Chapter III: Steam Engines

The first steam engine was an exceedingly simple affair. It had neither eccentric, cylinder, crank, nor valves, and it did not depend upon the pressure of the steam acting against a piston to drive it back and forth, because it had no piston.

It is one of the remarkable things in the history and development of mechanism, that in this day of perfected steam engines, the inventors of our time should go back and utilize the principles employed in the first recorded steam engine, namely, the turbine. Instead of pressure exerting a force against a piston, as in the reciprocating engine, the steam acted by impacting against a moving surface, and by obtaining more or less reaction from air-resistance against a freely discharging steam jet or jets.

The original engine, so far as we have any knowledge, had but one moving part, namely, a vertical tubular stem, to which was attached a cross or a horizontal tube.

The Original Engine.--Figure 8 is a side view of the original engine. The vertical stem A is pivoted to a frame B, and has a bore C which leads up to a cross tube D. The ends of the tube D are bent in opposite directions, as shown in the horizontal section, Fig. 9.

Steam enters the vertical stem by means of a pipe, and as it rushes up and out through the lateral tubes D, it strikes the angles E at the discharge ends, so that an impulse is given which drives the ends of the tube in opposite directions. As the fluid emerges from the ends of the tubes, it expands, and on contacting with the air, the latter, to a certain extent, resists the expansion, and this reacts on the tube. Thus, both forces, namely, impact and reaction, serve to give a turning motion to the turbine.

The Reciprocating Engine.--The invention of this type of engine is wrapped in mystery. It has been attributed to several. The English maintain that it was the invention of the Marquis of Worcester, who published an account of such an engine about 1650. The French claim is that Papin discovered and applied the principle before the year 1680.

In fact, the first actual working steam engine was invented and constructed by an Englishman, Captain Savery, who obtained a patent for it in 1698. This engine was so constructed as to raise water by the expansion and condensation of steam, and most engines of early times were devoted solely to the task of raising water, or were employed in mines.

Atmospheric Engines.--When we examine them it is difficult to see how we can designate them as steam engines. The steam did not do the actual work, but a vacuum was depended on for the energy developed by the atmospheric pressure.

A diagram is given, Fig. 10, showing how engines of this character were made and operated. A working beam A was mounted on a standard B, and one end had a chain C on which was placed heavy weights D. Near this end was also attached the upper end of a rod E, which extended down to a pump.

The other end of the working beam had a chain F, which supported a piston G working within a vertically-disposed cylinder H. This cylinder was located directly above a boiler I, and a pipe J, with a valve therein, was designed to supply steam to the lower end of the cylinder.

A water tank K was also mounted at a point above the cylinder, and this was supplied with water from the pump through a pipe L. Another pipe M from the tank conducted water from the tank to the bottom of the cylinder.

The operation of the mechanism was as follows: The steam cock N, in the short pipe J, was opened to admit steam to the cylinder, below the piston. The stem of the steam cock also turned the cock in the water pipe M, so that during the time the steam was admitted the water was shut off.

When the steam was admitted so that it filled the space below the piston, the cock N was turned to shut off the steam, and in shutting off the steam, water was also admitted. The injection of water at once condensed the steam within the cylinder so a partial vacuum was formed.

It will be remembered that as steam expanded 1700 times, the condensation back into water made a very rarified area within the cylinder, and the result was that the piston was drawn down, thus raising both the weight D and also the pump rod E. This operation was repeated over and over, so long as the cock N was turned.

The turning of the stem of this cock was performed manually,--that is, it had to be done by hand, and boys were usually employed for doing this. When, later on, some bright genius discovered that the valve could be turned by the machinery itself, it was regarded as a most wonderful advance.

The discovery of this useful function has been attributed to Watt. Of this there is no conclusive proof. The great addition and improvements made by Watt, and which so greatly simplified and perfected the engine, were through the addition of a separate condenser and air pump, and on these improvements his fame rests.

From the foregoing it will be seen that the weight D caused the piston to travel upwardly, and not the force of the steam, and the suction produced by the vacuum within the cylinder did the work of actuating the pump piston, so that it drew up the water.

The Piston.--From this crude attempt to use steam came the next step, in which the steam was actually used to move the piston back and forth and thus actually do the work. In doing so the ponderous walking beam was dispensed with, and while, for a long period the pistons were vertically-placed, in time a single cylinder was used, and a crank employed to convert the reciprocating into a circular motion.

