570

THE COLLIERY GUARDIAN.

September 17, 1915.

current which brought it, and the condition of its surface
a criterion of the length of time during which it was
under way. As has already been seen [11], the filling
of rock faults often contains individual fragments which
range up to hundreds of cubic yards in bulk, and from
their composition it is clear that if they had been
exposed to weather for more than a very few days, they
must have lost the sharpness of their angles, even if by
reabsorption of water they had not become so weak that
they collapsed throughout their bulk. There is no
evidence of ordered arrangement in their deposition, and
still less evidence that the materials were brought from
afar; and the only type of surface accumulation which
is at all like the rock fault jumble is the tangled mass
of debris formed by a succession of small landslides at
the foot of a collapsing cliff. Even in cliffs, however,
there is a certain amount of weathering, and surface
water soaking along the gaping joints in a fallen mass
of coal measure binds quickly brings them to a condition
of plastic clay, among which the blocks of sandstone
slowly founder.

In this connection it may be pointed out that the
slipping of landslides on steep slopes is due to the
inability of the wetted surfaces to support the shearing
stress required .to maintain them in position, and the
writer suggests that the structural resemblance between
the jumble of a landslip and the jumble of a rock fault
is due to their common origin by shearing movement.
In -the absence of any evidence of wetting of the rock
fragments in the .rock fault jumble, the simile becomes
incomplete. By those who think of rock fault as due to
earth movement, all tearing apart of fragments and all
crushing of the weaker rocks is supposed to have taken
place under the burden of later carboniferous sediments.
At the time of shearing most of the water contained in
the muddy sediments had been crushed out under the
weight of these later deposits, and the rocks as they were
found in .the zone of the rock fault were already compact
and hard. In this condition they were sheared, and only
those materials which came within the belt of crushing
could be involved in the crushed breccia. Crushed
under pressure, and remaining under pressure since the
time of the crushing, the agents of weathering have had
no access to the components of .the rock fault filling,
which accordingly have been able to preserve their
chemical constitution identical with that of the rocks
deposited with them and still remaining uncrushed in
the normal ground outside the rock fault area.

The schistose arrangement of the lens-like units [12]
within the rock fault aggregate has not been noted by
the supporters of the contemporaneous-erosion explana-
tion. It is due to a continuation of the tearing process
in debris accumulated by tearing. Not infrequently this
further tearing has resulted in the isolation of compound
masses of jumble (which may now be accepted as fault
breccia) among fragments of rock tom from the sides of
the moving rock fault at a later stage of the tearing.

The process by which the coal has come to ramify in
hair cracks and fissures among sandstone rock [13] is
difficult to understand, whatever hypothesis be adopted.
Drifted trees snagged among the sediment have been
suggested by the stream-course devotees; but branches
and roots of trees are circular in section, and, even
when compressed, can hardly be converted into spread-
ing films of wide extent. Quite small pieces of the coal,
moreover, possess not only the “ sline ” proper to our
coal seams, but have also the alternating bright and dull
bands which are accepted as the bedding planes of the
coal, and -these bands are found to be cut off sharply
at the surface where the coal joins the rock, just as
are the bedding planes at the faces of other masses in the
same jumble [14]. The fact remains that coal in the
rock fault jumble behaves curiously, and there is much
to be learnt about its mechanical properties before we
can pretend to understand the origin of the arrangements
which we are able to observe.

With respect to the geological age of rock faults and
their time relationship to the coming of “ sline ” and
joints [15], all are agreed that the displacement of the
coal at the rock fault was the earlier event.

Advocates of the formation of rock faults by earth
movement put the time of the thrusting ]a:e in the
carboniferous period, and personal!v the vvriter thinks
of the production of sline as a process hich followed
very closely upon the thrusting. That it had been com-
pleted before the permian period was tar advanced is
proved by the occurrence of pebbles of “ real Yorkshire ”
coal, which break up readily along their bord faces,
among the marl slate rocks of the Rossi iigtcn and the
Hatfield Main Colliery sinkings.

Direction of sline over the northern pnrt. of the coal
field is a subject of which Prof. Kendall is making a
special study, and the writer will only say here that in
collieries at which he has been able to obtain the
evidence, the bord faces in the coal lie almost exactly
at right angles to the crests of the “waves and breakers’’
which rock rolls and rock faults have effected in the
coal roof. The trend of the various rock faults, rock
rolls, and thrust planes in the workings of the Haigh
Moor seam between Normanton and Kippax is a little
east of north-east and west of south-west. The direc-
tion of the sline over the same district averages
north 42 degs. west, and the writer submits that in earth
pressures, which drove the rocks towards the north-west
against a deep-seated obstacle, may bo found a common
cause for the two sets of phenomena.

Conclusions.

