--------+-----------------------+----------+------------+-------------
| Pressure. | | |
Atmo- +-----------+-----------+ Volume |Temperature | Total
spheres.|Pounds per |Pounds per | in Cubic | of the Air | Increase of
|Square Inch|Square Inch| Feet. | throughout | Temperature.
| above a |above the | |the Process.| Degrees.
| Vacuum. |Atmosphere | | Degrees. |
| |(Gauge | | |
| |Pressure). | | |
--------+-----------+-----------+----------+------------+-------------
1.00 | 14.70 | 0.00 | 1.0000 | 60.0 | 00.0
1.10 | 16.17 | 1.47 | 0.9346 | 74.6 | 14.6
1.25 | 18.37 | 3.67 | 0.8536 | 94.8 | 34.8
1.50 | 22.05 | 7.35 | 0.7501 | 124.9 | 64.9
1.75 | 25.81 | 11.11 | 0.6724 | 151.6 | 91.6
2.00 | 29.40 | 14.70 | 0.6117 | 175.8 | 115.8
2.50 | 36.70 | 22.00 | 0.5221 | 218.3 | 158.3
3.00 | 44.10 | 29.40 | 0.4588 | 255.1 | 195.1
3.50 | 51.40 | 36.70 | 0.4113 | 287.8 | 227.8
4.00 | 58.80 | 44.10 | 0.3741 | 317.4 | 257.4
5.00 | 73.50 | 58.80 | 0.3194 | 369.4 | 309.4
6.00 | 88.20 | 73.50 | 0.2806 | 414.5 | 354.5
7.00 | 102.90 | 88.20 | 0.2516 | 454.5 | 394.5
8.00 | 117.60 | 102.90 | 0.2288 | 490.6 | 430.6
9.00 | 132.30 | 117.60 | 0.2105 | 523.7 | 463.4
10.00 | 147.00 | 132.30 | 0.1953 | 554.0 | 494.0
15.00 | 220.50 | 205.80 | 0.1465 | 681.0 | 621.0
20.00 | 294.00 | 279.30 | 0.1195 | 781.0 | 721.0
25.00 | 367.50 | 352.80 | 0.1020 | 864.0 | 804.0
--------+-----------+-----------+----------+------------+-------------
A cubic foot of free air at a pressure of one atmosphere (equal to 14.7
pounds above a vacuum) at a temperature of 60 degrees, when compressed
to twenty-five atmospheres, will register 367.5 pounds above a vacuum
(352.8 pounds gauge pressure), will occupy a volume of 0.1020 cubic
foot, will have a temperature of 864 degrees, and the total increase of
temperature is 804 degrees.
The thermal results of air compression and expansion are shown by the
accompanying diagram.
The horizontal and vertical lines are the measures of volumes, pressures
and temperatures. The figures at the top indicate pressures in
atmospheres above a vacuum, the corresponding figures at the bottom
denote pressures by the gauge. At the right are volumes from one to
one-tenth. At the left are degrees of temperatures from zero to 1,000
Fahrenheit. The two curves which begin at the upper left hand corner and
extend to the lower right are the lines of compression or expansion.
The upper one being the _Adiabatic_ curve, or that which represents the
pressure at any point on the stroke with the heat developed by
compression remaining in the air; the lower is the _Isothermal_, or the
pressure curve uninfluenced by heat. The three curves which begin at the
lower left hand corner and rise to the right are heat curves and
represent the increase of temperature corresponding with different
pressures and volumes, assuming in one case that the temperature of the
air before admission to the compressor is zero, in another sixty
degrees, and in another one hundred degrees.
Beginning with the adiabatic curve, we find that for one volume of air
when compressed without cooling the curve intersects the first vertical
line at a point between 0.6 and 0.7 volume, the gauge pressure being
14.7 pounds. If we assume that this air was admitted to the compressor
at a temperature of zero, it will reach about 100 degrees when the gauge
pressure is 14.7 pounds. We find this by following down the first line
intersected by the adiabatic curve to the point where the zero heat
curve intersects this same line, the reading being given in figures to
the left immediately opposite. If the air had been admitted to the
compressor at 60 degrees, it would register about 176 degrees at 14.7
pounds gauge pressure. If the air were 100 degrees before compression,
it would go up to about 230 degrees at this pressure. Following this
adiabatic curve until it intersects line No. 5, representing a pressure
of five atmospheres above a vacuum (58.8 lb. gauge pressure), we see
that the total increase of temperature on the zero heat curve is about
270 degrees, for the 60 degree curve it is about 370 degrees, and for
the 100 degree curve it is about 435 degrees. The diagram shows that
when a volume of air is compressed adiabatically to 21 atmospheres (294
lb. gauge pressure), it will occupy a volume a little more than
one-tenth; the total increase of temperature with an initial temperature
of zero is about 650 degrees; with 60 degrees initial temperature it is
800 degrees, and with 100 degrees initial it is 900 degrees. It will be
observed that the zero heat curve is flatter than the others, indicating
that when free air is admitted to a compressor cold, the relative
increase of temperature is less than when the air is hot. This points to
the importance of low initial temperature.
We have now seen that the economical production of compressed air
depends upon the following conditions:
(1) A low initial temperature.
(2) Thorough cooling during compression.
It has been demonstrated by experiments made in France that the power
required to compress moist air is less than that for dry air. A table
showing the power required to compress moist and dry air has been
prepared from the data of M. Mallard and shows that for five atmospheres
the work expended in compressing one pound of dry air is 58,500 foot
pounds, while that for moist air is 52,500 foot pounds. In expansion
also moisture in the air adds to the economy, but in both cases the
saving of power is not great enough to compensate for the many
disadvantages due to the presence of water. Mr. Norman Selfe, of the
Engineering Association of N.S.W., has compiled a table which shows some
important theoretical conditions involved in producing compressed air.