Fig. 11 shows a simple diagram of a steam engine, so arranged that the operation of the valves may be readily understood. The cylinder A has a steam chest B, which contains therein a slide valve C to cover the ports at the ends of the cylinder. This figure shows the crank turning to the right, and the eccentric D on the engine shaft is so placed, that while the crank E is turning past the dead center, from 1 to 2, the slide valve C is moved to the position shown in Fig. 12, thereby covering port F and opening port G.

It will be seen that the slide valve is hollowed within, as at H, and that the exhaust port I leads from this hollowed portion while the live steam from the boiler enters through pipe J and fills the space K of the chest.

In Fig. 11 live steam has been entering port F, thus driving the piston to the right. At the same time the exhaust steam at the right side of the piston is discharging through the port G and entering the hollow space within the slide valve. In Fig. 12 the conditions are reversed, and now live steam enters port G, and the exhaust passes out through port F.

When the engine crank reaches the point 3, which is directly opposite 1, the reverse action takes place with the slide valve, and it is again moved to its original position, shown in Fig. 12.

Importance of the Valve.--Every improvement which has been made in the engine has been directed to the valve. The importance of this should be fully understood. As the eccentric is constantly turning it is a difficult matter to so arrange the valve as to open or close it at the correct time, absolutely, and many devices have been resorted to to accomplish this.

Expanding the Steam.--As all improvements were in the direction of economizing the use of steam, it was early appreciated that it would be a waste to permit the steam to enter the cylinder during the entire period that the engine traveled from end to end, so that the valve had to be constructed in such a way that while it would cut off the admission of steam at half or three-quarters stroke, the exhaust would remain on until the entire stroke was completed.

Some engines do this with a fair degree of accuracy, but many of them were too complicated for general use. In the form of slide valve shown the pressure of the steam on the upper side, which is constant at all times, produces a great wearing action on its seat. This necessitated the designing of a type of valve which would have a firm bearing and be steam tight without grinding.

Balanced Valve.--One of the inventions for this purpose is a valve so balanced by the steam pressure that but little wear results. This has been the subject of many patents. Another type also largely used in engines is known as the _oscillating_ valve, which is cylindrical or conical in its structure, and which revolves through less than a complete revolution in opening and closing the ports.

Rotary Valve.--The rotary valve, which constantly turns, is employed where low pressures are used, but it is not effectual with high pressures. This is also cylindrical in its structure, and has one or more ports through it, which coincide with the ports through the walls of the engine, as it turns, and thus opens the port for admitting live steam and closing the discharge port at the same time or at a later period in its rotation.

Engine Accessories.--While the steam engine is merely a device for utilizing the expansive force of steam, and thus push a cylinder back and forth, its successful operation, from the standpoint of economy, depends on a number of things, which are rarely ever heard of except by users and engineers.

Many of these devices are understood only by those who have given the matter thorough study and application. To the layman, or the ordinary user, they are, apparently, worth but little consideration. They are the things, however, which have more than doubled the value of the steam engine as a motor.

Efficiency of Engines.--When it is understood that with all the refinements referred to the actual efficiency of a steam engine is less than 30 per cent. some idea may be gained of the value which the various improvements have added to the motor.

Efficiency refers to the relative amount of power which is obtained from the burning fuel. For instance, in burning petroleum about 14,000 heat units are developed from each pound. If this is used to evaporate water, and the steam therefrom drives an engine, less than 4200 heat units are actually utilized, the remaining 9800 heat units being lost in the transformation from the fuel to power.

The value of considering and providing for condensation, compression, superheating, re-heating, compounding, and radiation, and to properly arrange the clearance spaces, the steam jackets, the valve adjustments, the sizes of the ports and passages, and the governor, all form parts of the knowledge which must be gained and utilized.

How Steam Acts in a Cylinder.--Reference has been made to the practice of cutting off steam before the piston has made a full stroke, and permitting the expansive power of the steam to drive the piston the rest of the way, needs some explanation.

As stated in a preceding chapter the work done is estimated in foot pounds. For the purpose of more easily comprehending the manner in which the steam acts, and the value obtained by expansion, let us take a cylinder, such as is shown in Fig. 13, and assume that it has a stroke of four feet. Let the cylinder have a diameter of a little less than one foot, so that by using steam at fifty pounds pressure on every square inch of surface, we shall have a pressure of about 5000 pounds on the piston with live steam from the boiler.

