_The office of the air compressor is to store up air under high
pressures_, which can be utilized at a greater or less distance,
without sustaining any loss by condensation in the pipes, as is the
case of carrying steam in pipes long distances.
Air stored under pressure in a reservoir can be used expansively, in an
ordinary steam engine returning an equivalent amount of work that was
required to compress it—less the friction.
The admission of the air being through a single tube, it creates a
constant flow of air in one direction only, thus filling the cylinder
at each stroke with air at atmospheric pressure. This movement gives a
momentum to the air which causes it to fill the cylinder to its fullest
extent at each stroke.
_Air compressors may be driven in various ways_, but the most commonly
used are those which are directly connected to a steam engine, thus
doing away with intermediate machinery. When the air piston draws in
a charge of air, the air fills the cylinder at atmospheric pressure,
or a little below, and on the return stroke of the piston it has to be
compressed to the same pressure as in the receiver before it can lift
the delivery valve, and as the valve is held to its seat by a spring,
and also by its own weight, the pressure has to be considerably above
that of the receiver before the valve will lift. To overcome this the
valves are operated by mechanical means, which lifts them at a point of
the stroke, when the pressure in the cylinder corresponds with that of
the receiver.
[Illustration: FIG. 363.—See page 71.]
This arrangement avoids pounding of the valves as well as the noise
caused by the air when rushing at much higher pressure from the
cylinder into the receiver.
For the sake of economy, air compressors are compounded, as for
example, by drawing the air into a large cylinder and compressing it
to a certain stage, whence it passes into a smaller cylinder, which
compresses it to a much higher pressure.
In a simple compressor, for very high pressures, there is at the end
of the stroke a large volume of air left in the clearance space, which
expands on the return stroke, to atmospheric pressure, before another
charge of air can be drawn in.
But in the compound compressor, the air is delivered from the low
pressure receiver to the high pressure cylinder far above atmospheric
pressure, thus the remaining air need not expand so much and allows the
cylinder to take a larger volume of air. The load is also distributed
more evenly.
The following are valuable “points” relating to the care and management
of air compressors.
As in a steam pipe line, elbows should be avoided in an air pipe line
but unlike a steam pipe it should be larger.
A mistake is sometimes made in purchasing a compressor built for a low
altitude and trying to run it in a higher elevation; the machine then
experiences the same trouble that some people do, in not being able to
get breath enough under the changed conditions.
The use of cheap oils, especially in an air cylinder is a most serious
mistake, as the least tendency to gum will prevent the valves from
properly seating, and even with the best of oils, it is well to use a
small amount of mineral oil at times.
In localities where the water is bad, the water jacket will require
extra attention, as it gets as badly scaled like steam boilers,
principally due to a very slow or retarded circulation, which allows
the sediment to settle, and should the water supply be shut off, even
for a few minutes, the cylinder heat will bake it so hard as to give
considerable trouble. It is a good plan to put a good boiler compound
in the water jacket, and run the machine for some time without any
circulation.
[Illustration: FIG. 364. (See page 71.)] In this case good judgment
must be used not to run too long or too fast, as the cylinder will heat
very quickly and is liable to be damaged.
There are many emergency ways of stopping small pipe leaks; any good
sticky substance, such as tar, wax, tallow candles, or even chewing
gum, melted and applied on narrow strips of cloth and wound as a
bandage, will be found handy.
It should be remembered that leaks in an air pipe line are as bad as in
a steam pipe line, and should receive as much care.
_The theoretical operation of air compressors may be thus explained_:
If a tight cylindrical vessel, containing one cubic foot of air at
atmospheric pressure, be fitted with a piston which is free to move up
and down but yet perfectly tight, the air in the vessel will have no
means of escape, and the pressure within and without the vessel, both
being atmospheric, are balanced.
Now, if the piston should be loaded with a weight, the pressure on the
outside would be that due to the atmosphere, plus the weight, while the
pressure from the inside is simply equal to atmospheric pressure; thus
the piston is forced to descend, but as the air inside of the cylinder
has no means of escape, the volume it fills being diminished, its
pressure rises until the pressure under the piston balances that above
it.
If, for example, the area of the piston should be 100 square inches,
and the weight with which it is loaded be 100 pounds, assuming the
piston to be without weight, the pressure below will have to react
with an equal force to hold the piston stationary, which in this case
would be 1 pound to the square inch above atmospheric pressure, and
the piston would have to descend sufficiently to cause this increase
of pressure, which descent would be equal to 1/16 of the total fall
of the piston. By adding another 100 pounds above, the pressure would
rise to 2 pounds to the square inch. The cylinder is thus charged with
compressed air.
_Liquid Air_ is a marvelous result of compression. It liquefies at a
pressure of 573 pounds per square inch, at the reduced temperature
of -220° F.; at atmospheric pressure it boils at-312° F., at which
temperature it can be handled like water. _Air_ is the vapor of a
liquid, and acts in its properties like the vapor of other liquids.
Liquid air in color is like that of a blue sky on a cloudless day.
[Illustration: FIG. 365. (See page 71.)]