So much for the theory of compression. We now come to the practical
production of compressed air.
The first record that we have of the use of an air compressor is at
Ramsgate Harbor, Kent, in the year 1788. Smeaton invented this "pump"
for use in a diving apparatus. In 1851, William Cubitt, at Rochester
Bridge, and a little later an engineer, Brunel, at Saltash, used
compressed air for bridge work. But the first notable application of
compressed air is due to Professor Colladon, of Geneva, whose plans were
adopted at the Mont Cenis tunnel. M. Sommeiller developed the Colladon
idea and constructed the compressed air plant illustrated in Fig. 2.
[Illustration: FIG. 2.]
The Sommeiller compressor was operated as a ram, utilizing a natural
head of water to force air at 80 pounds pressure into a receiver. The
column of water contained in the long pipe on the side of the hill was
started and stopped automatically, by valves controlled by engines. The
weight and momentum of the water forced a volume of air with such shock
against a discharge valve that it was opened and the air was discharged
into the tank; the valve was then closed, the water checked; a portion
of it was allowed to discharge and the space was filled with air, which
was in turn forced into the tank. The efficiency of this compressor was
about 50 per cent.
At the St. Gothard tunnel, begun in 1872, Prof. Colladon first
introduced the injection of water in the form of spray into the
compressor cylinder to absorb the heat of compression.
[Illustration: FIG. 3.]
Fig. 3 illustrates the air cylinder of the Dubois-Francois type of
compressor, which was the best in use about the year 1876. This
compressor was exhibited at the Centennial Exposition and was adopted by
Mr. Sutro in the construction of the Sutro tunnel. A characteristic
feature seems to be to get as much water into the cylinder as possible.
The water which flooded the bottom of the cylinder arose from the
voluminous injection; this water was pushed into the end of the cylinder
and some of it escaped with the air through the discharge valve.
An improved pattern of this compressor is shown in Fig. 4.
[Illustration: FIG. 4.]
These illustrations are interesting from an historical point of view, as
indicating the line of thought which early designers of air-compressing
machinery followed. As the necessity for compressed air power grew,
inventors turned their attention to the construction of air-compressing
engines that would combine _efficiency_ with _light weight_ and _economy
of space and cost_. The trade demanded compressors at inaccessible
localities, and in many cases it was preferred to sacrifice isothermal
results to simplicity of construction and low cost.
It is evident that an air compressor which has the steam cylinder and
the air cylinder on a single straight rod will apply the power in the
most direct manner, and will involve the simplest mechanics in the
construction of its parts. It is evident, however, that this straight
line, or direct construction, results in an engine which has the
greatest power at a time when there is no work to perform. At the
beginning of the stroke steam at the boiler pressure is admitted behind
the piston, and, as the air piston at that time is also at the initial
point in the stroke, it has only free air against it. The two pistons
move simultaneously, and the resistance in the air cylinder rapidly
increases as the air is compressed. To get economical results it is, of
course, necessary to cut off in the steam cylinder, so that at the end
of the stroke, when the steam pressure is low, as indicated by the
dotted line (Fig. 5), the air pressure is high, as similarly indicated.
The early direct-acting compressor used steam at full pressure
throughout the stroke. The Westinghouse pump, applied to locomotives, is
built on this principle, and those who have observed it work have
perhaps noticed that its speed of stroke is not uniform, but that it
moves rapidly at the beginning, gradually reducing its speed, and seems
to labor, until the direction of stroke is reversed. This construction
is admitted to be wasteful, but in some cases, notably that of the
Westinghouse pump, economy in steam consumption is sacrificed to
lightness and economy of space.
[Illustration: FIG. 5.]
Many efforts were made to equalize the power and resistance by
constructing the air compressor on the crank shaft principle, putting
the cranks at various angles, and by angular positions of steam and air
cylinders. Several types are shown in Fig. 6.
[Illustration: FIG. 6.]
Angular positions of the cylinder involve expensive construction and
unsteadiness. Experience has conclusively proved that it does not pay to
build air compressors with vertical cylinders, and moreover we have
found out that there is nothing in the apparent difficulty in equalizing
the strains in a direct-acting engine. It is simply necessary to add
enough weight to the moving parts, that is, to the piston, piston rod,
fly wheel, etc., to cut off early in the stroke and secure rotative
speed with the most economical results and with the cheapest
construction. It is obvious that the theoretically perfect air
compressor is a direct-acting one with a conical air cylinder, the base
of the cone being nearest the steam cylinder. This, from a practical
point of view, is impossible. Mr. Hill, in referring to the fallacious
tendencies of pneumatic engineers to equalize power and resistance in
air compressors, says: "The ingenuity of mechanics has been taxed and a
great variety of devices have been employed. It is usual to build on the
pattern of presses which do their work in a few inches of the end of the
stroke and employ heavy fly wheels, extra strong connections, and
prodigious bed plates. Counterpoise weights are also attached to such
machines; the steam is allowed to follow full stroke, steam cylinders
are placed at awkward angles to the air-compressing cylinders and the
motion conveyed through yokes, toggles, levers; and many joints and
other devices are used, many of which are entire failures, while some
are used with questionable engineering skill and very poor results."
[Illustration: FIG. 7.]
