January 31, 1913.

THE COLLIERY GUARDIAN.

225

THE GENERATION AND USE OF COMPRESSED
AIR FOR MINING.♦

By George Blake Walker.

Air-compressing has been brought to the front in
recent years, more particularly by the increased use of
rock drills in mining and tunnelling work. On the
Rand in South Africa alone there are at the present
time many thousand rock drills at work, and the very
large propoition of them are percussive drills actuated
by compressed air. In connection with this remarkable
development has been witnessed the wide adaptation of
electricity to mining operations, and the introduction
of air-compressors driven by electric motors. While
the majority of these compressors are of the recipro-
cating type, several successful rotary machines are now
available, among which which is the Reavell, which is a
reciprocating single-acting compressor, the four
cylinders of which are arranged around a common
centre; and the turbine principle applied as an air-
compressor in conjunction with exhaust or mixed power
steam turbines running at very high velocities. The
extended use of coal-cutting machinery in recent years,
and the limitation of the use of electricity in coalmines
consequent on the provisions of the Coal Mines Act of
1911, have also brought about an increased demand for
air-compressors.

No doubt there are, even to-day, many crude air
plants in operation, but not so many as there used to be,
and they may be left out of account. Even with the
most modern compressors, however, it is not possible to
transmit air with the same economy as the electric
current, and in practice the loss from leakage is greatly
more in the former than in the latter system. Losses in
air compression and transmission are due chiefly to two
causes—friction and the loss of heat in the air from the
moment of compression. In all compressors there is loss
through drawing in the air through valve spaces and
expelling it through the outlet valves. Its whole course
is then one of friction through pipes (often too small in
diameter), and it finally has to overcome much friction
in the valves and cylinders of the motor. Air can be
used expansively, but not to the same extent as steam,
and the essential difference is due to the heat losses in
compression and expansion. If a theoretical diagram be
constructed, showing the expansion of air and steam at
an initial pressure of 75 lb. to the square inch, the
adiabatic expansion curve of the air shows that the
pressure is reduced to zero gauge pressure when the air
has expanded to three and three-quarter times the
initial volume, the mean effective pressure being 18’9 lb.
per square inch. At the end of the stroke the pressure
falls to 71b. below atmospheric pressure. The steam
curve, on the other hand, does not cut the atmospheric
line until the expansion reaches four and a-half times
the initial volume, and the mean effective pressure is
25’2 lb. per square inch. The lower mean pressure of
the air is due to the loss of heat during its expansion.*
Just as it is most difficult to keep the air cool during
compression, and so to fill the cylinder at the outside
temperature, so it is practically impossible in the case of
underground air motors to heat the air so as to prevent
its greater density during the expansion period, and
consequent loss of power. That is the reason why so
many air machines, notably coal-cutters, are run with
the air on for the greater part of the stroke. The ice
difficulty is naturally increased by expansion. From
Prof. Peele’s book, it appears that at a pressure of four
atmospheres (591b. per square inch), with complete
expansion, the theoretical efficiency in the motor is 0*78,
and with full pressure 0 67. This loss of efficiency is due
to expansion of the air in the cylinder alone, and takes
no account of the loss by friction of the air in the ports
and valves. There is the further loss due to piston
clearance. Prof. Peele states that the actual effect of
the cut-off in any given case is found by dividing the
sum of the cut-off, plus the clearance, by the cylinder
volume, plus clearance. For example, if the stroke be
10 in., with a cut-off of four-tenths and a clearance of
6 per cent., the actual volume of the cylinder, including
clearance, will be (10 X *06) 4-10 = 10’6. Then the
ratio of actual cut-off, plus the clearance, is 4 + 0*6 = 4*6,
and the working cut-off becomes 4*6 -4-10*6 = 0*434.

The consumption of free air required for such purposes
as coal-cutting machines, in which the cut-off is very
late (say, three-quarters), is very large. If, for instance,
a coal-cutter develops 25-horse power, and the pressure
at the face is 401b., it is, according to Mr. Webber,
25 X 17*1 = 427 5 cubic feet of free air per minute.

Compressed air has usually to be used at a considerable
distance from the compressor, and the friction in the

* Abstract of a paper read before the Midland Institute
of Mining, Civil and Mechanical Engineers.

f Compressed-air Plant for Mines, by Prof. Peele, p. 208,
quoting from Etude Theoretique sur les Machines d Air
Comprimd, by — Mallard.

pipe ranges amount to a very large factor. For a given j from the compressor. Moisture is deposited as the
length of pipe, when the volume of compressed air pressure of the air is reduced.

discharged and its initial pressure remain constant, the
loss of pressure is proportionate to the length of the
pipe. The more air used, the greater its velocity, and
the friction increases as the square of the velocity. The
same rule applies to the size of the pipes. The areas
of the sizes of pipes generally used in mines are as
follow:—

Diameter	Area.	Diameter	Area.
of pipe.	Square	of pipe.	Square
Inches.	inches.	Inches.	inches.
2	3*14	6	28*27
3	7-07	7	3848
4	12*56	8	50*26
5	19*63	9	63*61

A compressor having two cylinders 24 in. in diameter
by 36 in. stroke, and compressing the air to 60 lb. per
square inch (4 atmospheres), would produce theoretically

= 21*21 cubic feet of air per revolution at
4

60 lb. pressure, or at 50 revolutions per minute say
1,000 cubic feet per minute. The velocity of 1,000 cubic

Fig. 1.—Pair of Corliss Two-stage Air-

compressing Engines.

