentials, in this case, are,

Then, for a constant power applied to the fan shaft, the quantity of air in circulation varies as the 4/5 power of the power potential, which gives for the circulation after splitting

(c) Water gage (after splitting).—In the fan drift the gage is

w.g. =

0.00000002 × 34,2502 × 0.6892
5.2

3.1 in.

To find the gage at the shaft bottom it is necessary to deduct the potential factor for the two shafts from the total potential factor for the mine after splitting; thus

Then, since the gage is proportional to this potential factor, the gage at the bottom of the shaft is

Relative Variation of Factors.-The following relation of some of the more important factors in the ventilation of mines by means of centrifugal fans is based on the results of many experiments:

SECTION VII

PRACTICAL VENTILATION

CONDUCTING AIR CURRENTS, AIR BRIDGES GENERAL PLAN OF MINE-DISTRIBUTION OF AIR IN THE MINE-SPLITTING AIR CURRENTS-SYSTEMS OF VENTILATION SYSTEMS OF MINE AIRWAYS.

The first step, in the practical ventilation of a mine, is to determine the volume of air that will be required in order to maintain a pure and wholesome atmosphere in the mine workings. This will depend on conditions, such as the size and depth of the mine; thickness and inclination of the seam; character and quality of the coal; kind and quantity of gas generated; methods of working the seam and mining the coal. Aside from these conditions the volume of air must always be sufficient to meet the requirements of the mine law.

The second question to be determined is the general ventilating pressure or water gage, under which the mine is to be ventilated. This will depend on the possible extent and size of the workings and the power available. The water gage, in mining practice, varies from a fraction of an inch to 3 or 4 in., in this country; and higher gages are in use in the deep mines of Belgium and other countries. The best practice, however, employs such a system of mining that the required volume of air can be circulated under, say 1 or 2 in. of water gage. This can only be accomplished by so planning the mine, in the start, that it can be divided into separate ventilation districts. The number of ventilation districts should increase with the development of the mine. Each district is thus ventilated by a separate air split or current, which insures good air, besides reducing the water gage necessary for the ventilation of the mine.