In the diagram the piston moves forwardly to the right from 0 to 1, which represents a distance of one foot, so that the full pressure of the steam of the boiler, representing 5000 pounds, is exerted on the piston. At 1 the steam is cut off, and the piston is now permitted to continue the stroke through the remaining three feet by the action of the steam within the cylinder, the expansive force alone being depended on.

As the pressure of the steam within the cylinder is now much less and decreases as the piston moves along, we have taken a theoretical indication of the combined pressure at each six inch of the travel of the piston. The result is that we have the following figures, namely, 4000, 2700, 1750, 1000, 450 and 100. The sum of these figures is 10,000 pounds.

The piston, in moving from 0 to 1, moved one foot, we will say, in one second of time, hence the work done by the direct boiler pressure was 5000 _foot pounds_; and since the piston was moved three feet more by the expansion of the steam only, after the steam pressure was shut off, the work done in the three seconds required to move the piston, was an additional 5000 foot pounds, making a total of 10,000 foot pounds for four seconds, 150,000 foot pounds per minute, or about 45 horse power.

This movement of the piston to the right, represented only a half revolution of the crank, and the same thing occurs when the piston moves back, to complete the entire revolution.

Indicating the Engine.--We now come to the important part of engine testing, namely, to ascertain how much power we have obtained from the engine. To do this an indicator card must be furnished. A card to indicate the pressure, as we have shown it in the foregoing diagram would look like Fig. 14.

The essential thing, however, is to learn how to take a card from a steam engine cylinder, and we shall attempt to make this plain, by a diagram of the mechanism so simplified as to be readily understood.

In Fig. 15 we have shown a cylinder A, having within a piston B, and a steam inlet pipe C. Above the cylinder is a drum D, mounted on a vertical axis, and so geared up with the engine shaft that it makes one complete turn with each shaft revolution. A sheet of paper E, ruled with cross lines, is fixed around the drum.

The cylinder A has a small vertical cylinder F connected therewith by a pipe A, and in this cylinder is a piston H, the stem I of which extends up alongside of the drum, and has a pointed or pencil J which presses against the paper E.

Now, when the engine is set in motion the drum turns in unison with the engine shaft, and the pressure of the steam in the cylinder A, as it pushes piston B along, also pushes the piston H upwardly, so that the pencil point J traces a line on the ruled paper.

It will be understood that a spring is arranged on the stem I in such a manner that it will always force the piston H downwardly against the pressure of the steam.

Mean Efficiency.--We must now use a term which expresses the thing that is at the bottom of all calculations in determining how much power is developed. You will note that the pressure on the piston during the first foot of its movement was 10,000 pounds, but that from the point 1, Fig. 13, to the end of the cylinder, the pressure constantly decreased, so that the pressure was not a uniform one, but varied.

Suppose we divide the cylinder into six inch spaces, as shown in Fig. 13, then the pressure of the steam at the end of each six inches will be the figures given at bottom of diagram, the sum total of which is 30,000, and the figures at the lower side show that there are eight factors.

The figure 10,000 represents, of course, two six inch spaces in the first foot of travel.

The result is, that, if we divide the sum total of the pressures at the eight points by 8, we will get 3750, as the mean pressure of the steam on the piston during the full stroke of the piston.

In referring to the foot pounds in a previous paragraph, it was assumed that the piston moved along each foot in one second of time. That was done to simplify the statement concerning the use of foot pounds, and not to indicate the time that the piston actually travels.

Calculating Horse Power.--We now have the first and most important factor in the problem,--that is, how much pressure is exerted against the piston at every half revolution of the crank shaft. The next factor to be determined is the distance that the piston travels in one minute of time.

This must be calculated in feet. Let us assume that the engine turns the crank shaft at a speed of 50 revolutions a minute. As the piston travels 8 feet at each revolution, the total distance traveled is 400 feet.

If, now, we have a constant pressure of 3750 pounds on the piston, and it moves along at the rate of 400 feet per minute, it is obvious that by multiplying these two together, we will get the figure which will indicate how many pounds the steam has lifted in that time.

This figure is found to be 1,500,000, which means foot pounds, as we have by this means measured pressure by feet, or pounds lifted at each foot of the movement of the piston.