Fig. 7 illustrates the theory of Duplex Air Compressors. The hydraulic
piston or plunger compressor is largely used in Germany and elsewhere on
the Continent of Europe, but the duplex may be said to be the standard
type of European compressor at the present time. It is also largely used
in this country. Fig. 7 shows the four cylinders of a duplex compressor
in two positions of the stroke. It will be observed that each steam
cylinder has an air cylinder connected directly to the tail rod of its
piston, so that it is a direct-acting machine, except in that the
strains are transmitted through a single fly wheel, which is attached to
a crank shaft connecting the engines. In other words, a duplex air
compressor would be identical with a duplex steam engine were it not for
the fact that air cylinders are connected to the steam piston rods. The
result is, as shown in Fig. 7, that, at that point of the stroke
indicated in the top section, the upper right hand steam cylinder,
having steam at full pressure behind its piston, is doing work through
the angle of the crank shaft upon the air in the lower left hand
cylinder. At this point of the stroke the opposite steam cylinder has a
reduced steam pressure and is doing little or no work, because the
opposite air cylinder is beginning its stroke. Referring now to the
lower section, it will be seen that the conditions are reversed. One
crank has turned the center, and that piston which in the upper section
was doing the greatest work is now doing little or nothing, while the
labor of the engine has been transferred to those cylinders which a
moment before had been doing no work.
There are some advantages in this duplex construction, and some
disadvantages. The crank shafts being set quartering, as is the usual
construction, the engine may be run at low speed without getting on the
center. Each half being complete in itself, it is possible to detach the
one when only half the capacity is required. The power and resistance
being equalized through opposite cylinders, large fly wheels are not
necessary. Strange to say, the American practice seems to be to attach
enormous fly wheels to duplex air compressors. It is difficult to
justify this apparently useless expense in view of the facts shown in
Fig. 7. A fly wheel does not furnish power, nor does it add to the
economy of an engine except in so far as it enables it to cut off early
in the stroke, and to equalize the power and resistance. In other words,
a fly wheel is not a _source_ of power, and in many cases it is only a
means by which we accomplish rotative speed. It takes power to move
matter, and assuming that other conditions are equal, every engine that
carries a fly wheel that is larger than is necessary consumes a certain
number of foot pounds in turning so much metal around through space.
Were it possible to cut off at the same point and rotate as positively
without a fly wheel, it would be done away with entirely. Some straight
line air compressors are so constructed that the momentum of the piston
and other moving parts is nearly sufficient to equalize the strains
without a fly wheel; but the fly wheel is there because it insures a
definite length of stroke, and because it enables us to operate
eccentrics and to regulate the speed of the engine uniformly.
Objections to the duplex construction are: The strains are indirect,
angular and intermittent. It is necessary therefore to largely increase
the strength of parts; to add a crank shaft of large diameter with
enormous bearings, and to build expensive and very secure foundations.
Should the foundations settle at any point, excessive strains will be
brought upon the bearings, resulting in friction and liability to
breakage. A steam engine meets with a resistance on its crank shaft that
is uniform throughout the stroke; while an air compressor is subject to
a heavy maximum strain at the end of the stroke, hence the importance of
direct straight line connection between power and resistance.
[Illustration: FIG. 8.]
The friction loss on a duplex compressor seldom gets lower than 15 per
cent., while straight line compressors show as low a loss as 5 per cent.
Fig. 8 illustrates the Rand Duplex Air Compressor, a machine largely
used in America, especially in the Lake Superior iron mines. Fig. 9
illustrates a Duplex Compound Condensing Corliss Air Compressor built by
the Ingersoll-Sergeant Drill Company. This is a compressor made of the
best type of Corliss engine, with air cylinders connected to the tail
rods of the steam cylinders. One of these machines, of about 400 horse
power capacity, is now at work furnishing compressed air power for the
Brightwood Street Railway in Washington, D.C. Fig. 10 illustrates the
Norwalk direct-acting straight line air compressor, with compound air
cylinder. The chief purpose of compounding is to reduce the maximum
strain. This construction also adds to isothermal economy. The large
cylinder to the left determines the capacity of the compressor, the air
being compressed first to a low pressure (ordinarily about 30 pounds per
square inch), afterward passing through an intercooler, by which its
temperature is reduced, and then it is compressed still higher, even to
5,000 pounds per square inch if desired. The terminal strain, which is
so severe in air compressors, is here considerably reduced, as in this
case it is only equal to the area of the initial air piston multiplied
by its low air pressure.
[Illustration: FIG. 9.]
Economical results are attained with this compressor at low cost of
construction. The fly wheels are small, and the bearings narrow, because
the maximum strain is less, and the momentum of the piston and other
moving parts is such that most of the high initial steam power is taken
up in starting these parts and is afterward given out at the end of the
stroke, when the steam pressure is low. The strains are direct, and
expensive foundations are not required. Fig. 11 illustrates the
Ingersoll-Sergeant Compound Straight Line Air Compressor. This differs
from the one just described chiefly in that it is single-acting, while
the other is double-acting.
[Illustration: FIG. 10.]
By single-acting is meant that the air cylinders compress their
respective volumes of air _once_ every revolution. The air is admitted
to the large cylinder through the piston, is compressed to about 30
pounds, and on the return stroke the pressure is raised to almost any
point required, and in proportion to the diameter of the smaller
cylinder. Though single-acting, the capacity of one of these compressors
is about equal to that of the double-acting machine of the same cost of
construction. The initial air cylinder is made large enough to
correspond with the capacity of the smaller double-acting cylinder. The
strains are equalized because the area of the large cylinder multiplied
by its low pressure is exactly equal to that of the small cylinder
multiplied by its high pressure. The maximum strains are reduced
considerably below those which exist in compressors that do not compound
the air.
[Illustration: FIG. 11.]
