POWER FROM HEAT

THE TERM “water power” is a misnomer. There is no power inherent in water. It is gravity that makes water fall in cataracts or flow down a river bed, and hence it is the force of gravity that is responsible for the turning of our turbines and Pelton wheels. Water is merely the medium through which the force of gravity acts.

In the steam engine, water, again, serves as a medium and it is the energy of heat that is actually the working force. A closed vessel is partly filled with water and heat is applied to it. The water grows hotter and hotter until at a temperature of 212 degrees it begins to vaporize. The temperature of the water remains stationary until the space above the water is completely filled with steam at a pressure equal to that of normal air pressure. The steam gauge then registers zero. Continued application of heat raises the pressure of the steam and also raises the temperature of the water. When the steam gauge shows a pressure of 25 pounds per square inch, the water temperature is nearly 267 degrees; at 100 pounds it is over 337 degrees; at 200 pounds it is 387 degrees; at 500 pounds it is about 467 degrees. As steam is drawn off from the boiler a proportionate amount of water turns immediately into steam to take its place. If the boiler should burst, the entire mass of water would instantly flash into steam because its temperature is far above the boiling point at normal air pressure. That is why the explosion of a steam boiler is so violent. If the boiler were entirely filled with steam the effect of an explosion would not begin to be so destructive as if the boiler were half full of water. It is really the explosion of water that does such serious damage.

When the steam pressure in a boiler corresponds to the temperature of the water it is said to be saturated. The water in a boiler boils violently when the steam is drawn off rapidly and tiny droplets of water are carried off in the steam, producing what is known as wet steam. Steam that carries no water particles in suspension is called dry steam. When the steam is heated above the temperature of the water by means of an auxiliary heating device or by some peculiar construction of the boiler, it is called superheated steam.

In order to economize fuel it is highly important that as much water surface be exposed to the heat as possible. This may be done either by passing the heat in tubes through the water or by passing the water in tubes through the fire. Locomotive boilers are of the fire-tube type. The flaming gases of the furnace pass through the boiler through a series of tubes that lead to the stack. In the water-tube boilers it is highly important that a good and rapid circulation of water be maintained, otherwise there might be local generation of steam with serious consequences.

THE GIFFARD INJECTOR

As steam is used from a boiler the water is slowly exhausted and it must be replenished with a fresh supply. In early days of the steam engine, water was pumped in against the boiler pressure by the use of powerful pumps, but in 1858 a man named Giffard invented a most ingenious apparatus by which steam of the boiler was used to force water in directly against its own pressure. This seems like lifting oneself by one’s boot straps. When the injector was first invented it seemed so impossible for it to work that engineers would not accept it until it had repeatedly demonstrated its operativeness. Even after it was accepted and in common use its mysterious operation was a subject of discussion for years. A sectional view of a Giffard injector is shown in Figure 45. Steam from the boiler comes down the tube A and passes out in a jet from the nozzle B. A needle valve C may be moved into the nozzle to reduce or shut off the jet of steam. The jet enters a conical chamber D which has a tube E that runs down into the water reservoir. The steam jet blows the air out of chamber D, producing a partial vacuum which draws water up the tube E and into the chamber D. When the water reaches the steam jet it is driven out of the chamber across a short open space into a slightly diverging tube or receiving cone F and through a check valve G into the boiler. At H there is a glass window through which the action of the water jet as it rushes into the receiving tube may be watched. I is an overflow pipe leading back to the reservoir. When the steam flows into the cone B it gathers momentum and issues from the nozzle in a jet of high velocity. On striking the water it combines with the water and condenses, but at the same time it imparts its momentum to the water so that the water is given more than enough momentum to drive it into the boiler against the pressure in the boiler.

The pressure of steam is utilized to drive the piston of an engine or the steam may be set in motion by letting it issue from a nozzle, when its momentum may be employed to drive a steam turbine in the same way that a water jet drives a Pelton wheel but in either case it is heat that does the work. The steam expands as it moves the piston and as it passes through the nozzle its expansion is accompanied by a corresponding loss of heat.

As we have already noted, it was for the purpose of raising water that inanimate powers were first set to work. It was with the same object in view that steam was first employed.

