N. Hawkins

“There is this remarkable difference between bodies in a fluid and bodies in a solid form, namely, that every particle of a fluid is perfectly independent of every other particle. They do not cohere in masses, like the particles of a solid, nor do they repel one another, as is the case with the particles composing a gas. They can mingle among each other with the least degree of friction, and, when they press down upon one another by virtue of their own weight, the downward pressure is communicated in all directions, causing a pressure upwards, sideways, and in every possible manner. Herein the particles of a fluid differ from the particles of a solid, even when reduced to the most impalpable powder; and it is this which constitutes fluidity, namely, the power of transmitting pressure in every direction, and that, too, with the least degree of friction. The particles which compose a fluid must be very much smaller than the finest grain of an impalpable powder.”—Richard Green Parker, A. M.

PNEUMATICS.

Pneumatics treats of the mechanical properties and effects of air and similar fluids; these are called elastic fluids and gases, or aËriform fluids.

Hydro-pneumatics. This is a compound word formed from two Greek words signifying water and air; in its primary meaning it conveys the idea of the combined action of water and air or gas.

Air is the respirable fluid which surrounds the earth and forms its atmosphere. It is inodorous, invisible, insipid, colorless, elastic, possessed of gravity, easily moved, rarefied and condensed, essential to respiration and combustion, and is the medium of sound. It is composed by volume of 20.7 parts of oxygen and 79.3 of nitrogen, by weight, of 23 of oxygen and 77 of nitrogen. These gases are not chemically united, but are mixed mechanically. Air contains also ¹/₂₀₀₀ of carbon dioxide, some aqueous vapor, about one per cent. of argon, and small varying amounts of ammonia, nitric acid, ozone, and organic matter. The specific gravity of the air at 32° F is to that of water as 1 to 773, and 100 cubic inches of air at mean temperature and pressure weighs 30¹/₂ grains.

AËriform fluids are those which have the form of air. Many of them are invisible, or nearly so, and all of them perform very important operations in the material world. But, notwithstanding that they are in most instances imperceptible to our sight, they are really material, and possess all the essential properties of matter. They possess, also, in an eminent degree, all the properties which have been ascribed to liquids in general, besides others by which they are distinguished from liquids.

Elastic fluids are divided into two classes, namely, 1, permanent gases and, 2, vapors. The gases cannot be easily converted into the liquid state by any known process of art;* but the vapors are readily reduced to the liquid form either by pressure or diminution of temperature. There is, however, no essential difference between the mechanical properties of both classes of fluids.

As the air which we breathe, and which surrounds us, is the most familiar of all this class of bodies, it is generally selected as the subject of Pneumatics. But it must be premised that the same laws, properties and effects, which belong to air, belong in common, also, to all aËriform fluids or gaseous bodies.

There are two principal properties of air, namely, gravity and elasticity. These are called the principal properties of this class of bodies, because they are the means by which their presence and mechanical agency are especially exhibited.

Although the aËriform fluids all have weight, they appear to possess no cohesive attraction.

The pressure of the atmosphere caused by its weight is exerted on all substances, internally and externally, and it is a necessary consequence of its fluidity. When the external pressure is artificially removed from any part, it is immediately felt by the reaction of the internal air.

Heat insinuates itself between the particles of bodies and forces them asunder, in opposition to the attraction of cohesion and of gravity. It therefore exerts its power against both the attraction of gravitation and the attraction of cohesion. But, as the attraction of cohesion does not exist in aËriform fluids, the expansive power of heat upon them has nothing to contend with but gravity. Hence, any increase of temperature, expands an elastic fluid prodigiously, and a diminution of heat condenses it.

A column of air, having a base an inch square, and reaching to the top of the atmosphere, weighs about fifteen pounds. This pressure, like the pressure of liquids, is exerted equally in all directions.

The elasticity of air and other aËriform fluids is that property by which they are increased or diminished in extension, according as they are compressed. This property exists in a much greater degree in air and other similar fluids than in any other substance. In fact, it has no known limit, for, when the pressure is removed from any portion of air, it immediately expands to such a degree that the smallest quantity will diffuse itself over an indefinitely large space. And, on the contrary, when the pressure is increased, it will be compressed into indefinitely small dimensions.

The elasticity or pressure of air and all gases is in direct proportion to their density; or, what is the same thing, inversely proportional to the space which the fluid occupies. This law, which was discovered by Mariotte, is called “Mariotte’s Law.” This law may perhaps be better expressed in the following language; namely, the density of an elastic fluid is in direct proportion to the pressure which it sustains.

Air becomes a mechanical agent by means of its weight, its elasticity, its inertia and its fluidity.

The fluidity of air invests it, as it invests all other liquids, with the power of transmitting pressure; fluidity is a necessary consequence of the independent gravitation of the particles of a fluid. It may, therefore, be included among the effects of weight.

The inertia of air is exhibited in the resistance which it opposes to motion, which has already been noticed under the head of Mechanics. This is clearly seen in its effects upon falling bodies, as will be exemplified in the experiments with the air-pump.

The great degree of elasticity possessed by all aËriform fluids, renders them susceptible of compression and expansion to an almost unlimited extent. The repulsion of their particles causes them to expand, while within certain limits they are easily compressed. This materially affects the state of density and rarety under which they are at times exhibited.

It may here be stated that all the laws and properties of liquids (described under the heads of Hydrostatics and Hydraulics) belong also to aËriform fluids.

The chemical properties of both liquids and fluids belong peculiarly to the science of Chemistry, and are, therefore, not to any extent, considered in this volume.

The air which we breathe is an elastic fluid, surrounding the earth, and extending to an indefinite distance above its surface, and constantly decreasing upwards in density. It has already been stated that the air near the surface of the earth bears the weight of that which is above it.

Being compressed, therefore, by the weight of that above it, it must exist in a condensed form near the surface of the earth, while in the upper regions of the atmosphere, where there is no pressure, it is highly rarefied. This condensation, or pressure, is very similar to that of water at great depths in the sea.

Besides the two principal properties, gravity and elasticity, the operations of which produce most of the phenomena of Pneumatics, it will be recollected that as air, although an invisible is yet a material substance, possessing all the common properties of matter, it possesses also the common property of impenetrability.

The Thermometer is an instrument to indicate the temperature of the atmosphere. It is constructed on the principle that heat expands and cold contracts most substances. The thermometer consists of a capillary tube, closed at the top and terminating downwards in a bulb. It is filled with mercury which expands and fills the whole length of the tube or contracts altogether into the bulb, according to the degree of heat or cold to which it is exposed. Any other fluid which is expanded by heat and contracted by cold, may be used instead of mercury.

As it has been proved by experiment that 100 cubic inches of air weighs 30¹/₂ grains, it will readily be conceived that the whole atmosphere exercises a considerable pressure on the surface of the earth. The existence of this pressure is shown by the following experiments. On one end of a stout glass cylinder, about 10 inches high, and open at both ends, a piece of bladder is tied quite air-tight. The other end, the edge of which is ground and well-greased, is pressed on the plate of the air-pump, Fig. 331. As soon as the air in the vessel is rarefied by working the air-pump, the bladder is depressed by the weight of the atmosphere above it, and finally bursts with a loud report caused by the sudden entrance of air.

The preceding experiment only serves to illustrate the downward pressure of the atmosphere. By means of the Magdeburg hemispheres, Figs. 332 and 333, the invention of which is due to Otto von Guericke, burgomaster of Magdeburg, it can be shown that the pressure acts in all directions. This apparatus consists of two hollow brass hemispheres of 4 to 4¹/₂ inches diameter, the edges of which are made to fit tightly, and are well greased. One of the hemispheres is provided with a stop-cock, by which it can be screwed on to the air-pump, and on the other there is a handle. As long as the hemispheres contain air they can be separated without any difficulty, for the external pressure of the atmosphere is counterbalanced by the elastic force of the air in the interior. But when the air in the interior is pumped out by means of an air-pump, the hemispheres cannot be separated without a powerful effort.