The advantage of the single-acting air cylinder over the double is that
it compresses a volume of free air only once every revolution, hence
there is a better chance to cool the air during compression. The
cylinders have time to impart to the water jackets the heat produced by
compression and are kept cooler. The large air head of the initial
cylinder is jacketed, also adding to isothermal economy.
[Illustration: FIG. 12.]
Fig. 12 illustrates the Ingersoll-Sergeant Piston Inlet Cold Air
Compressor. This a straight line direct-acting engine, with steam and
air pistons connected to a single rod through a crosshead which connects
with two fly wheels. The strains are direct and the power and resistance
are equalized by the inertia of the crosshead, piston, rods, and fly
wheels. The Meyer's adjustable cut-off is used on the steam cylinder.
The air cylinder is provided with a tail rod tube through which all the
air is admitted into the cylinder.
[Illustration: FIG. 13.--AN AUTOMATIC AND ADJUSTABLE REGULATOR AND
UNLOADING DEVICE APPLIED TO INGERSOLL-SERGEANT AIR COMPRESSORS.]
Fig. 13 illustrates an unloading device and regulator applied to the
Ingersoll-Sergeant compressor.
The purpose of this unloading device is to maintain a uniform air
pressure in the receiver and a uniform speed of engine, notwithstanding
the consumption of the air, and to do this without waste of power or
attention on the part of the engineer. A weighted valve of safety valve
pattern is attached to the air cylinder, and is connected with the air
receiver, and with a discharge valve on each end of the air cylinder,
also with a balanced throttle valve in the steam pipe. When the pressure
of the air gets above the desired point in the receiver, the valve is
lifted and the air is exhausted from behind the discharge valves, thus
letting the compressed air at full receiver pressure into the cylinder
at both ends, and balancing the engine. At the same instant the
compressed air is exhausted from the little piston connected with the
balanced steam valve and the steam is automatically throttled, so that
only enough steam is admitted to keep the engine turning around, or to
overcome the friction, no work being done.
[Illustration: FIG. 14.]
When the compressor is unloaded, it is evident that the function of the
air piston is merely to force the compressed air through the discharge
valves and passages from one end to the other until more compressed air
is required, this being indicated by a fall in the receiver pressure.
The weighted valve now closes and the small connecting pipes are
instantly filled with compressed air; the steam valve automatically
opens, and the compression goes on in the regular way. Another function
of this device is to prevent the compressor from stopping or getting on
the center. Direct-acting compressors are liable to center when doing
work at slow speed.
[Illustration: FIG. 15. PISTON INLET VALVE OPERATED BY THE NATURAL LAWS
OF MOMENTUM.]
Fig. 15 illustrates the Ingersoll-Sergeant Air Cylinder and Piston.
Fig. 16 shows the piston inlet valve, situated at G in Fig. 15. Two of
these valves are placed in each piston of a double-acting air cylinder,
the piston being hollow and the free air being admitted through a
tail-rod pipe, letter E, Fig. 15. JJ are water jacket passages for
cooling the air during compression. Owing to the absence of inlet
valves, large water jackets are provided, not only around the cylinder
itself, but through the heads. As the heat of compression is greater
near the end of the stroke, the advantage of a cool head is manifest. H
H are the discharge valves through which the compressed air is forced.
[Illustration: FIG. 16. PISTON INLET VALVE OPERATED BY THE NATURAL LAWS
OF MOMENTUM.]
The most interesting feature of this cylinder is the piston inlet valve.
It is evident that this valve being attached to the piston needs no
springs or other connections, but is opened and closed exactly at the
right time by its natural inertia. With only about ¼ of an inch throw of
valve a large area is opened, through which the free air is drawn. The
valve is made of a single piece of composition metal and is practically
indestructible. Its construction is such that it fills the clearance
spaces to a greater extent than is usual in air compressors. A singular
feature is that indicator cards taken on these cylinders show a free air
line in some cases a little above the atmospheric line. Poppet valve
compressors almost invariably show a slight vacuum, due to several
causes, mainly the duty performed in compressing the springs of the
valves, but the vacuum is also influenced by insufficiency of valve
area, hot air cylinders, etc. This cylinder gives its full volume of
air, and apparently a little more at times, because the air is admitted
by a concentrated inlet in which free _air is always moving in one
direction_. After it has been started, the speed of the compressor is
such that the air attains a momentum due to its velocity and density;
this serves a useful purpose in piling up the free air in the cylinder
before the inlet valve closes on the return stroke.
[Illustration: FIG. 17.--COMBINED STEAM AND AIR INDICATOR CARD:
Taken from a 16x18 Sergeant piston inlet air compressor, meyer's cut-off
at 3/10. Steam at 58 lb.; air pressure, 77 lb.; total engine friction, 5
per cent.]
Fig. 17 illustrates a combined steam and air indicator card taken from
one of these cylinders. It will be observed that with steam and air
cylinders equal in diameter and stroke, an air pressure of 77 pounds is
reached with a steam pressure of only 58 pounds. The reason for this is
plainly shown in the cards, their areas being nearly equal. What is made
up in the air card by high pressure is represented in the steam card by
greater volume. The indicated efficiency deduced from these cards is 95
per cent., that is, the area of the air card divided by the area of the
steam card, representing the resistance divided by the power, results in
95 per cent. While several cards have been taken on the cylinders
showing a loss by friction of only 5 per cent., yet on the average the
best practice shows a loss of 6 per cent. or an efficiency of 94 per
cent. This result indicates an almost perfect proportion between power
and resistance, and good workmanship in air-compressing machinery. It is
difficult to conceive an engine of this size being worked with a less
expenditure for friction than 5 or 6 per cent. Were it possible to
retain the heat which is in the air, and which is represented by the
space between the dotted isothermal curve and the actual curve, we might
attain high efficiency in using compressed air power, but it is evident
that the power represented by the area of this space will be lost by
radiation of heat before it is used in an engine situated several
hundred feet away.
