March 20, 1913.

THE COLLIERY GUARDIAN.

593

a receiver was that the velocity was reduced, and by
this reduction of velocity the water was allowed to
collect. Lower down in the paper the outlet valves of
the mushroom form were mentioned. This was rather
misleading. The mushroom form meant that they must
have a stalk projecting into some space, and there they
had a clearance, but they could not have the stalk
sticking either into the cylinder or into the clearance
space, otherwise they got that clearance magnified.
A table, by Prof. Peele, was given showing the ratios
of actual horse-power required to compress one cubic
foot of free air at different pressures. The table only
gave the loss as low as 15 per cent., but there were
compressors made to-day in which the losses were as low
as 10 per cent., and even lower. Therefore the loss at
100 pounds pressure, which was stated in the table at
0’2080, should be, worked on modern lines, 0T85 per brake
horse-power. He (the speaker) contended that there were
many cases of expediency where it was desirable to have an
inbye compressor. He added that every colliery manager
ought to bear in mind the almost universal practice
adopted by makers of percussion drills, of understating
the amount of air used for those drills. In conclusion,
the speaker again referred to the difficulty of under-
standing the characteristics of compressed air. So few
engineers, he said, really understood that air was abso-
lutely the reverse of steam. The first fundamental
principle of the efficiency of compressed air lay in the
reduction of the clearances ; the second in avoiding the
dissipation of the heat during the compression. If
someone could devise a means whereby the heat could
be retained, carried along the pipe, and used expansively,
then they would begin to get that efficiency from com-
pressed air that they got from electricity and other
means. Until they did that, they were dissipating their
heat, no matter how they reduced their clearances. Six
or seven years ago, Mr. Reavell told the institute that he
was taking this matter up, but the speaker was sorry to
say that that gentleman had not had the success he
hoped to meet with in delivering the air hot and using
it hot, from the fact that there was no latent heat. If
they had to carry the air any distance, they lost the
heat before it got to the machine. Of course, by using
hot air they would also do away with the moisture, the
freezing up, and a whole lot of trouble.

Mr. H. Rhodes said the paper was very interesting,
and the remarks of Mr. Abell were equally so. Every-
one was quite aware that, the lower the pressure at
which they could use compressed air, the greater the
efficiency, but there was one point which must not be
forgotten. If they were going to use a very low pres-
sure they must have good big pipes. In the thin seams
that some people had to work, the difference between
a 6 inch cast iron pipe and a 3-inch wrought iron or
steel pipe was of importance from the point of view of the
space occupied. They were too much apt to think about
the actual efficiency of the compression, rather than the
efficiency and the economy of maintaining their
installation. He remembered a case very well where
they had a compressor which used to do about 60
pounds. A mile and a half inbye, they got to the
end of their tether. Then they put down a two-stage
compressor, pressing to 80 pounds, and, on the same
pipes, they got about three times as big a pressure at
the far end as they used to get when pressing only about
50 pounds. The question of 10 or 20 per cent was not
very much, as long as they were getting the work done.
The paper was not extraordinarily enthusiastic on the
subject of inbye compressors, and his experience con-
firmed that attitude in every way. The difficulty that
arose in the case he had referred to was not that of
supply, nor of water to keep the compressor cool, but
they found that the cost of arranging the water supply
and cooling the water, came to more than the actual
cost of the compressor, although the machine itself was
just at the bottom of the shaft.

Mr. C. Snow referred to the statement in the paper
that “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 underside,
it takes a somewhat higher pressure to lift the valve
than the pressure in the mains. This means some loss.”
What he was going to say might sound rather heretical,
but he thought he could prove it. Theoretically
some loss would occur, as stated, but in practice no
increase in the pressure required to expel the air from
the cylinder, due to unequal areas, could be detected by
the indicator. In a series of diagrams taken on the
72 in. by 42 in. air cylinder at the South Kirkby Colliery
by the engineer (Mr. Fowler) at speeds varying from
1 revolution per minute to 11, the pressure in the
cylinder did not exceed that in the mains. From
12 revolutions per minute to 32, the pressure required to
expel the air rose from zero to 12*7 lb. to the square