The Barometer is an instrument to measure the weight of the atmosphere, and thereby to indicate the variations of the weather, etc. It consists of a long glass tube, about thirty-three inches in length, closed at the upper end, and filled with mercury. The tube is then inverted in a cup or leather bag of mercury, on which the pressure of the atmosphere is exerted. The following experiment, which was first made in 1643, by Torricelli, a pupil of Galileo, gives an exact measure of the weight of the atmosphere.

A glass tube is taken, about a yard long and a quarter of an inch internal diameter, Fig. 334. It is sealed at one end, and is quite filled with mercury. The aperture, C, being closed by the thumb, the tube is inverted, the open end placed in a small mercury trough, and the thumb removed. The tube being in a vertical position, the column of mercury sinks, and, after oscillating some time, it finally comes to rest at a height, A, which at the level of the sea is about 30 inches above the mercury in the trough.

The mercury is raised in the tube by pressure of the atmosphere on the mercury in the trough. There is no contrary pressure on the mercury in the tube, because it is closed; but, if the end of the tube be opened, the atmosphere will press equally inside and outside the tube, and the mercury will sink to the level of that in the trough. It has been shown that the heights of two columns of liquid in communication with each other are inversely as their densities; and hence it follows that the pressure of the atmosphere is equal to that of a column of mercury the height of which is 30 inches. If, however, the weight of the atmosphere diminishes, the height of the column which it can sustain must also diminish.

Why a vacuum gauge is graduated in inches instead of in pounds is thus explained. Take a tube say 35 inches long, closed at one end, filled with mercury and inverted with its open end in a bowl containing the same liquid.

The atmosphere will exert on the surface of the mercury in the bowl a pressure of about 15 pounds per square inch and this pressure will be transmitted to that in the tube so that the upward pressure inside the tube at the level of the mercury in the bowl will be 15 pounds per square inch.

Below the surface the pressure increases, due to the depth of mercury, but the weight of mercury inside the tube below the level in the bowl counteracts the weight of that outside so that the upward pressure per square inch at the surface line is 15 pounds per square inch inside the tube no matter how much or little it is submerged. In the upper end of the tube the mercury has dropped away, leaving a complete vacuum.

The 15 pounds will force the mercury up into the tube until the column is high enough to balance that pressure. One cubic inch of mercury weighs about half a pound. It would take two cubic inches to weigh a pound and a column two inches high to exert a pressure of one pound per square inch of base, or a column 30 inches high to balance the pressure of 15 pounds.

If instead of a perfect vacuum there was a pressure of two pounds in the upper end of the tube the column would have to balance a pressure of 15-2 = 13 pounds and would be 26 inches high. As the absolute pressure in the top of the tube gets greater, that is to say, as the difference between that pressure and that of the atmosphere or the so-called vacuum gets less, the column of mercury gets lower, and its height is a measure of the completeness of the vacuum.

Hero’s fountain, which derives its name from its inventor, Hero, who lived at Alexandria, 120 B.C., depends on the elasticity of the air. It consists of a brass dish, D, Fig. 335, and of two glass globes, M and N. The dish communicates with the lower part of the globe, N, by a long tube, B; and another tube, A, connects the two globes. A third tube passes through the dish, D, to the lower part of the globe, M. This tube having been taken out, the globe, M, is partially filled with water; the tube is then replaced and water is poured into the dish. The water flows through the tube, B, into the lower globe, and expels the air, which is forced into the upper globe; the air, thus compressed, acts upon water, and makes it jet out as represented in the figure. If it were not for the resistance of the atmosphere and friction, the liquid would rise to a height above the water in the dish equal to the difference of the level in the two globes.

The fountain in vacuo, Fig. 336, shows an interesting experiment made with the air-pump, and shows the elastic force of the air. It consists of a glass vessel, A, provided at the bottom with a stop-cock, and a tubulure which projects into the interior. Having screwed this apparatus on the air-pump, it is exhausted, and the stop-cock being closed, it is placed in a vessel of water, R. By opening the stop-cock, the atmospheric pressure upon the water in the vessel makes it jet through the tubulure into the interior of the vessel, as shown in the drawing.

ON GASES.

Gases are bodies which, unlike solids, have no independent shape, and, unlike liquids, have no independent volume. Their molecules possess almost perfect mobility; they are conceived as darting about in all directions, and are continually tending to occupy a greater space. This property of gases is known by the names expansibility, tension, or elastic force, from which they are often called elastic fluids.

Gases and liquids have several properties in common, and some in which they seem to differ are in reality only different degrees of the same property. Thus, in both, the particles are capable of moving; in gases with almost perfect freedom; in liquids not quite so freely, owing to a greater degree of viscosity. Both are compressible, though in very different degrees.

If a liquid and a gas both exist under the pressure of one atmosphere, and then the pressure be doubled, the water is compressed by about the ¹/₂₀₀₀₀ part while the gas is compressed by one-half. In density there is a great difference; water, which is the type of liquids, is 770 times as heavy as air, the type of gaseous bodies, while under the pressure of one atmosphere. A spiral spring only shows elasticity when it is compressed; it loses its tension when it has returned to its primitive condition. A gas has no original volume; it is always elastic, or in other words, it is always striving to attain a greater volume; this tendency to indefinite expansion is the chief property by which gases are distinguished from liquids.

Matter assumes the solid, liquid, or gaseous form according to the relative strength of the cohesive and repulsive forces exerted between their molecules. In liquids these forces balance; in gases repulsion preponderates.

By the aid of pressure and of low temperatures, the force of cohesion may be so far increased in many gases that they are readily converted into liquids, and we know now that with sufficient pressure and cold they may all be liquified. On the other hand, heat, which increases the vis viva of the molecules, converts liquids, such as water, alcohol and ether or gas into the aËriform state in which they obey all the laws of gases. The aËriform state of liquids is known by the name of vapor, while gases are bodies which, under ordinary temperature and pressure, remain in the aËriform state.

In describing exclusively the properties of gases, we shall, for obvious reasons, refer to atmospheric air as their type.

Expansibility of Gases. This property of gases, their tendency to assume continually a greater volume, is exhibited by means of the following experiment:—A bladder, closed by a stop-cock and about half full of air, is placed under the receiver of the air pump, Fig. 337, and a vacuum is produced, on which the bladder immediately distends.

This arises from the fact that the molecules of air flying about in all directions press against the sides of the bladder. Under ordinary conditions, this internal pressure is counterbalanced by the air in the receiver, which exerts an equal and contrary pressure. But when this pressure is removed, by exhausting the receiver, the internal pressure becomes evident. When air is admitted into the receiver, the bladder resumes its original form.

The compressibility of gases is readily shown by the pneumatic syringe, Fig. 338. This consists of a stout glass tube closed at one end, and provided with a tight-fitting packed piston. When the rod of the piston is pressed down in the cube, the air becomes compressed into a smaller volume; but as soon as the force is removed the air regains its original volume, and the piston rises to its former position.

Weight of Gases. From their extreme fluidity and expansibility, gases seem to be uninfluenced by the force of gravity: they nevertheless possess weight like solids and liquids. To show this, a glass globe of 3 or 4 quarts’ capacity is taken, Fig. 339, the neck of which is provided with a stop-cock, which hermetically closes it, and by which it can be screwed on the plate of the air-pump.

The globe is then exhausted, and its weight determined by means of a delicate balance. Air is now allowed to enter, and the globe again weighed. The weight in the second case will be found to be greater than before, and if the capacity of the vessel is known the increase will obviously be the weight of that volume of air.