HERO’S STEAM ENGINE

To be sure the first steam engine was invented by Hero, the disciple of Ctesibius, in the second century before Christ, and it was not a water-raising machine. It consisted of a hollow sphere (see Figure 46), mounted to rotate and fed with steam that entered it through the journals. The steam issued from opposite sides of the sphere through two bent tubes. The steam in issuing from the tubes reacted against the tubes, pushing them back and causing the sphere to revolve.

Every action is accompanied by an equal and opposite reaction. A bullet fired from a gun kicks the gun back. In order to push the bullet out of the barrel, the powder must have something to push against and the push against the gun is equal to the push against the bullet. Even if there were no bullet to be pushed the discharge of the powder would react against the gun. In order to push its own gases out of the gun, the powder must push back against the gun.

In Hero’s engine, the steam, in order to push itself out of the sphere, must push against the bent tubes. We have a similar reaction motor in the revolving lawn sprinkler. The water issuing from the bent arms of the sprinkler forces them back and causes them to revolve. Were the arms radial and not bent the reaction would be there just the same, but it would be exerted against the center of the wheel and hence there would be no rotation.

Reaction has nothing to do with the pressure of the atmosphere as so many people imagine. A gun would kick just as hard and the lawn sprinkler or Hero’s engine would operate just as well and as fast in a perfect vacuum. It was to demonstrate the principle of reaction that Hero built his steam engine. As far as we know it was never put to useful work and remained merely a scientific toy.

NEWCOMEN’S ATMOSPHERIC ENGINE

The first practical use of steam power was applied to the pumping of water from the deep mines of Cornwall. Newcomen’s engine, which antedated Watt’s by several decades, was known as an atmospheric engine. It consisted of a cylinder fitted with a piston, but the top of the cylinder was open. The piston was connected to one end of a lever or walking beam, the other end of which was connected by a chain with the plunger of a pump situated at the bottom of the mine shaft. Little power was required to lift the piston because it was counterbalanced by the chain and plunger attached to the opposite end of the walking beam. The real work was done in pushing the piston down and thereby raising the pump plunger. Steam under low pressure was let into the cylinder under the piston to raise it and then a jet of water was sprayed into the steam-filled cylinder. This condensed the steam, producing vacuum and the atmospheric pressure acting on the upper surface of the piston forced the piston down, raising the pump plunger. The engine ran very slowly, making only about 15 strokes per minute. Later it was improved, producing about 30 strokes per minute.

WATT’S STEAM ENGINE

Watt was the first man to build an engine in which the real work was done by the pressure of steam as in modern steam engines. As long as the steam engine was used for pumping water the reciprocating piston could be connected directly to the pump plunger and it was unnecessary to exert power on both sides of the piston, but in 1782 Watt obtained a patent on a double-acting engine. Steam was admitted first on one side of the piston and then on the other. The walking beam was connected by means of a connecting rod to a crank on a shaft that carried a flywheel, and the seesaw motion of the beam was converted into rotary motion of the wheel. This opened up new industrial opportunities for the steam engine.

The steam was exhausted from the cylinder into a condenser, i.e., a chamber, in which a spray of water converted the steam into water, producing a vacuum, thereby relieving the piston of back pressure and virtually adding that much more power to the steam operating on the opposite side of the piston. A pump driven by the engine drew the water and the air liberated from the condensed steam out of the condenser.

Watt invented an ingenious governor to control the flow of steam to the engine and insure a uniform motion. (See Figure 47.) This governor consisted of a pair of levers hinged to a revolving shaft and each provided with a ball weight at its free end. The shaft was revolved by the engine and if the engine tended to run too fast the balls were thrown out by centrifugal action. In so doing they operated a throttle valve cutting down the steam supply. When the engine slowed down the balls would drop, admitting more steam. In this way the speed of the engine was kept within close limits. The ball governor in improved form is still widely used in stationary engines.

