Scientific American Supplement, No. 799, April 25, 1891 by Various
V >>
Various >> Scientific American Supplement, No. 799, April 25, 1891
Pages:
1 |
2 | 3 |
4 |
5 |
6 |
7 |
8
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 1/4 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.
Pages:
1 |
2 | 3 |
4 |
5 |
6 |
7 |
8