When the atoms or particles which constitute a body are so balanced by a system of attractions and repulsions that they resist any force which tends to change the figure of the body, they will possess a property, known by the name of elasticity. Elasticity, therefore, is the property which causes a body to resume its shape after it has been compressed or expanded.

Pressure exerted by Gases. Gases exert on their own molecules, and on the sides of vessels which contain them, pressures which may be regarded from two points of view. First, we may neglect the weight of the gas; secondly, we may take account of its weight. If we neglect the weight of any gaseous mass at rest, and only consider its expansive force, it will be seen that the pressures due to this force act with the same strength on all points, both of the mass itself and of the vessel in which it is contained.

It is a necessary consequence of the elasticity and fluidity of gases that the repulsive force between the molecules is the same at all points, and acts equally in all directions.

If we consider the weight of any gas, we shall see that it gives rise to pressures which obey the same laws as those produced by the weight of liquids. Let us imagine a cylinder, with its axis vertical, several miles high, closed at both ends and full of air. Let us consider any small portion of the air enclosed between two horizontal planes. This portion must sustain the weight of all the air above it, and transmit that weight to the air beneath it, and likewise to the curved surface of the cylinder which contains it, and at each point in a direction at right angles to the surface. Thus the pressure increases from the top of the column to the base; at any given layer it acts equally on equal surfaces, and at right angles to them, whether they are horizontal, vertical, or inclined.

The pressure acts on the sides of the vessel, and it is equal to the weight of a column of gas whose base is this surface, and whose height its distance from the summit of the column. The pressure is also independent of the shape and dimensions of the supposed cylinder, provided the height remain the same.

For a small quantity of gas the pressures due to its weight are quite insignificant, and may be neglected; but for large quantities, like the atmosphere, the pressures are considerable, and must be allowed for.

Diffusion of gases.—Liquids mixed together, gradually separate, and lie superimposed in the order of their densities, and the surfaces of the separation of the liquids are horizontal. But when gases are mixed, they present other conditions of equilibrium, as follows.

1.—A homogeneous and persistent mixture is formed rapidly, so that all parts of the same volume are composed of the same proportions of the mixed gases.

2.—In a mixture of gases, the pressure (or elastic force), exercised by each of the gases, is the same as it was when alone.

3.—The rapidity with which the diffusion takes place, varies with the specific gravity of the gases. The more widely two gases differ in density, the quicker the process of intermixture.

Evaporation.—This is the slow formation of vapor from the surface of a liquid. The elastic force of a vapor which saturates a space containing a gas (like air), is the same as in a vacuum. The principal causes which influence the amount and rapidity of evaporation are as follows.

1st.—Extent of a surface. As the evaporation takes place from the surface, an increase of surface evidently facilitates evaporation.

2d.—Temperature. Increasing the elastic force of vapor, has a most important influence on the rapidity of evaporation; therefore the temperature of ebullition marks the maximum point of evaporation.

3d.—The quantity of the same liquid already in the atmosphere exercises an important influence on evaporation. The atmosphere can absorb only a certain amount of vapor, and evaporation ceases entirely when the air is saturated, but it is greatest when free from vapor, that is perfectly dry.

4th.—Renewal of the air. If currents of air are continually removing the saturated atmosphere from above the surface of a liquid, evaporation takes place most rapidly, since new portions of air, capable of absorbing moisture, are presented to it. Evaporation is therefore more rapid in a breeze than in still air.

5th.—Pressure on the surface of the liquid influences evaporation, because of the resistance thus offered to the escape of the vapor. That is to say—water boils more freely in an open vessel than within a steam boiler under pressure. Hence, the necessity for having large steam disengaging surfaces to prevent priming or lifting of the water when the boiler is forced beyond its rated capacity.

HAND AIR PUMPS.

The use of compressed air has become very general through the use of small hand pumps; the cylinder of these must be smooth, and the plunger is usually packed with a cup leather packing.

Fig. 340 shows a gas fitter’s air proving pump. The gauge is attached to any opening into the system of pipes to be tested, with a rubber hose leading to the pump. By working the pump the air is forced into the pipes; upon stopping the pump if the hand upon the gauge remains stationary there are no leaks in the system. If there are leaks the hand of the gauge will gradually return to the zero mark.

Fig. 341 shows a Portable Tire Air Pump, which can be used by hand or affixed to a wall or bench; it is of the lever type, with 2 × 8 cylinders, fitted with check valve and extra heavy rubber tubing. As the leverage on the piston-rod increases the resistance on the piston also increases, thereby securing the powerful leverage of the well-known “toggle-joint” principle as the piston finishes its stroke; thus the best possible results are obtained.

Fig. 342 illustrates a Hand Lever Air Pump with cylinder 3¹/₄ × 6¹/₄; its capacity—one stroke—is 36 cubic inches. The greatest pressure it is intended to operate against is 150 lbs. to the square inch. In operation this design has the advantage of the leverage of the toggle-joint indicated above.

Fig. 343 exhibits a Hand Air Pump which has the same dimensions as that just described, screwed to the floor. Its particular advantage is the fact that the motion of the lever is natural and easy being horizontal and still retaining the advantages of the toggle-joint.

AIR AND VACUUM PUMPS.

An air pump is an apparatus for, 1, the exhaustion; 2, compression or transmission of air.

A vacuum pump is an apparatus consisting of, 1, a chamber or barrel; 2, a suction pipe with a valve to prevent return flow; 3, a discharge pipe which has a valve which is closed when the chamber is emptied and, 4, a steam induction pipe provided with a valve that is opened when the chamber is filled with water and closed when the chamber is filled with steam.

It is not right to call an air pump a vacuum pump, as the latter does not move air alone; it removes water, vapor and air from the condenser to form a vacuum. An air pump is designed to pump air alone.

A vacuum is a space entirely devoid of matter. That is, it is a space that contains nothing—no oxygen, no hydrogen, no air, no water, no pressure. It is for this reason that a perfect vacuum in practice is very difficult to obtain, especially as applied in a steam engine, as a liquid when in the presence of a vacuum generally gives off some vapor, owing to the fact that the surface is more or less in tension, besides its usual evaporative quality. Among all the liquids it has been found that mercury, on account of its very high specific gravity, can be best used to produce a vacuum and maintain it, and it is for this reason that the words “vacuum” and “inches of mercury” are synonymous.

The particular feature that makes steam valuable in producing a vacuum is the fact that when it is condensed, it decreases 1600 times in volume and except for this small quantity of water and some vapor which even cool water gives off in a vacuum, a perfect vacuum would be established and it is only necessary to draw off the condensed steam and vapor by proper apparatus to enable the vacuum to be maintained which the condensation has created. The apparatus for doing this is called the air pump and the reservoir in which this condensation takes place is called the condenser.

The condensation of steam in the condenser is effected in two ways. The exhaust steam either meets in direct contact the water which is to condense it, or, the steam impinges upon cool metallic surfaces the temperature of which is kept down by circulating cool water through them. In the first case the condensed steam and the condensing water meet and mingle. The condenser is an iron pot or shell into which the steam is exhausted and the cooling water enters it in the form of a sheet or spray. Such condensers are called jet condensers for this reason, and the cooling water is called the injection. All water that is used for condensing steam is therefore called the injection water.

When the exhaust steam strikes cool surfaces and is condensed by those surfaces, such condenser is called a surface condenser. The cooling surface is usually a series of pipes or tubes made of brass or copper to secure a rapid transfer of heat. These tubes are usually tinned inside and outside to prevent corrosion and in marine practice are made ⁵/₈ in diameter. In most cases, condensation is effected by bringing the exhaust steam in contact with the outside of the tubes, the circulating water being inside.