These indicator cards show at a glance that heat is responsible for the
important air losses, and that so far as the design of the compressing
engine is concerned, we have attained a point very near perfection. All
the devices, past, present and future, on which inventors spend so much
time, and in the development of which capitalists are innocently
inveigled, _aim to save this six per cent. loss!_ We hear a good deal
about "Centrifugal Air Compressors," "Rotaries," "Plunger Pumps," etc.,
designs involving expensive complications without any heat advantage,
and which seem to be based upon the "iridescent dream" of a large loss
in the present method of compressing air. Here we have a simple engine,
compact and complete in itself, capable of high speed without injury,
constructed on the basis of our best steam engine practice, which
produces compressed air power at a loss of only six per cent.
Clearance is not taken into consideration in the foregoing figures, but
clearance is very much more of a _bete noir_ in theory than in practice.
The early designers, as shown in the "Dubois-Francois" illustrations,
Figs. 3 and 4, regarded clearance loss as a very serious matter. Even at
the present time some air compressor manufacturers admit water through
the inlet valves into the air cylinder, not so much for the purpose of
cooling as to fill up the clearance space. A long stroke involving
expensive construction is usually justified by the claim that a large
saving is effected by reduced clearance loss. Let us see what the effect
of this clearance is. Assuming that we have an air compressor which
shows an isothermal pressure line, there would be some loss of power due
to clearance space, because we would have a certain volume of air upon
which work was done and heat produced, that heat having been absorbed
and the air being retained in the cylinder and not serving any useful
purpose. But let us assume that we have a compressor which shows an
adiabatic pressure line. We now have the air in the clearance space
acting precisely as a spring, compressed at each stroke, retaining its
heat of compression, and giving it out against the air piston at the
point when the stroke is reversed. There is no loss of power in such a
case as this, but, on the contrary, the air spring is useful in
overcoming the inertia of the piston and moving parts. The best air
compressors give a result about midway between the isothermal and the
adiabatic, and the net loss of _power_ directly due to clearance is so
small as to be practically unworthy of consideration.
It must not be inferred from the preceding remarks that the designer of
an air compressor may neglect the question of clearance. On the
contrary, it is a very important consideration. If we assume a large
clearance space in the end of an air cylinder of a compressor which is
furnishing air at a high pressure, we may readily conceive that space to
be so large, and that pressure so high, that the entire volume of the
cylinder would be filled by the air from the clearance space alone, and
the compressor would take in no free air and would, of course, produce
no compressed air.
Loss in _capacity_ of air compressors by clearance is in direct
proportion to the pressure.
Owing to the loss of capacity by clearance space at high pressures, it
is important that compound air cylinders should be used for furnishing
air at high pressure. With compound air cylinders the air is compressed
to alternate stages of pressure in the different cylinders, and the
clearance loss is thus reduced because of the reduced density of the air
in the clearance spaces. In ordinary practice air compressors deliver
the air at less than 100 pounds pressure, so that with a properly
designed air cylinder the clearance space is so small that the capacity
of the compressor is not materially affected.
Two systems are in use by which the heat of compression is absorbed, and
the difference between one and the other is so distinct that air
compressors are usually divided into two classes (1) wet compressors,
(2) dry compressors.
A _wet_ compressor is that which introduces water directly into the air
cylinder during compression.
A _dry_ compressor is that which introduces no water into the air during
compression.
_Wet_ compressors may be subdivided into two classes.
(1) Those which inject water in the form of a spray into the cylinder
during compression.
(2) Those which use a water piston for forcing the air into confinement.
The injection of water into the cylinder is usually known as the
Colladon idea. Compressors built on this system have shown the highest
isothermal results, that is, by means of a finely divided spray of cold
water the heat of compression has been absorbed to a point where the
compressed air has been discharged at a temperature nearly equal to that
at which it was admitted to the cylinder. The advantages of water
injection during compression are as follows:
(1) Low temperature of air during compression.
(2) Increased volume of air per stroke, due to filling of clearance
spaces with water and to a cold air cylinder.
(3) Low temperature of air immediately after compression, thus
condensing moisture in the air receiver.
(4) Low temperature of cylinder and valves, thus maintaining packing,
etc.
(5) Economical results, due to compression of moist air (see table 3).
TABLE 3.--SHOWING THE RELATIVE QUANTITY OF WORK REQUIRED TO COMPRESS A
GIVEN VOLUME AND WEIGHT OF AIR, BOTH DRY AND MOIST--ALSO RELATIVE
VOLUMES WITH AND WITHOUT INCREASE OF TEMPERATURE FROM COMPRESSION.