Fig. 2.—Plan of Cover of Air Cylinder,
Showing Arrangement of Air-valves.

(Walker Compressor.)





Fig. 3.—Air-cylinder Cover, with Walker
Unloading Device.

3



feet of air per minute through a 6 in. pipe with an area
of 28*27 square inches is 5,094 ft. per minute. The
velocity through a 4 in. pipe would be 11,465 ft. per
minute; the friction is (2*25)2, or five times greater in
the 4 in. pipe than in the 6 in. pipe; and the loss of
pressure proportionately greater also. On the other
hand, as the area of a 9 in. pipe is more than twice as
great as that of a 6 in. pipe, the velocity would be less
than half, and the loss of pressure proportionately less.
It is evident, then, that the first cost of pipes of ample
size is true economy where compressed air is concerned.
Leakages are generally due to defective joints. They
are most frequently caused by the lifting of the floor, or
by falls of roof. They are less likely to suffer from the
former cause if the pipes are suspended from above.
Moisture in the air usually results in freezing at the
motor exhaust ports. It can, however, generally be got
rid of by passing the air through a receiver some distance

The valves of an air-compressor deserve the greatest
attention and consideration. The readiness with which
the air is admitted behind the retreating piston, and the
freedom with which the compressed volume is allowed
to leave the cylinder, determine largely the efficiency of
the machine. The objectaimed at is to fill the cylinder
completely, and to allow the whole volume of air when
compressed to escape from it. To secure the first, the
inlet valves must be as free as possible and of the
greatest possible area; but this is not in practice more
than 15 per cent, of the sectional area of the cylinder.
The contracted passages must be as short as possible.
The outlet valves should open as soon as the desired
pressure is reached, and should close immediately the
stroke is completed. Valves may be either automatic
(self-acting) or mechanically actuated. Inlet valves are
usually automatic, but where the air is admitted at
above atmospheric pressure they may be mechanically
operated. Outlet valves, if automatic, are lifted by the
pressure of the compressed air in the cylinder; but, as
the upper surface of the valve, which is usually of the
mushroom form, has a greater area than the under side,
it takes a somewhat higher pressure to lift the valve
than the pressure in the mains. This means some loss.
When the valve is opened mechanically—say, by a tappet
lever at about two-thirds of the stroke—the air in the
cylinder has not to lift the valve, and may flow out
freely. The valve spindle is fitted with a helical spring;
at the end of the stroke a trip releases the valve, and the
spring forces it instantaneously on to its seat. The way
in which this valve problem has been dealt with will be
seen in the descriptions of the various types of com-
pressors given later. In two-stage compressors the
inlet valves are usually operated mechanically.

Without going into the lengthy question of the benefit
of multiple-stage compression, it may be stated that up
to 60 lb. pressure there is not much practical advantage
in two-stage compression, but that for higher pressures

Fig. 4.—Peter Brotherhood Steam-driven
Air-compressor.

it is increasingly advantageous. It should, however, be
borne in mind that under mining conditions high
pressures mean proportionately increased losses from
leakage. With stage compression intermediate cooling
is possible, and is one of its great advantages.

The stresses on automatic valves of hard material
necessitate cushions and cataract arrangements some-
times with air, sometimes with oil. To minimise
clearance losses, a number of devices have been contrived,
of which the revolving valve of Schulz is typical. The
rotating valve opens a passage for the inlet air into the
cylinder and for the compressed air on the return
stroke. The rotary valve is supplemented by a drop
automatic valve. Plate valves have, however, recently
been introduced, and have many important advantages.
The valves simply consist of thin steel plates with
circular or parallel slits which close over corresponding
grids. They combine the possibility of a maximum
inlet or outlet area with a maximum thickness of the
contracted passages. These valves are used by Messrs.
Walker Brothers, Robey and Co., Beiliss and Morcom,
and Peter Brotherhood, and also Messrs. R. Meyer
and Co., of Mulheim-on-Ruhr; Borsig, of Berlin; and
other leading Continental makers.

There are certain practical difficulties in the working
of outlet valves, entirely mechanically, because of the
varying pressures in the receiver. Sometimes the
opening of the valve is governed in part by the air
pressure, a very small lift being allowed by the
controlling mechanism for affording the necessary
relief. Corliss valve gear is used for inlet valves in
high-speed compressors of the Riedler type.

To compress 300 cubic feet of air to a pressure of
50 lb. per square inch, according to Prof. Peele, would
be for 15 cent, losses 41*1-horse power, and for 20 per