In the surface condenser, as the circulation does not mingle with the condensed steam, the air pump has nothing to do with this water but is only required to pump out the condensed steam and air which enter the condenser; the pump which takes care of the circulation is called the circulating pump. When large quantities of water are used and the difference in level through which the water must be raised is slight as on board ship, centrifugal pumps are generally used.

In the jet type of condenser where the water acts directly on the steam, the injection water will cause a lower temperature with less water and less apparatus than a surface condenser. The amount of injection water varies from 20 to 30 times the weight of steam to be condensed in cool seasons and from 30 to 35 times the amount in summer season. With fresh water this can be pumped into the boiler when the oil is extracted from it. It is for this reason that surface condensers are universally used for sea-going vessels to avoid salt water. They are also much used on land in places where the feed water contains mineral salts and is injurious to the boiler.

In places where the cost of hydrant water is excessive, it is of importance to use the same injection water over and over again, but this cannot be done until the water is first cooled. There are numerous methods by means of which this is done. All of these methods utilize the principle of scattering the injection water in the way best calculated to bring the greatest surface in contact with the largest quantity of air so that evaporation may take place quickly and effectively.

This is sometimes done by pumping the water through a number of spray nozzles up into the air, allowing it to fall into a lake or cold well below, or, as is more usually the case, the injection water is allowed to descend in a tower in a fine state of division over tiles or wire gauze or corrugated surfaces. A current of air, either forced by a fan or drawn up through it, causes a vaporization of the film of warm water pouring over the different surfaces, and the air cooling and the evaporation combined withdraw the heat from the water so that when it reaches the bottom it is in condition to be used again.

Cooling towers are used with either jet or surface condensers and can be used either with or without a fan, depending upon the design. In general these towers usually lower the temperature of the water from 120 degrees to 80 degrees, which is sufficient to maintain a vacuum of about 26 inches. As they depend chiefly upon the results of evaporation to do the cooling, they work better on a dry day than when the air is humid.

The figures on the opposite page are designed to illustrate the use of an air pump in connection with a jet condenser; this combination is properly called a vacuum pump because it not only pumps air, but water and vapor as well. The steam end of this apparatus is described in Part One, page 324, of this work.

The air and vacuum end has a cylinder lined with composition-brass bored smooth; the piston has square rubber and canvas packing. The discharge—as shown in cut—is located sufficiently high, so that the cylinder retains a large portion of water. This forms a seal and causes the pump to work more advantageously than it would with air alone. A small pipe leads from the discharge chamber to the piston-rod stuffing-box. This contains a double packing and the water which flows through this small pipe forms a continuous seal around the piston-rod and thus prevents air from entering.

The injection water enters the elbow at the top and is drawn through an annular opening into the condenser. This opening may be regulated by the small hand wheel shown at the top end of the stem.

The exhaust from the steam end flows into the condenser through the pipe as may readily be observed—or escapes into the atmosphere by throwing the switch valve.

A ball-float attached to an air valve is located at the right hand of the condenser so that in case the pump should fail to operate from any cause, the injection water will lift the ball-float, which in turn will open the air valve and by discharging the vacuum will prevent the flooding of the engine cylinder with water.

It is a well-known fact that the atmosphere exerts what is usually termed “back pressure” of 14.7 pounds per square inch upon the piston area of a steam engine, also that water converted into steam, may be converted into its original state by condensation. Now, if this back pressure, which is, in reality, the weight of the surrounding atmosphere, be removed from the piston of a steam engine, the steam on the opposite side of the piston would have that much (14.7 lbs.) less work to do.

Applying this to steam engines means conveying the exhaust, or expanded steam, which would otherwise be allowed to escape into the open air, into a closed chamber, where it is met by a spray of cold water, which so rapidly absorbs the heat contained in the steam that it ceases to retain its gaseous form, and is again reduced to its original bulk as water. A great change has now taken place, and the steam is reduced to its liquid form. As this water of condensation only occupies about ¹/₁₆₀₀ of the space filled by the steam from which it was formed, the remainder of the space is vacant, and no pressure exists.

The difference in volume accounts for the atmospheric pressure on the outside of the chamber, and as the vacuum extends throughout the whole distance which the exhaust steam originally occupied, it, of course, is made available in the cylinder of the engine in the shape of a decreased pressure on the exhaust side of the piston; the atmospheric pressure remains constant, therefore we have the atmospheric pressure acting on one side of the piston, and absent on the other; the gain being 14.7 pounds per square inch, if a perfect vacuum could be secured. It amounts in average engineering practice to from 12 to 13 pounds, or 24 to 26 inches of mercury, as the graduations usually read on vacuum gauges.

Jet and Surface Condensers are further described and illustrated in a special allotted section of this work. The vacuum pump is usually of the reciprocating order, although other methods have been employed for emptying condensers, but not with equally satisfactory results.

The gain to be secured by using a condensing apparatus may be measured in two ways: first, by the decrease in fuel consumption over that necessary when running non-condensing, which will represent a constant decrease in running expenses; or, second, by the increase of power working quite up to its economical limit, in a non-condensing engine.

By the use of a condenser a further increase of power is realized in raising the mean effective pressure of steam within the engine cylinder without increasing the demand upon the boiler.

The application of a condenser to a steam engine increases its economy from 20% to 25% depending upon circumstances, while by compounding and condensing an economy of 35% to 40% is effected.

The vacuum pump shown in the engraving, Fig. 346, represents a single cylinder double acting vertical design having but one set of valves and those used exclusively for the discharge.

The suction port is in the middle of the cylinder, A, shown in the sectional view, Fig. 347. The piston, E, when it passes this port imprisons the water beyond it and pushes this water out of the discharge valves, D D, if the piston is rising, and out of the valves, C C, if the piston is descending. The main discharge pipe is attached to a flange at B.

This pump is made to work easily and steadily by adjusting the cushioning valves, F. F.

The discharge valves are reached through the holes provided for that purpose and covered by plates shown in the engraving, Fig. 347.

The main slide valve moves horizontally for the reason that if it moved up and down the force of gravity would seriously interfere with its regular action.

This slide is moved by a valve piston in the usual way. The parts of the valve may be inspected and adjusted by removing the cover held by the two studs shown.

The outline engraving, Fig. 348, shows a cross-compound double acting vacuum pump, six-inch high pressure, nine-inch low pressure cylinders, by eight-inch stroke, and two air cylinders, ten-inch diameter by eight-inch stroke.

They are piped up to run either high or low pressure, also to run independently by manipulating the cocks, C, and D, as directed in the engraving showing arrangement of valves, Fig. 349, page 41.

These pipes are simple in design and run direct to the boiler for live steam and convey the exhaust to the atmosphere or condenser as desired. On a recent test at a fair rate of speed the capacity of this pump was shown to be equivalent to taking care of a triple expansion engine of 2,000 I. H. P. On a further test this same pump on a basis of 20 lbs. weight of steam per I. H. P. per hour demonstrated its ability to take care of 3,000 I. H. P. triple expansion engine.

The advantages claimed for this pump are briefly as follows:

Unusual light weight and compactness.

There being NO SUCTION VALVES, working-beams, rock shaft and bearings, beam-links, etc., this pump is simple.

It is economical in the use of steam, by reason of compounding the steam cylinders; also clearance loss is reduced to a minimum by the perfect regulation that is secured by the valve gear described. Full stroke at any and all speeds can be readily maintained.

As the air pistons travel within a distance of less than ¹/₈ inch of the air cylinder heads, a high efficiency results. Although double-acting, the flow of water and vapors is always in one continuous direction—the same as in a single-acting air pump. Either side of pump can run independent of the other, which means a spare pump to be used in case of accident to the other side of this pump.