_______________________________________________________________________________________
| | |
|Compression at |Compression |
|a Constant |with |
|Temperature. |Increase of |
|Mariotte's Law. |Temperature. |
__|________________|__________________________________|________________________________
| | | | | | | | | | | | | | |
1|0.1 | | | | | | 20 | 68 |1.0 | | | 68 | | |
2|0.5 | 7199|1468|0.612| 7932|1618| 85.5|186 |1.222| 733|0.092|111 |3.0|23500|22500
3|0.333|11356|2316|0.459|13360|2725|130.4|267 |1.375|2004|0.150|135.5|4.0|37000|35000
4|0.25 |14260|2909|0.374|17737|3618|165.6|330 |1.495|3477|0.196|153.5|4.8|48500|45000
5|0.200|16580|3383|0.320|21209|4326|195.3|384 |1.595|4629|0.213|167 |5.4|58500|52500
6|0.167|18475|3768|0.281|24310|4959|220.5|429 |1.681|5835|0.240|179 |6.0|67000|60000
7|0.143|20038|4087|0.252|27048|5517|243.2|470 |1.758|7040|0.260|190 |6.4|75000|66000
8|0.125|21422|4370|0.229|29518|6021|263.6|506.5|1.828|8096|0.274| | | |
9|0.111| | |0.210| | |282 |539.6|1.891| | | | | |
10|0.100| | |0.195| | |299 |570.2|1.950| | | | | |
_______________________________________________________________________________________
| | | | | | | | | | | | | | |
1| 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14| 15 | 16
__|_____|_____|____|_____|_____|____|_____|_____|_____|____|_____|_____|___|_____|_____
Column Heading
1 Tension in Atmospheres.
2 Volume.
3 Work of Compression. Cubic Meters in Kilogram-meters.
4 Work of Compression. Cubic Feet in Foot Pounds.
5 Volume.
6 Work of Compression. (Dry.) Cubic Meters in Kilogram-meters.
7 Work of Compression. (Dry.) Cubic Feet in Foot Pounds. Deduced from 6.
8 Temperatures. (Dry.) Cent.
9 Temperatures. (Dry.) Fah.
10 Ratio of Greater to Less Temperature. Absolute.
11 Loss of Work in Compressing one Cubic Meter in Kilogram-meters.
By Increase of Temperature alone.
12 Percentage of Work of Compression Converted into Heat and Lost.
By Increase of Temperature alone.
13 Final Temperature if Water is used in Compression. Fah.
14 Percentage of Water to Air Required.
15 Foot Pounds to Compress One Pound Air. Dry.
16 Foot Pounds to Compress One Pound Air. With sufficient Moisture.
The first advantage is by far the most important one, and is really the
only excuse for water injection in air compressors. We have seen (table
3) that the percentage of work of compression which is converted into
heat and loss when no cooling system is used is as follows:
Compressing to 2 atmospheres loss 9.2 per cent.
" " 3 " " 15.0 " "
" " 4 " " 19.6 " "
" " 5 " " 21.3 " "
" " 6 " " 24.0 " "
" " 7 " " 26.0 " "
" " 8 " " 27.4 " "
We see that in compressing air to five atmospheres, which is the usual
practice, the heat loss is 21.3 per cent., so that if we keep down the
temperature of the air during compression to the isothermal line, we
save this loss. The best practice in America has brought this heat loss
down to 3.6 per cent. (old Ingersoll Injection Air Compressor), while in
Europe the heat loss has been reduced to 1.6 per cent. Steam-driven air
compressors are usually run at a piston speed of about 350 feet per
minute, or from 60-80 revolutions per minute of compressors of average
sizes, say 18" diameter of cylinder. Sixty revolutions per minute is
equal to 120 strokes, or two strokes per second. An air cylinder 18" in
diameter filled with free air once every half second, and at each stroke
compressing the air to 60 pounds, and thereby producing 309 degrees of
heat, is thus, by means of water injection, cooled to an extent hardly
possible with mere surface contact. The specific heat of water being
about four times that of air, it readily takes up the heat of
compression.
A properly designed spray system must not be confused with the numerous
devices applied to air cylinders, by means of which water is introduced.
In some cases the water is merely drawn in through the inlet valves. In
others it passes through the center of the piston and rod, coming in
contact with the interior walls of the air cylinder between the packing
rings. Introducing water into the air cylinder in _any other way, except
in the form of a spray, has but little effect in cooling the air during
compression._ On the contrary, it is a most fallacious system, because
it introduces all the disadvantages of water injection without its
isothermal influence. Water, by mere surface contact with air, takes up
but little heat, while the air, having a chance to increase its
temperature, absorbs water through the affinity of air for moisture, and
thus carries over a volume of saturated hot air into the receiver and
pipes, which on cooling, as it always does in transit to the mine,
deposits its moisture and gives trouble through water and freezing. It
is, therefore, of much importance to bear in mind that unless water can
be introduced _during compression_ to such an extent as to _keep down
the temperature of the air in the cylinder_, it had better not be
introduced at all.
If too little water is introduced into an air cylinder during
compression, the result is warm, moist air, and if too much water is
used, it results in a surplus of power required to move a body of water
which renders no useful service. The following table deduced from
Zahner's formula gives the quantity of water which should be injected
per cubic foot of air compressed in order to keep the temperature down
to 104 degrees Fah.
_________________________________________________________________________
| | |
| |Weight of water |Weight of water
| |to be injected at |to be injected at
|Heat units devel-|68° Fah. to keep |68° Fah. to keep
Compression |oped in 1 lb. |the temperature at|the temperature at
by atmosphere |free air by |104° Fah. in lbs. |104° Fah. in lbs. of
above a volume.|compression. |of water and per |water for 1 cubic
| |lb. of free air. |foot of free air.
_______________|_________________|__________________|____________________
| | |
2 | 3.702 | 0.734 | 0.056
3 | 5.867 | 1.664 | 0.089
4 | 7.406 | 1.469 | 0.113
5 | 8.598 | 1.701 | 0.131
6 | 9.570 | 1.891 | 0.145
7 | 10.398 | 2.063 | 0.158
8 | 11.109 | 2.204 | 0.167
9 | 11.740 | 2.329 | 0.179
10 | 12.301 | 2.440 | 0.188
11 | 12.813 | 2.542 | 0.195
12 | 13.278 | 2.634 | 0.202
13 | 13.706 | 2.719 | 0.209
14 | 14.102 | 2.798 | 0.215
15 | 14.471 | 2.871 | 0.223
_______________|_________________|__________________|____________________
Objections to water injection are as follows:
(1) Impurities in the water, which, through both mechanical and chemical
action, destroy exposed metallic surfaces.