“_Denys Papin_ was the first to propose and make, in 1653, an actual
trial of the transmission of power to a distance by compressed
air. It was the fertile and mechanical brain of Papin that first
conceived the idea of the pneumatic tube for transmitting parcels
by air pressure.” Historical Note by Hiscox.
If now the bottom of the cylinder should be connected by means of a
pipe to another vessel of larger capacity called a receiver, the pipe
having been closed by a valve in it during compression, and the valve
should be opened, the piston would at once commence a further descent,
the compressed air escaping into the receiver, until the pressure in
the receiver and cylinder is equalized, or the piston reaches the
bottom of the cylinder, which it will do, if the receiver is large
enough. Then the valve closes, stopping communication between cylinder
and receiver, and the piston is drawn upward; at the same time air is
again admitted to the cylinder by another valve, which closes when the
piston reaches the top, and the same operation is again repeated.
The receiver can thus be charged with compressed air and by loading the
piston very heavy the pressure can be raised quite high.
Now, if the piston, instead of being loaded by weights, be connected to
the piston rod of a steam engine, or by means of a connecting rod to a
crank (which is rotated by a belt or some other driving mechanism, and
the valves be operated automatically, as the valves on a water pump),
the simple apparatus is converted into a perfect air compressor, _which
really is nothing else than an air pump_, and the air can be pumped
into the receiver against a high pressure the same as water is forced
into an elevated reservoir by a pump.
As air is a compressible gas, it acts a little different in the air
cylinder from the almost incompressible water in a pump.
To lift the valves of an air compressor by the compressed air pressure
in the cylinder (added to the pressure of their springs besides the
receiver pressure), the air would have to be compressed considerably
above the receiver pressure before it would lift the valve which allows
it to flow from the cylinder into the receiver, and then the valve
would not open freely as a pump valve, but would chatter, causing a
disagreeable noise, and damaging the valve.
_To avoid this, the valves of an air compressor are operated by
mechanical means._ Some devices operate the valve directly as soon as
the pressure in the cylinder reaches that of the receiver, while others
simply release it of the spring pressure, the valve itself being lifted
by the air itself. Such devices generally give the valves a full free
opening, without noise.
[Illustration: FIG. 366.]
_The blowing engine_ is almost identical with the air compressor. The
chief difference between them being the ratio of steam cylinder to air
cylinder. While the air compressor furnishes a comparatively small
amount of air at very high pressures, _the blowing engine delivers a
very large volume at lower pressures_.
Blowing engines are mainly used in large blast furnaces, smelting works
and foundries, to furnish the air pressure for cupolas, air furnaces
and smelting ovens.
In Fig. 366 is shown a blowing engine of very large size; the steam
cylinder is 42 inches in diameter, the air cylinder 84 inches, and the
stroke 60 inches.
_The valve gear is of the Reynolds-Corliss type._ The piston rod is
attached to a cross-head extending through the guides, which are
formed by the frame, with wrist pins upon each end, from which the two
connecting rods are suspended with their lower ends connected to the
cranks, as shown in Fig. 366. There are two air piston rods attached to
the main piston and held to the cross-head by nuts at points near the
guides.
The crank shaft carrying the flywheels, which also form the cranks
attached to the ends of this shaft, is located below the steam
cylinder. This construction is of the return connecting rod engine
design, to economize space.
Both the air and steam valve gears are worked from eccentrics on an
auxiliary shaft, driven from the main shaft by bevel gears underneath
the steam cylinder.
_The “Imperial” air compressor_ is presented herewith in Figs. 367 and
368.
The “Imperial” compressor is especially designed for use in machine
shops, foundries and other industrial establishments where it is not
convenient to use a steam driven compressor.
The machine has two vertical, single-acting cylinders, each employing
long trunk pistons that act as guides for the lower ends of the
connecting rods. By this design, the height of the machine is reduced,
stuffing-boxes and crossheads are eliminated, and a minimum number of
bearings required. The cranks are set opposite to each other, so that
when the piston on one side is ascending, the other side is descending.
[Illustration: FIG. 367.]
The machine is made with duplex cylinders for the low pressures used
in sand blast work and the like, and with either duplex or compound
cylinders for higher pressures. In the compound type, an intercooler
is supplied, through which the air passes from the low pressure to the
high pressure cylinder.
_The air cylinders_ are water-jacketed and provided with hooded heads,
so that air may be supplied to them from outside the compressor-room;
the cylinders are cast in one piece with the frame.
[Illustration: FIG. 368.]
_The air-valves_, both inlet and outlet, are of the poppet type, fitted
with light springs, and work vertically. On account of their position
at the bottom of the cylinder, they are well lubricated, and, acting
vertically, they have little tendency to wear out of line with their
seats.
_The air intake passage_ is tapped to receive a supply pipe leading
from out-of-doors, or from some place where cool and clean air is
obtainable. The compressed air is discharged into a passage which is
tapped for a pipe to convey it to the air-receiver.
_All parts of the compressor are easily accessible_ for inspection,
adjustment, or repair. The air-heads may be removed without disturbing
any of the pipe connections. The valves may be taken out by unscrewing
the bonnets.