Referring to the accompanying table of tests, page 41, it may be claimed that with an average of 36 double strokes per minute this pump handled at the rate of upwards of 26,000 pounds of feed-water per hour, which on a basis of 20 lbs. engine economy shows this vacuum pump capable of taking care of a 1,300 I. H. P. engine at this very moderate speed. By comparing the power required to drive this pump (which aggregated 1.18 I. H. P.) to the I. H. P. of an engine of the power here represented it is apparent that this pump did its work on less than one-eleventh of one per cent. of the I. H. P. of said engine, which is a very excellent showing.

Table No. 4 also shows a very excellent vacuum maintained under extreme duty.

Table.

The Deane Vacuum Pump. There are a number of novel features exhibited in the construction of this pump; the cylinder has four ports, that is to say two steam or admission ports and two compression or cushioning ports.

Referring to the engraving, page 42, Fig. 6, shows the main valve to be a plain D slide,—directly under it the same valve is shown in section. The projection on the back of this valve fits into the valve piston. The secondary valve, 5, surrounds the main valve and contains two plain slide valves, one on each side. Referring to 3, it will be noted that one of these valves admits steam while the other allows the steam to escape after having done its work of moving the valve piston.

A longitudinal section of this secondary valve and steam cylinder are shown, in 2.

In the engraving, 1, it is shown that the cylinder for each end of the valve piston is jacketed with live steam so that the cylinder itself heats up as quickly as the valve piston, hence the piston cannot stick in the cylinder due to unequal expansion of valve and seat.

The supplemental valve ports are shown in section 4.

To set the valve of this pump: Remove the steam chest, place the piston at mid stroke with the lever, plumb, then set the stem at its mid position with the secondary valve in place. See that the tappets measure equal distances either side of the tappet block.

The water end of this pump consists of a cylinder with valve chambers as shown. The piston rod has two stuffing-boxes, which makes a water seal around the rod so that no air can enter the cylinder, as the chamber between the two stuffing-boxes is kept constantly filled with water. It will be noticed that the suction pipe enters the pump in such a position in relation to the valves that both suction and discharge valves are perpetually immersed in water.

When this pump is pumping air only, there is sufficient water left within the valve chambers to provide a water seal under all working conditions. The valves in this pump are easily reached for inspection or repairs, a hand-hole being provided for each valve, with proper covers, which are easily and quickly replaced.

The Worthington Vertical Beam Vacuum Pump with condenser attached is shown in Fig. 351.

This is a pump of great simplicity and strength. The figure shows a compound engine for using high pressure steam; these machines can be built with simple steam cylinders of equal diameters, but they are not recommended except in special cases; for example where the steam pressure is very low. Each side of the pump end is single-acting, the buckets being of the form used for years in detached air pumps in marine service. The two sides are connected together by a beam and links attached to the cross-heads. As one side comes down and does little work, the other side makes an up-stroke and does full duty in emptying the condenser to which the suction is attached.

The condensing chamber is usually placed at the rear and connects directly with the channel plate at the bottom of the pump. The opening shown in front is for the discharge water.

The steam cylinders are so arranged that either piston may be examined by removing its cylinder head, without disturbing the other cylinders. The valves are of the Corliss, or semi-rotative, type and the high-pressure cylinders are provided with cut-off valves to assure the desired ratio of expansion.

The interior of each air cylinder may be inspected by removing the plates shown in front, near the middle. There are also two plates at the top for inspection of the discharge valves. The four machinery steel columns form a light but very strong frame allowing free access to the working parts.

The next four cuts show Dean Brothers’ twin cylinder air pumps with their special steam valve gear. They are made for and supplied with either surface or jet condensers. See Fig. 352.

The arrangement of the valve gear is such that steam will be applied at the upper end of one piston at the same instant that it begins to act on the lower end of the other. By this device steam is so controlled in the steam chests that no pressure comes on the main pistons, until the moment that both are ready to move, after having reached the full limit of their stroke, thereby securing an exactly uniform, but opposite, motion of the pistons. Fig. 354 is a sectional elevation of the steam cylinder and steam chest; Fig. 353, a front elevation; Fig. 355, a section of the air cylinder, and Fig. 352, an exterior perspective view of the pump.

Each steam cylinder has its own steam piston, piston rod, valve movement, steam chest, etc. A sleeve, a, is rigidly attached to each piston rod, and connected to this sleeve is a lever, b, the outer end of which connects with a link, c, which in turn is connected to a sleeve, d, loosely mounted upon the valve rod between collars, e. The valve rod, f, operates the auxiliary slide valve and admits the steam from above and below the auxiliary piston. This piston has attached thereto the main slide valve, which admits and exhausts steam alternately from above and below the main steam pistons. Any movement of the main piston communicates movement in the opposite direction to the sleeve, d, which moves the valve rod only when it strikes one or the other of the collars. As there is considerable lost motion between the sleeve and the collars, the main steam piston will be nearing the end of its stroke when the valve rod begins to move.

Extending through the ends of the steam chests are short piston rods, g, which are connected to a centrally pivoted vibrating lever, h, mounted on a pivot. When the main steam piston has moved from the top to the bottom of the steam cylinder, the corresponding valve rod has moved in the opposite direction and the auxiliary slide valve has moved upward, opening the port, i, to steam and the port, k, to the exhaust port. At the moment the main steam piston has completed its downward stroke the auxiliary piston is forced upward and carries with it the main slide valve, l. This opens the main steam port and exhaust port, which reverses the movement of the main piston. When the main piston reaches the upward limit of its stroke the auxiliary valve has moved downward, opening the port, k, to steam and the port, i, to the exhaust, causing the auxiliary piston to move downward, thus reversing the movement of the main valve and piston.

By this arrangement the valve operating piston, m, is held at all times immovably at one end of the stroke, except when the main piston is nearing the end of its stroke and is ready to reverse. Supposing the left-hand main piston has not quite reached the upper limit of the stroke, the steam would still be on the lower side of its auxiliary or main valve operating piston and the exhaust open to the other side. We now have steam on the bottom side of both auxiliary pistons, and as they are of equal diameters and are connected by the lever, h, they are balanced and cannot move the main steam valves. The right-hand main steam piston must wait until the left-hand piston has completed its stroke before it can reverse, and consequently the movement of the main pistons will always be in opposite directions, and neither can reverse until both have completed their stroke.

There are three ways that this apparatus may be operated: First, the pumps may be operated in conjunction with each other, as is hereinbefore described. Second, the lever, h, may be detached from the auxiliary or main valve operating pistons, and the two pumps may then run independently of each other or in the ordinary and well-known manner, each performing its own independent work. Third, by further detaching the link, c, on one of the valve gears the auxiliary slide valve, n, will remain at rest and the corresponding pump will not move while the other pump continues to operate. These are important features, because, as in case of accident, it may be necessary to use one pump while the other is disabled, and in some cases it may be desirable to operate the pumps independently. The engineer will appreciate this feature, as the stoppage of an air pump is a serious matter.

The piston rods are separable at the crossheads. The crossheads are of steel. The steam cylinders and pump cylinders are connected by six heavy steel stretcher rods. Adjusting valves are fitted to steam cylinders for controlling motion of pistons. The valve gear is provided with a special lever adjustment by which the length of stroke of pistons may at any time be changed, even while the pump is running.

In Fig. 356 is shown a form of independent vacuum pump, with its condenser, built by the Conover Mfg. Co.

This apparatus consists of a jet condenser with air pump, boiler feed pump, and engine to drive both, combined as one machine. The air pump is a single acting bucket plunger pump, driven by a crank shaft, turned by the engine, which is a single cylinder compound automatic cut-off engine, and also drives the boiler feed pump; it is of the trunk pattern, and the small space around the trunk on the top side of the piston forms the high pressure cylinder. Steam is admitted to the high pressure side, at boiler pressure, and is cut off and expanded and exhausted into the receiver, whence it is admitted under the bottom side of the piston, where it is again cut off and expanded, finally exhausting into the condenser.