USING THE HEAT IN THE STEAM

Another of Watt’s inventions which has proved of highest importance in steam engineering was the cut off. In his first engine the piston was subjected to the full boiler pressure throughout its stroke, and the steam that poured out of the exhaust port of the cylinder at the end of the piston stroke was almost as hot as that which entered the cylinder at the beginning of the stroke. The heat that goes out in the exhaust represents just so much wasted energy. Watt realized this and so he invented a valve which would cut off the flow from the boiler before the piston had completed its stroke. Then the steam back of the piston would continue to expand because of the heat within it and would keep on pushing the piston. Of course, the pressure would gradually diminish and there would be less power in the stroke than if the full boiler pressure were pushing the piston all the way, but this loss of power is offset by the saving in steam and in the fuel used to heat the steam. The point at which the cut-off takes place depends largely upon the pressure of the steam. If the steam is cut off when the piston has made only one-fifth of its stroke, one-fifth as much steam will be used at each stroke as would be the case if the steam were used nonexpansively. However, in actual practice the expansion of the cut-off steam instead of being five times would be only about four times, because of the clearance that must be allowed between the piston and the end of the cylinder. If steam of 100 pounds absolute pressure is used, the average pressure throughout the piston will be only about 57 per cent of the full pressure of the steam in the boiler, but each pound of steam will actually do .57 × 4 = 2.28 times as much work as it would if used nonexpansively. All sorts of valve gear have been invented to admit steam quickly and cut it off at the proper point to produce the most efficient result.

In low-pressure cylinders in order to prevent loss of heat through the wall of the cylinder, the latter is steam-jacketed. In other words, there is an outer casing surrounding the cylinder and between this casing and the cylinder steam is admitted to keep the cylinder walls hot.

In order to make full use of the heat in steam it is, in some engines, sent through a series of two, three, and even four cylinders. The exhaust from one cylinder goes into a second larger cylinder. From here after doing work on a piston it discharges into a third still larger cylinder and from that may be led into a fourth cylinder. The cylinders must be progressively larger to allow for the expansion of the steam.

The ordinary steam engine labors under the disadvantage of having to start and stop its pistons at the end of each stroke. Every body possesses inertia, whether it be moving or at rest. If it be at rest, it takes much more energy to set it in motion than to keep it moving. In fact it would keep on moving without further expenditure of energy were there no friction and no forces acting against it. In order to stop the body energy must be expended to overcome its inertia. The more rapidly a body is started and the more quickly it is stopped, the more work must be done in overcoming its inertia. In a steam engine not only the piston but other parts connected to it may be required to reciprocate several hundred times per minute. A great deal of energy is uselessly expended in starting and stopping these parts.

Many efforts have been made to produce a rotary engine in which the piston rotates instead of reciprocating, thus doing away with the work of overcoming inertia. However, there are serious obstacles to the construction of such an engine, and as yet no truly efficient and practical rotary engine has been built.

DE LAVAL’S STEAM TURBINE

However, there is another type of engine in which the steam is applied continuously and all the parts revolve. Such an engine was the reaction turbine invented by Hero, to which reference has already been made. Modern turbines, however, are of very different construction. They resemble the Pelton wheels and turbines used in developing water power, differing from them mainly in the fact that use is made of the expansive energy of steam which is lacking in water. In the De Laval steam turbine a wheel is used which has a series of curved buckets all around its periphery that are closed at the outer end by a circular rim. (See Figure 48.) Steam is directed against this bucket, not tangentially as in a Pelton wheel, but from the side. Several steam nozzles are employed and as the steam jets strike the buckets and sweep around their curved surfaces they react against the buckets and drive the wheel around. In order to operate efficiently the velocity of the steam must be very high and the wheel must also turn at high speed. When steam flows through a diverging nozzle its velocity is greatly accelerated by its expansive effort. Such nozzles are used in the De Laval turbines and the steam issues from them with a velocity which may be higher than that of a rifle bullet. The buckets are forged and the hard-scale surface is left on them; otherwise they would wear away quickly under the action of the powerful jets of steam.