inch. This seemed to prove very clearly that the excess
pressure found was due entirely to friction in the air
outlet, and that unequal areas between the upper and
lower surfaces of the valves had nothing whatever to do
with it. In this particular case, if the unequal areas of
the valve had caused an increase in pressure, a uniform
excess pressure would have been observed at all revolu-
tions between 1 and 11. Of course, if a valve or valves
giving a larger outlet area had been used, increase in the
pressure required to expel the air would have been
absent until a much greater number of revolutions per
minute had been attained. With a valve giving an
infinitely large area of outlet passage, the pressure
in the cylinder would be uniform with that in the
main at any speed of the compressor, however great.
It might be noted that excess pressure due to the
velocity of the air through the outlet passages did not
commence until a speed of 97 ft. per second was
attained. This seemed to prove that the speed of 60 to
65 feet per second referred to in Mr. Walker’s paper, as
stated by a certain firm, was unnecessarily low, and that
satisfactory results could be attained at a 50 per cent,
higher speed, with a properly designed valve. In
further proof that inequality in the areas of the upper
and lower surfaces of the outlet valve of an air cylinder
had practically nothing to do with the excess pressure,
it might be stated that the new valves which had given
the results already described had a difference of 28*6 per
cent, in area between these surfaces, whereas the valves
they replaced gave an excess pressure varying from
19 to 30 pounds at only 22 revolutions per minute, with
only 6’4 per cent, difference in area. He did not
advance any theory as to why there was no excess
pressure caused by the inequality of areas between the
upper and lower surfaces of the valve. He merely
pointed out that practical tests they had made proved
that the excess pressure did not exist. Theoretically,
he quite agreed with the statement in the paper that it
ought to be there.

Mr. H. Jenkins, referring to the cooling of inbye
compressors, said in the cases of compressors which
they had installed difficulty had manifested itself when
they got over 350 cubic feet. Below this there had not
been much trouble. He believed many attempts had
been made to cool inbye compressors by means of
radiators, similar to those used in motor car work, but
these were rendered ineffective owing to dust clogging
up between the pipes and choking them up. There was
not much difficulty in a pit where there was much
water, but it was in connection with places where there
was no water present, in the shape of water being
pumped, that it cropped up. With regard to pressures,
the one which they had found most suitable for rock
drill, and more particularly hammer-drill work, was
about 75 pounds per square inch. They usually reckoned
about 80 to 85 pounds on the compressor—depending on
the distance, of course, between the compressor and the
machines; but the pressure of 75 pounds gave the best
results when reckoning air consumption per inch of
rock drilled, and was the most general pressure in use,
he thought, where compressors were put in for rock-
drill purposes only. Any higher pressure resulted in
higher air consumption without a corresponding increase
of drilling speed. There was also a good deal of waste
power in hammer drills, due in some cases to the want
of ample lubrication of the cylinders. If they took a
test of a hammer drill with the cylinder practically dry
they would get a 20 per cent, increased air consumption
over that taken when there was ample lubrication
present. He was speaking more of hammer drills that
had had some little wear. The fact was perhaps due to
the oil forming a sort of film round the piston, and
making a more tight joint. Mr. Abell said that air
consumptions were usually given wrongly as regarded
various percussive and other machines used in connec-
tion with the compressors. The speaker had found
that the most simple way of settling the point was to
introduce an airmeter.

Mr. H. Walshe maintained that the volumetric
efficiency of an air-compressor had very little effect on
the operating efficiency of a compressor. The only
effect the volumetric efficiency had was that they did
not get the full cylinderful of air every time. What air
was left in the clearance space at the end of a passage
stroke was practically got back out of it on the
expansion stroke. He was speaking now of high-speed
compressors working with automatic valves. Another
statement of Mr. Abell’s with which he could not agree
was that the light plate valves were designed to reduce
the clearance space. As a matter of fact the clearance
spaces (he was speaking now of high - speed com-
pressors) in the ports and cylinders had been put in
since the introduction of this class of valve. It was not
possible to get a clearance space so small. To counter-
act the disadvantage—if there was any disadvantage at