To those who have followed the writer’s argument so
far, it will have become apparent that, in his opinion,
rock faults, like other faults, are displacements due to
earth movement, and that their filling must be accepted
as a true fault breccia. Evidence which he will
assemble in Part II. of his paper goes to show that the
rock faults of the district described are among the
earliest permanent records of the effects of earth move-
ments which have compressed the district after the
consolidation of the coal measures.

Compressed Air for Coal - Cutters.*

By SAM MAYOR.

The widely extending application of mechanical
appliances at the coal face affords a field of usefulness
in which, in the majority of gaseous mines, compressed
air has at present no rival. It is important to note that
formerly compressed air was used mainly in a compara-
tively small number of motors of considerable size near
the pit bottom, whereas modern conditions require trans-
mission of air power long distances in-bye, and its
distribution to a large number of motors individually
of comparatively small size; these new condition^ are
less favourable to efficiency in the use of compressed
air.

The collieries where the use of compressed air for coal
cutters is indispensable are, for the greater part, those
which are working the deeper seams. As the expense
of sinking and equipping deep shafts limits the number
of these, and renders it desirable to work as large an area
as possible from the same centre, it is clear that in such
cases the distances to the working faces to which com-
pressed air must be transmitted are considerable.

Until quite recent years the occasion of the first intro-
duction of a coal cutter was usually the failure to work
some seam by other means, and the adoption of a
machine was contingent upon a real or an imaginary-
surplus of power from an existing air-compressing plant.
Not infrequently the first introduction of coal cutters
still follows a similar course; the coal cutter is an
after-thought so far as the air compressors are concerned.
From the extensive adoption of longwall machine mining
a new condition has emerged, and this has not yet had
the attention which its importance merits. This now
condition is the delivery, on a scale formerly unknown,
of compressed-air power to remote coal faces for
operating coal cutters, conveyors, etc.

The proposition that wherever the underground con-
ditions permit, electricity should be adopted in
preference to compressed air, will probably not bo
challenged. The systems should' not be considered as
rivals. There is a place for each, and the place for
compressed air is where electricity cannot be used. Com-
pressed air is not deprecated on account of its low
efficiency; on the contrary, it will be shown that com-
pressed air has not received fair play, that the efficiencies
obtained in present practice are a great deal lower than
they might be, and that the efficiency of the system
ought to be substantially improved.

The prevailing indifference to the conditions upon
which economy essentially depends, and the scale of
extravagance and waste that result from neglect of these
conditions, are almost incredible. Those concerned
with the applications of coal cutters have exceptional
opportunities of informing themselves as to the condi-
tions of compressed air power supply at the coal face
in our mines.

The writer has during the last three years directed
an extensive series of investigations which have been
conducted by members of his firm’s technical staff at
a large number of collieries in different parts of the
country.

Inefficiency.

As we are now concerned only with the efficiency of
the compressed air system, we may for our present
purpose define “ efficiency ” as—

Brake-horse-power hours expended by
coal-cutter motors during a shift_______	.

Brake-horse-power hours delivered to — ciency.
air-compressors during a shift

All losses of energy during the shift or other stated
period, mechanical and thermal, in the compressors, the
piping, and the coal-cutter motors, are covered by this
expression.

The results the investigation upon which this paper
is based indicate that the efficiency of the compressed
air system in in-bye service under normal everyday con-
ditions of working, is between 5 and 10 per cent. At
the majority of collieries the efficiency probably more
nearly approaches the lower than the higher limit.

Load. Factor.—The load factor—that is, the ratio
between the working load and the full load capacity of
the plant—has greater influence on the efficiency of a
compressed-air system than on any other method of
power distribution. At less than three-quarter full load,
the efficiency of the compressor falls, but it falls at a
much lower rate than the ratio of loss in the transmitting
and distributing pipe system. Leakage continues as at
full load, and is even increased by the rise of pressure
in the piping. The load factor depends upon the char-
acter of the service and upon local conditions. Where
electrical power is used for a variety of purposes at
collieries, the load factor is usually between 35 and 40
per cent.; it rarely exceeds 50 per cent. The conditions
of air power supply for coal cutters are so various that
no figure useful for general application can be given.
It may be said, however, that if the load factor is low,
there is serious loss of efficiency; and if the load factor
is too high—in other words, if the compressing plant is
overtaxed—the pressure falls; and, as is pointed out
later, the air-consumption per unit of work by the air
motors is largely increased, and other and more serious
consequences result. The efficiency of the compressed
air system is, in this and in other respects, peculiarly
sensitive to departure from the conditions most favour-
able to it.

Consequences of Cow Efficiency.—Efficiency in the
use of power is not the prime desideratum at a colliery;
if it were, compressed air would have no place in our
mines. The plea here is not for the highest attainable
efficiency at all costs, but only for such degree of
efficiency as is “ reasonably practicable.’’ That degree

* From a paper read before the Institution of Mining
Engineers.

depends in each case upon the local conditions, and is a
compromise between cost of power on one hand, and cost
of plant maintenance on the other.