The turbine wheel may revolve at a speed of 30,000 revolutions per minute. Such a tremendous speed has its disadvantages. If a wheel is to run smoothly it must revolve on its center of gravity. A lopsided wheel, or one that is mounted a little off center, produces a pounding action which imposes a serious strain upon the bearings and the revolving parts. The wheel tries to turn on its own center of gravity and will do so if permitted to. It is impossible to balance a wheel so perfectly that the axis it turns on passes exactly through its center of gravity. At ordinary speeds this slight eccentricity is so slight that it is practically negligible, but when we have to deal with 30,000 revolutions per minute the least divergence between the center of rotation and the center of gravity will produce dangerous strains. For this reason the wheel of the De Laval turbine is mounted on a flexible shaft and on floating bearings, so that it will automatically find and turn on its own center of gravity. In order to utilize the power developed in the wheel, gearing must be used to step down the speed.

PARSONS EXPANSION TURBINE

The Parsons steam turbine resembles a water turbine in its action. Instead of having a few nozzles directing steam against the buckets there is a nozzle for each bucket, and instead of a single wheel there is a series of them through which the steam passes, successively passing through a set of nozzles between each wheel. The nozzles are formed of blades on the periphery of stationary wheels. These blades are curved in the opposite direction to the blades on the revolving wheels, as shown in Figure 49. Steam in passing through the ring of curved stationary blades is divided up into a series of jets which strike the curved blades of the first wheel. In passing through this wheel the direction of the stream is reversed, and it enters between the second set of stationary blades, which turn it back again and direct it against the next wheel. Thus the steam pursues a sinuous course through the series of wheels. To allow for the expansion of the steam the blades are made progressively longer and the wheels of progressively larger diameter from the inlet to the exhaust end of the engine.

The Curtis turbine combines the De Laval and the Parsons principles. The steam enters through a series of nozzles which are of the expanding type, then it goes through a series of moving and stationary blades, as in the Parsons turbine, from which it enters another set of expanding nozzles and gains velocity and momentum before passing through the second series or stage of moving and fixed blades. (See Figure 50.)

Steam turbines are particularly adapted for use in electric power plants. The speed of rotation of the Parsons and Curtis types is much lower than that of the De Laval and hence the electric generators may be directly driven by them without the interposition of any gearing. They can be built of larger power than the reciprocating engines because they are so economical of space.

A good comparison of turbine versus reciprocating engines is offered by the 74th Street power station of the Manhattan Elevated Railway, New York. This station, which was completed in 1901, was equipped with eight huge reciprocating engines, each developing 8,000 horsepower normally, and capable of delivering a maximum of 12,500 horsepower. The whole plant, therefore, had a maximum capacity of 100,000 horsepower. Gradually these units have been giving way to steam turbines of much higher power, and in 1919 there was installed one powerful turbine which alone was capable of developing as much power as the entire plant of 1901. This is a triple-compound turbine comprising one high-pressure turbine and a low-pressure turbine at each side. Steam enters the high-pressure turbine at 205 pounds pressure to the square inch and then exhausts into the low-pressure turbines, passing from them into condensers which operate under 29 inches vacuum. Each turbine drives a separate generator and the combined horsepower of the whole unit is about 100,000, while the floor space occupied is only 50 by 52 feet.

The economy of space and of fuel offered by the steam turbine is of great value in the power plants of ships, and this form of prime mover has been installed on modern high-speed passenger liners and also on high-speed war vessels. While in certain respects the turbine is ideal for such service, there are two handicaps which must be overcome. In the first place, the most efficient speed for the turbine is considerably higher than the efficient speed of the propeller and some means must be provided for stepping down the speed. In the second place, the turbine cannot be as economically controlled as a reciprocating engine and its direction of rotation cannot be reversed, so that difficulties are encountered in maneuvering the ship in harbors. It is no simple matter to gear down the high speed and enormous power of a turbine, However, an elaborate system of gearing has been provided for this purpose which has proved satisfactory even in powerful battle cruisers. The British battle cruisers with a power plant of 134,000 horsepower are driven by geared turbines. To reverse the propellers separate low-power turbines are used.

A more attractive system of control is to have the turbines drive electric generators and then use the electric power to drive the propellers through motors mounted on the propeller shafts. The electric power can easily be controlled from the bridge and the propellers may be reversed by reversing the motors. However, the disadvantage of the electric system is that it occupies a great deal of space, particularly in plants running over 100,000 horsepower.