(2) Wear of cylinder, piston and other parts, due directly to the fact
that water is a bad lubricant, and as the density of water is greater
than that of oil, the latter floats on the water and has no chance to
lubricate the moving parts.
(3) Wet air arising from insufficient quantity of water and from
inefficient means of ejection.
(4) Mechanical complications connected with the water pump, and the
difficulties in the way of proportioning the volume of water and its
temperature to the volume, temperature and pressure of the air.
(5) Loss of power required to overcome the inertia of the water.
(6) Limitations to the speed of the compressor, because of the liability
to break the cylinder head joint by water confined in the clearance
spaces.
(7) Absorption of air by water.
Before the introduction of condensing air receivers, wet air resulting
in freezing was considered the most serious obstacle to water injection;
but this difficulty no longer exists, as experience has conclusively
demonstrated that a large part of the moisture in compressed air may be
abstracted in the air receiver. Even in the so-called dry compressors a
great deal of moisture is carried over with the compressed air, because
the atmosphere is never free from moisture. This subject will be
referred to more fully when treating of the transmission of compressed
air.
By far the most serious obstacle to water injection, and that which
condemns the wet compressor, is the influence of the injected water upon
the air cylinder and parts. Even when pure water is used, the cylinders
wear to such an extent as to produce leakage and to require reboring.
The limitation to the speed of a compressor is also an important
objection. The claim made by some that the injected water does not fill
the clearance spaces, but is aerated, does not hold good, except with an
inefficient injection system. The writer has increased the speed of an
air compressor (cylinders 12 in. and 12 in. by 18 in., injection air
cylinder) ten revolutions per minute by placing his fingers over the
orifice of the suction pipe of the water pump. The boiler pressure
remained the same, the cut-off was not changed and the air pressure was
uniform, hence this increase of speed arose from the fact that the water
was restricted and the clearance spaces were filled with compressed air,
which served as a cushion or spring. While the volume of compressed air
furnished by this compressor would be somewhat reduced by the
restriction of the water, yet the increase in speed which was obtained
without any increase of power fully compensated for the clearance loss.
Mr. John Darlington, of England, gives the following particulars of a
modern air compressor of European type:
"Engine, two vertical cylinders, steam jacketed, with Meyer's expansion
gear. Cylinders, 16.9 inches diameter, stroke 39.4 inches; compressor,
two cylinders, diameter of piston, 23.0 inches; stroke 39.4 inches;
revolutions per minute, 30 to 40; piston speed 39 to 52 inches per
second, capacity of cylinder per revolution, 20 cubic feet: diameter of
valves, viz., four inlet and four outlet, 5½ inches; weight of each
inlet valve, 8 lb.; outlet, 10 lb.; pressure of air, 4 to 5 atmospheres.
The diagrams taken of the engine and compressor show that the work
expended in compressing one cubic meter of air to 4.21 effective
atmospheres was 38,128 lb. According to Boyle and Mariotte's law it
would be 37,534 lb., the difference being 594 lb., or a loss of 1.6 per
cent. Or if compressed without abstraction of heat, the work expended
would in that case have been 48,158. The volume of air compressed per
revolution was 0.5654 cubic meter. For obtaining this measure of
compressed air, the work expended was 21,557 pounds. The work done in
the steam cylinders, from indicator diagrams, is shown to have been
25,205 pounds, the useful effect being 85½ per cent. of the power
expended. The temperature of air on entering the cylinder was 50 degrees
Fah., on leaving 62 degrees Fah., or an increase of 12 degrees Fah.
Without the water jacket and water injection for cooling the temperature
it would have been 302 degrees Fah. The water injected into the
cylinders per revolution was 0.81 gallon."
We have in the foregoing a remarkable isothermal result. The heat of
compression is so thoroughly absorbed that the thermal loss is only 1.6
per cent.; but the loss _by friction of the engine_ is 14.5 per cent.,
and the net economy of the whole system is no greater than that of the
best American dry compressor, which loses about one-half the theoretical
loss due to heat of compression, but which makes up the difference by a
low friction loss.
The wet compressor of the second class is the water piston compressor,
Fig. 18.
[Illustration: FIG. 18.--HYDRAULIC AIR COMPRESSOR.]
The illustration shows the general type of this compressor, though it
has been subject to much modification in different places. In America, a
plunger is used instead of a piston, and as it always moves in water the
result is more satisfactory. The piston, or plunger, moves horizontally
in the lower part of a U shaped cylinder. Water at all times surrounds
the piston, and fills alternately the upper chambers. The free air is
admitted through a valve on the side of each column and is discharged
through the top. The movement of the piston causes the water to rise on
one side and fall on the other. As the water falls the space is occupied
by free air, which is compressed when the motion of the piston is
reversed, and the water column raised. The discharge valve is so
proportioned that some of the water is carried out after the air has
been discharged. Hence there are no clearance losses.
This hydraulic compressor seems to have a certain charm about it, which
has resulted in its adoption in Germany, France and Belgium, and by one
of the largest mines in the United States. Its advantages are _purely
theoretical_, and without certain adjuncts which have been in some cases
applied to it, even the _theory_ is a very bad one.