all in the extra clearance space—they had to have a
cylinder slightly larger for the same amount of air—to
counteract that they had the very much more efficient
operation of the valve itself. A previous speaker had
pointed out that the indicator would not show the
pressure drop amongst these valves, and this bore out
his own experience exactly. The principal source of loss
in an air-com pressor was the heating of air. But to say
that the water-cooling of a compressor and the applica-
tion of inter-coolers to reduce the temperature of the
outgoing air was causing a loss of efficiency, was quite
a mistake. It improved the efficiency. The gain would
be in reducing the temperature of the outlet air to about
the atmospheric; that was the best system to work on.
As far as the volumetric efficiency of the compressor
was concerned, that was a thing that really had to do
with design, and people did not sell compressors now on
cylinder volume but on a guarantee of the amount of air
delivered. The volumetric efficiency was affected very
largely by the inlet pipes of a compressor. It was
possible to put inlet pipes for a certain length, which
would bring about a volumetric efficiency of 100 per
cent, and over. That sounded extraordinary, because
they could not get something for nothing in this world,
but they could get more than the full cylinderful if
they put on an inlet pipe of the correct length and
diameter, which would induce an air surge at the inlet*
He exhibited a number of diagrams, showing the effect
of an inlet pipe where the air was induced to flow in.
He thought there was a good deal more to be said in
favour of the high-speed compressor. The high-speed
compressors designed by up-to-date high-speed engine
makers were all on the forced-lubrication system. They
required less attention, and ran for a long time without
oil. He had figures by him which he got through from
an installation in South Africa—four 1,000-horse power
air-compressors at New Modderfontein. The engineer
there wrote to tell them that he used 1 pint of oil every
24 hours, and he (the speaker) thought that would bear
comparison with anything. There was also the saving
in buildings and foundations, and the accessibility, to
be considered. He thought the high-speed type were
just as accessible as the slow-speed, not to mention that
the high-speed vertical compressor was built with its
bed and one solid foundation was all that it required,
while a slow-speed horizontal compressor wanted lining
up, and wanted some keeping in line, too.

Mr. P. O. Davis said that, with regard to the com-
parative efficiency of small electrically-driven com-
pressors inbye and large compressors on the pithead, he
did not think that, taking into consideration the losses
in electrical transmission, the loss in generation on the
pitbank, the transmission and the reconversion into
mechanical energy in the motor below, there was really
very much in it. Of course there were other reasons
why perhaps inbye compressors should be used, and one
was the question of leakage in air pipes. But in cases
where pipes had been laid above ground, or underground,
the various experiments that had been made on them
had shown that there was very little actual loss due to
leakage. There was a 20-mile compressed air trans-
mission at work in South Africa now where he believed
the actual loss due to leakage did not run to more than
about 4 per cent, on the whole transmission. Turning
to the question of moisture present in air, he said the
actual effect of moisture on the efficiency of compression
was practically negligible ; but moisture in itself was a
rather important thing to be taken into consideration
with regard to the design of a pipe line, as provision had
to be made for intercepting it at low-lying portions
of the line. With regard to the formation of ice
in compressed air tubes, he had always had the
opinion—though he might be wrong in this—that
it was not due so much to ice in the form of humidity
as to entrained moisture, or moisture which could be
separated out, and therefore the provision of a water
separator in front of a compressed-air tool would tend
to reduce ice formation. Of course, in tropical climates
the effect of moisture became very considerable. In one
case in India, he believed, the trouble had been cured
by putting a desiccator, filled with calcium chlorate, in
front of the inlet. A point which had been brought out
by some of the other speakers, and which he thought
was very important, was the measurement of air for
compressors. He thought that when tests with com-
pressors were published, the means by which the air was
measured should be definitely stated. Perhaps the term
“ delivery coefficient ” would meet the requirements.
This volumetric efficiency, dr delivery coefficient, was a
very indefinite quantity, unless they had carried out
actual tests of the air taken out, and compared these
with the indicator diagram. Of course, there were
several ways of measuring the air actually taken out.
One method was to pump out all the receivers. Another
which was very valuable, and was used for turbo-