The piston makes the down stroke when the air pump makes the up stroke; and it will be seen by referring to the cut that the engine does nearly all its work when making the downward stroke. When steam is acting on the top side of piston at high pressure, the vacuum at the same time is pulling on the full area of the piston underneath.

When the engine makes the up stroke, the steam at low pressure from the receiver acts to push the piston up; and as the air pump is doing no work then, being on its down stroke, the only work of the engine is to keep the machine up to speed.

It will thus be seen that the engine is suited to meet the demand of the large power on one stroke, and very little on the other, thus adapting itself admirably to its requirements.

The valves are of the Corliss type, and do not trip; the cut-off being set by hand, does not require to be changed or altered, as the speed is controlled by a throttling governor.

Fig. 357 shows a cross section through the steam cylinder of this vacuum pump.

The Edwards air and vacuum pump is shown in Figs. 358, 359 and 360, in which it may be perceived that both foot and bucket valves are dispensed with; the only valves used are those which in other pumps are known as head or discharge valves.

The following brief description of its leading features will be understood by reference to the illustrations: Fig. 358 is a sectional view through the center of the air pump, but the piston and rod are shown as a full view.

The action of this pump is as follows: the condensed steam flows continuously by gravity from the condenser into the base of the pump, and is there dealt with mechanically by the conical bucket working in connection with a base of similar shape. Upon the descent of the bucket the water is projected silently and without shock at a high velocity through the ports into the working barrel (see Fig. 359). The rising water is followed by the rising bucket, which closes the ports, and, sweeping the air and water before it, discharges them through the valve at the top of the barrel.

It may be said that however slowly an ordinary air pump with foot and bucket valves may be running, the pressure in the condenser has to be sufficiently above that in the pump to lift the foot valves, overcome the inertia of the water, and drive the water up through the valves into the barrel where the water is dealt with mechanically. The higher the speed of the older type of pump the greater is the pressure required to overcome these resistances owing to the very short space of time available, and as any increase of pressure in the condenser is accompanied by a corresponding increase of back pressure in the low pressure cylinder, hence the absence of the valves referred to allows a higher speed of the plunger. The elimination of the foot valves it is claimed gives from ¹/₂ to 1 inch better vacuum.

Another advantage claimed for this pump is that clear air inlets are maintained—see Figs. 359 and 360. Under ordinary working conditions, when the bucket descends and the ports open, there is no obstruction between the condenser and the pump; the air has a free entrance while immediately afterward the water is injected into the barrel at a high velocity. Thus, instead of obstructing the entrance of the air, the water tends to compress that already in the barrel, and to entrain or carry in more air with it.

The bucket or piston is a hollow casting with water grooves instead of packing rings.

The valve seat is constructed with a rib between each valve and a lip around the outer edge, so that each valve stands in its own water and is separated from the others. This forms a ready means of testing the relative tightness of each valve.

The cast iron working barrel is lined with brass.

The pump rod is Tobin bronze, and valve plate and valves of composition. These pumps are either single, twin, triplex, and are steam, electric or belt driven, for stationary, marine or sugar plantation service.

The steam driven pumps are built with either single or compound steam cylinders, fitted with new and improved valve gear, and with their arrangement of fly-wheels, insures smooth running, making full strokes free from vibration.

AIR COMPRESSORS.

Compressed air is air compressed by mechanical force into a state of more or less increased density. The power obtained from the expansion of greatly compressed air in a cylinder, on being set free is used in many applications as a substitute for steam or other force as in operating drills, shop tools and engines which are driven by the elastic force of compressed air.

A compressor is a machine usually driven by steam by which air is compressed in a receiver so that its expansion may be utilized as a source of power at distances where an ordinary engine could not be conveniently used.

The compressor proper comprises two sets of valves, usually designed to be opened automatically by excess of pressure under them and to be closed by gravity or by the action of springs when the pressures become equal. The inlet valves open just after the piston commences its stroke, when the expansion of the compressed air remaining in the cylinder behind the piston has lowered the pressure above the valves. They close at the end of the intake stroke, just as the piston comes to rest. The outlet valve lifts during the compression stroke, at about the time the rising pressure in the cylinder becomes equal to that in the outlet passage above the valves; and they close when the flow of air ceases as the piston completes its stroke.

Any of the accurately fitted steam engine valve gears may be used for compressors, observing only that the compressor is in every way a reversed steam engine.

Compressed air is already used in the operation of
1. Cranes, hoists and motors of all types and of all capacities.
2. Portable drilling, reaming and tapping machines.
3. Riveters and stay-bolt cutters, calking and chipping tools.
4. Shop tools of all kinds.
5. Air brakes.
6. Sand blasts.
7. Rock drills and coal mining machines.
8. Pneumatic locomotives and street cars.

and also for the following diversified uses,
1. Pumping water, sewage, oil and acids.
2. Raising sunken vessels.
3. Refrigerating and ice making.
4. Transmitting messages through pneumatic tubes.
5. Cleaning carpets and railroad cars and seats.
6. Sinking caissons and driving tunnels through silt and soft earth.
7. Tapping iron furnaces.
8. Transmitting power for all purposes.

The office of the air compressor is to store up air under high pressures, which can be utilized at a greater or less distance, without sustaining any loss by condensation in the pipes, as is the case of carrying steam in pipes long distances.

Air stored under pressure in a reservoir can be used expansively, in an ordinary steam engine returning an equivalent amount of work that was required to compress it—less the friction.

The admission of the air being through a single tube, it creates a constant flow of air in one direction only, thus filling the cylinder at each stroke with air at atmospheric pressure. This movement gives a momentum to the air which causes it to fill the cylinder to its fullest extent at each stroke.

Air compressors may be driven in various ways, but the most commonly used are those which are directly connected to a steam engine, thus doing away with intermediate machinery. When the air piston draws in a charge of air, the air fills the cylinder at atmospheric pressure, or a little below, and on the return stroke of the piston it has to be compressed to the same pressure as in the receiver before it can lift the delivery valve, and as the valve is held to its seat by a spring, and also by its own weight, the pressure has to be considerably above that of the receiver before the valve will lift. To overcome this the valves are operated by mechanical means, which lifts them at a point of the stroke, when the pressure in the cylinder corresponds with that of the receiver.

This arrangement avoids pounding of the valves as well as the noise caused by the air when rushing at much higher pressure from the cylinder into the receiver.

For the sake of economy, air compressors are compounded, as for example, by drawing the air into a large cylinder and compressing it to a certain stage, whence it passes into a smaller cylinder, which compresses it to a much higher pressure.

In a simple compressor, for very high pressures, there is at the end of the stroke a large volume of air left in the clearance space, which expands on the return stroke, to atmospheric pressure, before another charge of air can be drawn in.

But in the compound compressor, the air is delivered from the low pressure receiver to the high pressure cylinder far above atmospheric pressure, thus the remaining air need not expand so much and allows the cylinder to take a larger volume of air. The load is also distributed more evenly.

The following are valuable “points” relating to the care and management of air compressors.

As in a steam pipe line, elbows should be avoided in an air pipe line but unlike a steam pipe it should be larger.

A mistake is sometimes made in purchasing a compressor built for a low altitude and trying to run it in a higher elevation; the machine then experiences the same trouble that some people do, in not being able to get breath enough under the changed conditions.

The use of cheap oils, especially in an air cylinder is a most serious mistake, as the least tendency to gum will prevent the valves from properly seating, and even with the best of oils, it is well to use a small amount of mineral oil at times.

In localities where the water is bad, the water jacket will require extra attention, as it gets as badly scaled like steam boilers, principally due to a very slow or retarded circulation, which allows the sediment to settle, and should the water supply be shut off, even for a few minutes, the cylinder heat will bake it so hard as to give considerable trouble. It is a good plan to put a good boiler compound in the water jacket, and run the machine for some time without any circulation.