As heretofore stated, we must now use the value of a horse power, so that we may measure the foot pounds by it. If we had a lot of wheat in bulk, and we wanted to determine how much we had, a bushel measure would be used. So with power. The measure, as we have explained, is 33,000, and 1,500,000 foot pounds should give as a result a little over 45 horse power.

Condensation.--We now come to the refinements in engine construction,--that which adds so greatly to the economy of operation. The first of these is condensation. The first reciprocating engine depended on this to do the actual work. In this age it is depended upon simply as an aid.

The first thing however that the engineer tries to do is to prevent condensation. This is done by jacketing the outside of the cylinder with some material which will prevent radiation of heat, or protect the steam within from being turned back into water by the cool air striking the outside of the cylinder.

Atmospheric Pressure.--On the other hand, there is a time when condensation can be made available. The pressure of air on every square inch of surface is 14-3/4 pounds. When a piston moves along and steam is being exhausted from the cylinder, it must act against a pressure of 14-3/4 pounds on every square inch of its surface.

The problem now is to get rid of that back pressure, and the old type engines give a hint how it may be done. Why not condense the steam discharged from the engine cylinder? In doing so a vacuum is produced on the exhaust side of the piston, at the same time a pressure is exerted on its other side.

The Condenser.--Thus the condenser is brought into existence, as an aid. By jacketing condensation is prevented; it is fought as an enemy. It is also utilized as a friend. It is so with many of the forces of nature, where man for years vainly fought some principle, only to find, later on, that a friend is more valuable than a foe, and to utilize a material agency in nature is more economical than to fight it.

Pre-heating.--The condenser does two things, both of which are of great value to the economical operation of the engine. For the purpose of rapidly converting the steam back into water as it issues from the engine cylinder, water is used. The steam from the cylinder has a temperature of 212 degrees and upwards, dependent on its pressure.

Water, ordinarily, has a temperature of 70 degrees, or less, so that when the steam strikes a surface which is cooled down by the water, it is converted back into liquid form, but at a temperature less than boiling water. The water thus converted back from the steam gives up part of its heat to the water which cools the condenser, and the water from the condenser, as well as the water used to cool the condenser, are thus made available to be fed into the boiler, and thus assist in again converting it into a steam.

The economy thus lies in helping the coal, or other fuel, do its work, or, to put it more specifically, it conserves the heat previously put out by the coal, and thus saves by using part of the heat over again.

Superheaters.--Another refinement, and one which goes to the very essence of a heat motor, is the method of superheating the steam. This is a device located between the boiler and the engine, so that the steam, in its transit from the boiler to the engine, will be heated up to a high degree, and in the doing of which the pressure may be doubled, or wonderfully increased.

This may be done in an economical manner in various ways, but the usual practice is to take advantage of the exhaust gases of the boiler, in the doing of which none of the heat is taken from the water in the boiler.

The products of combustion escaping from the stacks of boilers vary. Sometimes the temperature will be 800 degrees and over, so that if pipes are placed within the path of the heated gases, and the supply steam from the boiler permitted to pass through them a large amount of heat is imparted to the steam from a source which is of no further use to the water being generated in the boiler.

Compounding.--When reference was made to the condensation of steam as it issued from the boiler, no allusion was made to the pressure at which it emerged. If the cylinder was well jacketed, so that the amount of condensation in the cylinder was small, then the pressure would still be considerable at the exhaust. Or, the steam might be cut off before the piston had traveled very far at each stroke, in which case the exhaust would be very weak.

In practice it has been found to be most economical to provide a high boiler pressure, and also to superheat the steam, but where it is not superheated, and a comparatively high boiler pressure is provided, compounding is resorted to.

To compound steam means to use the exhaust to drive a piston. In such a case two cylinders are placed side by side, one, called the high pressure cylinder, being smaller than the low pressure cylinder, which takes the exhaust from the high pressure.

The exhaust from the second, or low pressure cylinder may then be supplied to a condenser, and in that case the mechanism would be termed a compound condensing engine. If a condenser is not used, then it is simply a compound engine.

Triple and Quadruple Expansion Engines.--Instead of using two cylinders, three, or four, are employed, each succeeding cylinder being larger than the last. As steam expands it loses its pressure, or, stated in another way, whenever it loses pressure it increases in volume. For that reason when steam enters the first cylinder at a pressure of say 250 pounds, it may exhaust therefrom into the next cylinder at a pressure of 175 pounds, with a corresponding increase in volume.

