Motors Part 4
The figure 10,000 represents, of course, two six inch s.p.a.ces 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 a.s.sumed 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 a.s.sume 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 a.s.sist 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 pa.s.s 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.
[Ill.u.s.tration: _Fig. 16. Compound Engine._]
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.
[Ill.u.s.tration: Fig. 16a. Relative Piston Pressures.]
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.
[Ill.u.s.tration: Fig. 17. Changing Pressure into Velocity]
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.
[Ill.u.s.tration: _Fig. 18. Reaction against Air._]
[Ill.u.s.tration: _Fig. 19. Reaction against Surface._]
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.
[Ill.u.s.tration: Fig. 20. Turbine Straight Blades.]
Let us a.s.sume 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.