In this case good judgment must be used not to run too long or too fast, as the cylinder will heat very quickly and is liable to be damaged.

There are many emergency ways of stopping small pipe leaks; any good sticky substance, such as tar, wax, tallow candles, or even chewing gum, melted and applied on narrow strips of cloth and wound as a bandage, will be found handy.

It should be remembered that leaks in an air pipe line are as bad as in a steam pipe line, and should receive as much care.

The theoretical operation of air compressors may be thus explained:

If a tight cylindrical vessel, containing one cubic foot of air at atmospheric pressure, be fitted with a piston which is free to move up and down but yet perfectly tight, the air in the vessel will have no means of escape, and the pressure within and without the vessel, both being atmospheric, are balanced.

Now, if the piston should be loaded with a weight, the pressure on the outside would be that due to the atmosphere, plus the weight, while the pressure from the inside is simply equal to atmospheric pressure; thus the piston is forced to descend, but as the air inside of the cylinder has no means of escape, the volume it fills being diminished, its pressure rises until the pressure under the piston balances that above it.

If, for example, the area of the piston should be 100 square inches, and the weight with which it is loaded be 100 pounds, assuming the piston to be without weight, the pressure below will have to react with an equal force to hold the piston stationary, which in this case would be 1 pound to the square inch above atmospheric pressure, and the piston would have to descend sufficiently to cause this increase of pressure, which descent would be equal to ¹/₁₆ of the total fall of the piston. By adding another 100 pounds above, the pressure would rise to 2 pounds to the square inch. The cylinder is thus charged with compressed air.

If now the bottom of the cylinder should be connected by means of a pipe to another vessel of larger capacity called a receiver, the pipe having been closed by a valve in it during compression, and the valve should be opened, the piston would at once commence a further descent, the compressed air escaping into the receiver, until the pressure in the receiver and cylinder is equalized, or the piston reaches the bottom of the cylinder, which it will do, if the receiver is large enough. Then the valve closes, stopping communication between cylinder and receiver, and the piston is drawn upward; at the same time air is again admitted to the cylinder by another valve, which closes when the piston reaches the top, and the same operation is again repeated.

The receiver can thus be charged with compressed air and by loading the piston very heavy the pressure can be raised quite high.

Now, if the piston, instead of being loaded by weights, be connected to the piston rod of a steam engine, or by means of a connecting rod to a crank (which is rotated by a belt or some other driving mechanism, and the valves be operated automatically, as the valves on a water pump), the simple apparatus is converted into a perfect air compressor, which really is nothing else than an air pump, and the air can be pumped into the receiver against a high pressure the same as water is forced into an elevated reservoir by a pump.

As air is a compressible gas, it acts a little different in the air cylinder from the almost incompressible water in a pump.

To lift the valves of an air compressor by the compressed air pressure in the cylinder (added to the pressure of their springs besides the receiver pressure), the air would have to be compressed considerably above the receiver pressure before it would lift the valve which allows it to flow from the cylinder into the receiver, and then the valve would not open freely as a pump valve, but would chatter, causing a disagreeable noise, and damaging the valve.

To avoid this, the valves of an air compressor are operated by mechanical means. Some devices operate the valve directly as soon as the pressure in the cylinder reaches that of the receiver, while others simply release it of the spring pressure, the valve itself being lifted by the air itself. Such devices generally give the valves a full free opening, without noise.

The blowing engine is almost identical with the air compressor. The chief difference between them being the ratio of steam cylinder to air cylinder. While the air compressor furnishes a comparatively small amount of air at very high pressures, the blowing engine delivers a very large volume at lower pressures.

Blowing engines are mainly used in large blast furnaces, smelting works and foundries, to furnish the air pressure for cupolas, air furnaces and smelting ovens.

In Fig. 366 is shown a blowing engine of very large size; the steam cylinder is 42 inches in diameter, the air cylinder 84 inches, and the stroke 60 inches.

The valve gear is of the Reynolds-Corliss type. The piston rod is attached to a cross-head extending through the guides, which are formed by the frame, with wrist pins upon each end, from which the two connecting rods are suspended with their lower ends connected to the cranks, as shown in Fig. 366. There are two air piston rods attached to the main piston and held to the cross-head by nuts at points near the guides.

The crank shaft carrying the flywheels, which also form the cranks attached to the ends of this shaft, is located below the steam cylinder. This construction is of the return connecting rod engine design, to economize space.

Both the air and steam valve gears are worked from eccentrics on an auxiliary shaft, driven from the main shaft by bevel gears underneath the steam cylinder.

The “Imperial” air compressor is presented herewith in Figs. 367 and 368.

The “Imperial” compressor is especially designed for use in machine shops, foundries and other industrial establishments where it is not convenient to use a steam driven compressor.

The machine has two vertical, single-acting cylinders, each employing long trunk pistons that act as guides for the lower ends of the connecting rods. By this design, the height of the machine is reduced, stuffing-boxes and crossheads are eliminated, and a minimum number of bearings required. The cranks are set opposite to each other, so that when the piston on one side is ascending, the other side is descending.

The machine is made with duplex cylinders for the low pressures used in sand blast work and the like, and with either duplex or compound cylinders for higher pressures. In the compound type, an intercooler is supplied, through which the air passes from the low pressure to the high pressure cylinder.

The air cylinders are water-jacketed and provided with hooded heads, so that air may be supplied to them from outside the compressor-room; the cylinders are cast in one piece with the frame.

The air-valves, both inlet and outlet, are of the poppet type, fitted with light springs, and work vertically. On account of their position at the bottom of the cylinder, they are well lubricated, and, acting vertically, they have little tendency to wear out of line with their seats.

The air intake passage is tapped to receive a supply pipe leading from out-of-doors, or from some place where cool and clean air is obtainable. The compressed air is discharged into a passage which is tapped for a pipe to convey it to the air-receiver.

All parts of the compressor are easily accessible for inspection, adjustment, or repair. The air-heads may be removed without disturbing any of the pipe connections. The valves may be taken out by unscrewing the bonnets.

Table
of parts of the Imperial Compressor.

The Norwalk standard compressor is shown in Figs. 361 and 362, the latter being a longitudinal section; Fig. 361 is a perspective view; the two compressors are driven by a single steam cylinder having an adjustable cut-off. The air valves are operated by a positive crank motion.

A view of a Pelton water wheel operating a compressor it shown in Fig. 363. The cut represents a compound air compressor in which the valves are operated mechanically. The water which drives the wheel enters through the pipe and nozzle secured in the wheel pit, as represented.

Fig. 364 exhibits a belted duplex air compressor built by Allis-Chalmers & Co.

Fig. 365 shows a vertical duplex compressor driven by a belt.

All the latter, as may be seen by the engravings, have the positive valve motion operated by an eccentric. In selecting an air compressor the following points need consideration: 1, Number of cubic feet of free air required per minute; 2, Altitude, i.e., the number of feet above the sea level; 3, Steam pressure and air pressure.

The use of compressed air for operating mining pumps, while having advantages in some cases, is not to be recommended in all, particularly on account of the low efficiency of the plant as a whole. The loss due to leaks is serious, and the long line of piping with its numerous joints causes much trouble, delay and expense.

In Fig. 369 is shown a direct acting steam single air compressor; simplicity in its construction is a leading feature and there are few parts in this pump that are liable to wear.

This apparatus is designed for working pressures up to twenty pounds; it is intended for use in oil refineries, smelting works, blast furnaces and in all situations where compressed air of medium pressure is required. They are variously used for sand blasts, ventilating purposes, and for pneumatic deliveries.

The steam end and valve motion are the regular Deane pattern, assuring positive operation. The air cylinder is provided with a water jacket.

