Chapter 3

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.

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.

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.

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.

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.

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."

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.