The chief claim for this water piston compressor is that its piston is
also its cooling device, and that the heat of compression is absorbed by
the water. So much confidence seems to be placed in the isothermal
features of this machine that usually no water jacket or spray pump is
applied. Mr. Darlington, who is one of the stanch defenders of this
class of compressors, has found it necessary to introduce "spray jets of
water immediately under the outlet valves," the object of which is to
absorb a larger amount of heat than would otherwise be effected by the
simple contact of the air with the water-compressing column. Without
such spray connections, it is safe to say that this compressor has
scarcely any cooling advantages at all, so far as air cooling is
concerned. Water is not a good conductor of heat. In this case only one
side of a large body of air is exposed to a water surface, and as water
is a bad conductor, the result is that a thin film of water gets hot in
the early stage of the stroke and little or no cooling takes place
thereafter. The compressed air is doubtless cooled before it gets even
as far as the receiver, because so much water is tumbled over into the
pipes with it, but to produce economical results the cooling should take
place _during compression_.
Water and cast iron have about the same relative capacity for heat at
equal volumes. In this water piston compressor we have only one cooling
surface, which soon gets hot, while with a dry compressor, with water
jacketed cylinders and heads, there are several cold metallic surfaces
exposed on one side to the heat of compression, and on the other to a
moving body of cold water.
But the water piston fraternity promptly brings forward the question of
speed. They say that, admitting that the cooling surfaces are equal, we
have in one case _more time_ to absorb the heat than in the other. This
is true, and here we come to an important class division in air
compressing machinery--_high speed and short stroke_ as against _slow
speed and long stroke_. Hydraulic piston compressors are subject to the
laws that govern piston pumps, and are, therefore, limited to a piston
speed of about 100 feet per minute. It is quite out of the question to
run them at much higher speed than this without shock to the engine and
fluctuations of air pressure due to agitation of the water piston. The
quantity of heat produced, that is, the degree of temperature reached,
depends entirely upon the conditions in the air itself, as to density,
temperature and moisture, and is entirely independent of speed. We have
seen that it is possible to lose 21.3 per cent. of work when compressing
air to five atmospheres without any cooling arrangements. With the best
compressors of the dry system one-half of this loss is saved by water
jacket absorption, so that we are left with about 11 per cent., which
the slow moving compressor seeks to erase. We are quite safe in saying
that the element of _time alone_ in the stroke of an air compressor
could not possibly effect a saving of more than half of this, or 5½ per
cent. Now, in order to get this 5½ per cent. saving, we reduce the speed
of an air-compressing engine from 350 feet per minute to 100 feet per
minute. We must, therefore, in one case have a piston area _three and
one-half_ times that of the other in order to get the _same capacity of
air_, and in doing this we build an engine of enormous proportions with
heavy moving parts. We load it down with a large mass of water, which it
must move back and forth during its work, and thus we produce a
percentage of friction loss alone equal to twice or even three times the
5½ per cent. heat loss which is responsible for all this expense in
first cost and in maintenance, but which really is not saved after all
unless water injection in the form of spray also forms a part of the
system.
It is obvious that cost of construction and maintenance have much to do
with the commercial value of an air compressor. The hydraulic piston
machine not only costs a great deal more in proportion to the power it
produces, but it costs more to maintain it, and it costs more to run it.
It is not an uncommon thing to hear engineers speak of the hydraulic
piston compressor as the "most economical" machine for the purpose, but
that it is so "expensive" and takes up so much room, and requires such
expensive foundations that, unless persons are "willing to spend so much
money," they had better take the next best thing, a high speed machine.
We hear of "magnificent air-compressing engines, the largest in the
country," and pilgrimages are made to see these artificial wonders when,
not unlike the old pyramids, they represent a pile of inert matter--a
monument to moneyed kings.
The hydraulic piston compressor has one solitary advantage, and that is,
it has no dead spaces. It was conceived at a time when dead spaces were
very serious conditions--were positive specters! Valves and other
mechanism connected with the cylinder of an air compressor were once of
such crude construction that it was impossible to reduce the clearance
spaces to a reasonable point, and, furthermore, the valves were heavy
and so complicated that anything like a high speed would either break
them or wear them out rapidly, or derange them so that leakages would
occur. But we have now reduced inlet and discharge valves and all other
moving parts connected with an air cylinder to a point of extreme
simplicity. Clearance space is in some cases destroyed altogether by
what is, as it were, an elastic air head which is brought into direct
contact with the piston. All this reduces clearance to so small a point
that it has no influence of any consequence. The moving parts are made
extremely simple, even arriving at a point where inlet valves are opened
and closed by their natural inertia. Mr. Sturgeon, of England, has
applied a most ingenious and successful inlet valve, which is opened and
closed by the friction of the air piston rod through the gland. We have,
therefore, reached a point at which high speed is made possible.
Long-stroke air compressors are evidently objectionable on the basis of
greater expense of construction. All the parts must be larger and
heavier. The fly wheels are increased enormously in diameter and weight,
and the strength of bearings must be enlarged in proportion. It is
difficult to equalize power and resistance in air compressors with long
strokes. The speed will be jerky, and when slow, the fly wheel rather
retards than assists in the work of compression. This action tends to
derange the parts and makes large bearings a necessity. The piston in a
long-stroke compressor travels through considerable space before the
pressure reaches a point where the discharge valve opens, and after
reaching that point it has to go on still further against a prolonged
uniform resistance. This makes rotative speed difficult. During the
early part of the stroke, the energy of the steam piston must be stored
up in the moving parts, to be given out when the steam pressure has been
reduced through an early cut-off. With a short stroke and a large
diameter of steam cylinder we are able to get steam economy or early
cut-off and expansion without the complications of compounding.
* * * * *
[Continued from SUPPLEMENT, No. 793, page 12677.]