A notable difference between electricity and com-
pressed air, as systems of transmission and conversion
of energy, is that maintenance of the efficiency of an
electric plant is an absolute condition of its service; with
compressed-air plant, the result of neglect is a gradual
decrease of efficiency in imperceptible stages, and from
the machines diminished output to which the men
accommodate their activities. This docility of the com-
pressed-air motor is not a merit but a vice, and is
mainly responsible for the profligate extravagance of the
system. Submissiveness is a quality of the inefficient,
and is out of place in a pit, where retaliation by prompt
stoppage is the sure corrective to neglect.

Measurement.

Neglect of Measurement.—Practice in transmission,
underground distribution, and application of compressed-
air power has not shared in the progress which has
marked the recent history of kindred branches of
engineering. This is due chiefly to neglect to make
proper use of the instruments that are available for
measuring, under service conditions, the pressures and
volumes of the compressed air that is being dealt with.

Instruments for Compressed Air.—There is no reason
why the efficiency of a compressed air plant should not
be ascertained and kept under continuous observation;
in fact, there are special reasons why this course should
be followed, because the tendency of compressed air
plant towards inefficiency is so persistent that there is
more need for such checks than on any other power
system. Further, it is not only feasible, but quite
simple, to furnish the air compressing plant at any
colliery with measuring instruments, by which the rela-
tion between the energy expended in compressed air
and the work done by it may be determined. The cost
is moderate, and, compared with the economy to be
gained, it is insignificant. The instruments required
are of two kinds, namely:—(1) Pressure gauges; and
(2) rate of ” flow meters.

The pressure gauge is a simple, inexpensive, easily
applied, and familiar piece of apparatus, and is too well
known to require description here. The pressure
recorder is a pressure gauge of the type which, by a
diagram upon a chart, makes a graphic record of the
history of the pressure, for a period such as a working
shift. It not only shows the character, extent, and
duration of fluctuations of pressures, but also the times
when the fluctuations occur, and thus gives clues by
which the causes of irregularity may be traced and
removed. Such an instrument is not suitable for per-
manent use underground, but taps might be provided
at suitable places in the pipe system, and at every gate
end valve, so that a recording instrument might be
readily connected for testing purposes. The incessant
changes and extensions of air piping, imposed by the
perpetual recession from the shaft bottom of the coal
faces, render it extremely important to keep under
continuous, or at least periodic observation, the effect of
these changes upon the air pressure.

Air Meters. — The most notable example of the
measurement of compressed air is on the Rand, where
about four years ago the establishment of a large power
station for compressing air and selling it to the indi-
vidual mines was the occasion for an independent
investigation of the problem of air measurement, and
of production »of meters in which both sellers and pur-
chasers of the compressed air power should have
confidence. The types chiefly used for compressed air
are :—(1) The Pitot tube; (2) the Venturi tube; (3) the
swinging gate meter; (4) the displacement meter.
There are several variants of these, but the types are
distinctive.

Corrections for Pressure and Temperature. — If the
pressure and temperature of the compressed air to be
measured were constant, the flow meters could indicate
upon their dials the rate of flow in cubic feet per minute
of free air or of compressed air. But, under service
conditions at collieries, the fluctuations of pressure, and
to a less degree of temperature, of the air to be
measured, preclude the calibration of the scale of the
instrument directly in cubic feet of air. It is there-
fore necessary that the pressure and temperature should
be noted, and, together’ with the indication of the
pointer, referred to a scale from which the equivalent
cubic feet per minute are ascertained. Meters of the
types just described are made in several styles suitable
for different conditions of service.

Rato of flow meters indicate by a pointer on a dial or
a scale. These are the simplest, and are best suited for
underground purposes, such as measuring the rate of
flow in a pipe at any given time, or for testing the rate
of air consumption of any machine.

Chart recording meters make a graphic record of the
rate of flow upon a chart, thus providing a diagram-
matic view of the fluctuations of flow throughout a shift
or a day. A small air turbine operates the clock which
moves the chart. These instruments may be furnished
with automatic compensation for variations of pressure
and temperature, so that the record may be made directly
in terms of volume or of weight of air: this style of meter
is; in effect, an energy meter corresponding to a watt
meter, and is most suitable for use on the surface. It
is suggested that such a meter should be fitted to the
main air pipe between the air receiver and the shaft at
every colliery.

Integrating meters record, upon a series of dials, the
total volume of air that has passed through the metered
pipe during any time interval. When fitted with com-
pensation for pressure and temperature, this also is an
energy meter, and corresponds to the gas and electricity