To receive this increased volume, without causing a sensible back pressure on the first cylinder, the second cylinder must be larger in area than the first; in like manner when it issues from the exhaust of the second cylinder at 125 pounds pressure, there is again an increase in volume, and so on.

Examine Fig. 16, which shows a pair of cylinders, A being the high, and B the low pressure cylinders, the exhausts of the high pressure being connected up with the inlets of the low pressure, as indicated by the pipes, C D.

The diagram does not show the valve operations in detail, it being sufficient to explain that when the valve E in the pipe C is closed, the valve F, at the other end of the cylinders, in the pipe D, is closed. The same principle is employed in the triple and quadruple expansion engines, whereby the force of the steam at each exhaust is put to work immediately in the next cylinder, until it reaches such a low pressure that condensation is more effective than its pressure.

The diagram, as given, is merely theoretical, and it shows the following factors:

First: The diameter of each piston.

Second: The area of each piston in square inches.

Third: The steam pressure in each cylinder.

Fourth: The piston pressure of each cylinder.

It will be seen that an engine so arranged is able to get substantially the same pressure in each of the second, third and fourth cylinders, as in the first (see Fig. 16a), and by condensing the discharge from the fourth cylinder a most economical use of steam is provided for. The Steam Turbine.--We must now consider an entirely new use of steam as a motive power. Heretofore we have been considering steam as a matter of pressure only, in the development of power. It has been observed that when the pressure of steam decreases at the same temperature it is because it has a greater volume, or a greater volume results.

When steam issues from the end of a pipe its velocity depends on its pressure. The higher the pressure the greater its velocity. The elastic character of steam is shown by its action when ejected from the end of a pipe, by the gradually enlarging area of the discharging column.

In a reciprocating engine the power is derived from the pressure of the steam; in a turbine the power results from the impact force of the steam jet. Such being the case velocity in the movement of the steam is of first importance.

Pressure and Velocity.--To show the effectiveness of velocity, as compared with pressure, examine Fig. 17. A is a pipe discharging steam at a pressure of 100 pounds. To hold the steam in the pipe would require a pressure of 100 pounds against the disk B, when held at 1, the first position.

Suppose, now, the disk is moved away from the end of the pipe to position 2. The steam, in issuing forth, strikes the disk over a larger area, and in escaping it expands, with the result that its velocity from 1 to 2 is greater than the movement of the steam within the pipe that same distance.

The disk is now moved successively to positions 3, 4, 5, and so on. If we had a measuring device to determine the push against the disk at the various positions, it would be found that there is a point at some distance from the end of the pipe, at which the steam has the greatest striking force, which might be called the focal point.

A blow pipe exhibits this same phase; the hottest point is not at the end of the pipe, but at an area some distance away, called the focal point of heat.

The first feature of value, therefore, is to understand that pressure can be converted into velocity, and that to get a great impact force, the steam must be made to strike the hardest and most effective blow.

When a jet of steam strikes a surface it is diverted or it glances in a direction opposite the angle at which it strikes the object. In directing a jet against the blades of a turbine it is impossible to make it strike squarely against the surface.

Let us assume that a wheel A, Fig. 20, has a set of blades B, and a steam jet is directed against it by the pipe C. It will be seen that after the first impact the steam is forced across the blades, and no further force is transferred to them.

Form of Blades.--The blades are therefore so curved, that the steam after the first impact cannot freely pass along the blade, as it does on a straight blade, but imparts on every element of the curved-back blade, thereby giving up continually part of its speed to the blade.

This is clearly shown in Fig. 21, where the pipe D ejects the stream of steam against the concaved blades E. Many modifications have been made in the shapes of these blades, all designed to take advantage of this action.

Compounding the Jet.--We may extend the advantages gained by this form of blades, and diverting the course of the jet, so that it will be directed through a series of wheels, each of which will get the benefit of the moving mass from the pipes.

Such a structure is shown in Fig. 22, in which three bladed wheels A, B, C, are caused to rotate, a set of stationary blades D, E, being placed between the three moving wheels, but the stationary blades are disposed in reverse directions. When the steam from pipes F, F, impinges against the blades of the first wheel A, it is directed by the stationary blade D to the next wheel B, and from the stationary blade E to the blades of the next wheel C, thus, in a manner somewhat similar to the compounding effect of the steam engine, utilizes the pressure which is not used at the first impulse.