A power wall or post air compressor is shown in Figs. 370-372. The machine is single acting and is recommended where little space is available, as it can be bolted to the wall or to a post, or on the under side of the ceiling. The crank shaft and connecting rod are of cast steel. The bearings are babbitted and adjustable. The piston is of the trunk form, carrying a pin for the connecting rod, and is of extra length to act as a guide for the lower end of the connecting rod. The valves are of the poppet type. These compressors are extensively used in electric power stations for supplying air for removing dust from electric machinery, in bicycle shops for inflating pneumatic tires, maintaining a supply of air in pressure tanks, and for various purposes where a limited supply of air is needed.

These compressors are of the “Blake” design and the following particulars will be of interest.

With increase in altitude the barometric or atmospheric pressure falls from 14.7 pounds per square inch at sea level to about 10 pounds at 10,000 feet above sea level. Since the density of the air decreases with its pressure it is obvious that at such an altitude the total weight of air handled by a given displacement is considerably less than at sea level; and that to fill any volume—a rock drill cylinder, for instance—with air compressed to 90 pounds, a greater free-air displacement will be necessary than would be required at sea level. The relative capacities of a given displacement to do work—as through rock drills or pumps—at varying altitudes are figured in the following table:

Capacities at Varying Heights above Sea Level.

The fact that the heating effect of compressing air from an initial pressure of 10 pounds absolute to 90 pounds gauge pressure is theoretically equivalent to that of compression to 132 pounds at sea level, makes a two-stage arrangement more imperative in high level work than under ordinary conditions.

The two-stage or multi-stage system of air compression is used generally for high pressure work. The system is most usefully employed between 40 and 120 pounds gauge pressure. For the moderate working pressure of 90 to 100 lbs., the two-stage compression has demonstrated its efficiency chiefly for the reason, that the heat generated in the last half of the stroke of a single compressor is by the two-stage process greatly reduced.

Further compounding, for pressures above 100 pounds, becomes quite necessary to secure the advantages named hereafter; the two-stage has proved advantageous up to 500 lbs., three-stage up to 1,000 lbs., and four-stage compression up to 3,000 lbs.

As the pressures increase, however, the machines become more and more complicated, owing not only to the greater power required, but also to the heating of the air during compression. The use of water-jackets for cooling the air in the compression cylinders is general, but this does not effect thorough cooling, as only a small portion of the air in the cylinder comes in contact with the jacketed parts. This difficulty has led to the use of compound machines, in which case inter-coolers are generally used between the different stages of compression, which cause the air to shrink in volume between the stages.

Briefly summed up, the chief advantages of multi-stage over single-stage compression are:

1. Lower average temperature, resulting in lower average pressure, and permitting the compression of the same volume of air with less expenditure of energy.

2. Increased safety and ease of lubrication. When high final temperatures prevail, part of the lubricating oil vaporizes, and wear on the piston and cylinder becomes rapid. Under exceptional circumstances the combination of air and oil vapor may reach the proportions of an explosive mixture, and if the compression temperature passes its flash point damage may result. Such accidents are, however, very rare even in single-stage work; in multi-stage compression, with proper intercooling, they are impossible.

3. Greater effective capacity in free air. The final pressure in the low pressure cylinder is much lower than in a single-stage machine, and the air confined in the clearance spaces when expanded down to atmospheric pressure occupies comparatively little space. Consequently the inflow of air through the suction valves begins at an earlier point in the stroke.

4. The air delivered by a two-stage or multi-stage compressor is dryer than that furnished by a single cylinder. Under constant pressure the power of air to hold watery vapor decreases with its temperature, and during its passage through the inter-cooler much of the original moisture in the air is precipitated. Consequently less trouble is experienced from condensation in the discharge pipe.

A properly designed inter-cooler should reduce the air in the cylinders to the temperature of the outside air. The economy of compressing in several stages—or, in other words, compound compressors—is shown from the fact that in compressing air up to 100 lbs. the heat loss reaches about 30 per cent. By compressing in two stages, this loss is cut down to less than half; and in four stages, it is reduced to four or five per cent. It is evident, therefore, that the higher the pressure required the more essential is the use of compound machines.

The inter-cooler is the vital feature of the two-stage or multi-stage machine. In this construction the air is partially compressed in one cylinder; it is then passed through an inter-cooler where it is cooled and finally is compressed to the desired degree in the second or other additional cylinders.

An inter-cooler is shown in Fig. 373. The cooling surface consists of a nest of small brass water tubes. These tubes break up the stream of air entering the cooler, while their thin walls insure rapid conduction. The receiver volume formed by the connecting pipes and inter-cooler body results in a nearly uniform discharge pressure in the low-pressure cylinder. The air being outside of the tubes encounters practically no frictional resistance, and its slow passage allows time for cooling. A pocket, with gauge glass attached, is so placed as to catch any precipitated moisture which might otherwise enter the high-pressure cylinder.

An after-cooler is shown in Fig. 374. This serves to reduce the temperature of the air after the final compression.

The heat of compression, as may be judged from the foregoing, relating to inter and after-coolers is a feature of interest. The temperature to which it finally attains depends, 1, upon the initial temperature; 2, upon the degree of compression, or in other words, the amount of work expended upon the compression.

The extent of this heating is shown in the following table, for dry air when compression is performed with no cooling.

The Norwalk compound compressor is shown in outline by the cut 375. The large air cylinder on the left determines the capacity of the compressor; for illustration assume its piston at 100 square inches area; the small air cylinder can have an area of thirty-three and one-third square inches.

The small piston only encounters the heaviest pressure; at 100 pounds pressure the resistance to its advance is 3,333 pounds. The resistance against the large piston is its area multiplied by the pressure which is caused by forcing the air from the large cylinder into the smaller cylinder. In this case it is thirty pounds per square inch. But as this thirty pounds pressure acts on the back of the small piston, and hence assists the machine, the net resistance to forcing the air from the large into the small cylinder is equal to the difference of the area of the two pistons multiplied by the thirty pounds pressure. This is sixty-six and two-thirds by thirty, and equals 2,000 pounds.

Hence 2,000 pounds, the resistance to forcing the air from the larger into the smaller cylinder, plus 3,333 pounds, the resistance in the smaller cylinder to compressing it to 100 pounds, is the sum of all the resistances in the compound cylinders at the time of greatest effort. This is 5,333 pounds. By thus reducing the work to be done at the end of the stroke, more work is done in the first part, and the resistance is made nearly uniform for the whole stroke.

THE AIR LIFT PUMP.

The Air Lift is one of the simplest methods of raising water from underground sources. The main principle of its operation may be stated thus: air under pressure is conveyed into the lower end of the water pipe through a suitable foot piece.

City and town water works, asylums and hospitals, plantations, railway water tanks, irrigation, private country houses, pumping mines, ice manufactories, breweries, cold storage and packing houses, textile mills, dye works, bleacheries, sewerage installations, dry docks, seaside water works, stock farms; in fact, everywhere that clear and abundant water is needed are opportunities for the application of the Air Lift System of pumping water.

Nor is it alone for securing public water supplies that the Air Lift is of special value. Within recent years the question of an abundant and pure water supply for manufacturing, irrigation and other uses has become one of equal importance. After an extensive experience, two general systems have been devised for utilizing the water which lies immediately below the surface of the ground. One, known as the Deep-well Pump System, for which a working pump is used, and the other the Air Lift System, which employs compressed air to raise the water, either by means of its inherent expansive force or the difference in specific gravity between compressed air and water.

Theory of the Air Lift. Opinions differ as to the true theory of the Air Lift. A common Air Lift case is one where there is a driven well in which the water has risen approximately near the surface. In this well is placed a large pipe for the discharge of the water, which is known as an “eduction pipe.”