IN the previous chapter we have mentioned that the heat value of coal is measured by the number of heat-units it contains, and that each heat-unit represents 772 foot-pounds of work, or the energy required to raise 772 pounds one foot. According to the figures given, each pound of coal contains an enormous amount of possible work energy. The operating of the locomotive, and of all other steam-engines, is a process of transforming the heat energy of coal into mechanical work. In some kinds of engines driven by hot air or gas, the operation of converting heat into work is done without the use of steam. A greater proportion of the heat energy can be utilized in that way; but there are mechanical obstacles which prevent such systems from being used where much power is required.

Steam, the vapor of water, has been found the most convenient medium for transforming the energy of coal into the useful work of pulling railroad trains, and of driving other kinds of machinery. Water has the greatest heat-absorbing capacity of any known substance, which makes it an excellent means of converting heat into work; but it has some peculiarities which readily lead to great loss of energy if not carefully controlled. If we follow the circle of operations which the burning of coal for steam-making purposes sets going, we shall meet at every move heat-losses which show us why so small a portion of the entire heat energy of coal reaches the crank-pins that turn the wheels of the engine. But an intelligent study of the losses will also help an engineer to restrain them to the lowest possible limit.

Suppose we take one pound of water at a temperature of 40 degrees Fah., and apply heat to it in an open vessel. If we put a thermometer in the water, we shall find that the temperature will rise rapidly till it reaches 212 degrees, the boiling-point at the pressure of the atmosphere (at sea level). Then the mercury stops rising, but the water keeps absorbing the heat and turning into steam. It takes rather more than 5½ times the quantity of heat to evaporate the whole of the pound of water into steam that it took to raise the temperature from the tank temperature to the boiling-point; for, although it is not shown by the thermometer, the converting of the pound of water from the boiling-point into steam uses up 965.7 heat-units, that being called the latent heat of steam at atmospheric pressure. In raising the water to the boiling-point—from 40 degrees to 212 degrees—172 heat-units were used, and in vaporizing the water 965.7 units, making in all 1137.7 heat-units, which are expended in evaporating one pound of water under the pressure of the atmosphere alone, which is 14.7 pounds to the square inch. Steam formed under this light pressure fills 1644 times the space occupied by the water it was made from. The volume of steam varies nearly inversely as the pressure, so that when the steam is generated under the pressure of two atmospheres, it fills only 822 times the space that the water did. Every step in the increase of pressure reduces the volume of the steam in like proportion. Steam at 150 pounds per square inch gauge pressure is only 173 times the volume of the water. Steam gauge pressure is the pressure above the atmosphere; absolute pressure is reckoned from the vacuum line.

If the pound of water, instead of being left to boil in an open vessel, had been put into a boiler where a pressure of 165 pounds absolute was put upon it, that being equal to a gauge pressure of 150 pounds, the result would have been different. When heat was now applied, the mercury would keep rising till the temperature of 365.7 degrees was reached before the water would begin to boil. To raise it to the boiling-point under this pressure, 330.4 heat-units would be put in the water, and then the addition of 855.1 more heat-units would convert the whole pound of water into steam, the total expenditure of heat being 1185.5 heat-units. From this it will be seen that while the generating of steam at atmospheric pressure, which gives no capacity to speak of for doing work, calls for an expenditure of 1137.7 heat-units, raising the steam to the high gauge pressure of 150 pounds takes only 1185.5 heat-units. Steam of 100 pounds gauge pressure uses up 1177 heat-units, so that it takes very little more heat to raise the steam to the higher pressure where it has the power of doing much more work than to the lower pressures. A study of these facts will show why it is most economical to use steam of high pressure.

Steam formed in ordinary boilers, where only sufficient heat is applied to evaporate the water, is called saturated steam. It is also sometimes spoken of as dry steam, or anhydrous steam. Saturated steam contains only just sufficient heat to maintain it in a gaseous condition, and the least abstraction of heat causes a portion of the steam to fall back into water when it loses its power of doing work. This is why it is important that steam cylinders and passages should be well protected from cold. The condensation of steam that goes on in badly lagged cylinders wastes a great deal of fuel.

When heat is applied to steam that is not in contact with water, the steam absorbs more heat and is said to be superheated. Superheated steam has a greater energy than saturated steam in proportion to the amount of heat added. The practical advantage of superheated steam is, that it does not turn into water in the cylinder so readily as saturated steam.

Having got steam raised to 150 pounds gauge pressure, which is almost 165 pounds absolute, the next move is to use it to the best advantage, so that the greatest possible amount of work will be got out of every pound of steam generated. In ordinary circumstances, the higher the temperature of steam admitted into the cylinders of a steam-engine, and the lower the temperature at which it is passed out by the exhaust, the greater will be the economy, if the reduction of temperature has been due to the conversion of heat into mechanical work.

That the steam passed into the cylinders may be used to the best possible advantage, the ordinary practice is to cause the expansive force of the steam to do all the work practicable. As has been already mentioned in a former chapter, high-pressure steam is like a powerful spring put under compression, and is ever ready to stretch out when its force is directed against anything movable. In that way it pushes the piston when the valve is cutting off admission of steam before the end of the stroke is reached. We shall try to show how such practice is economical.

To find out what is going on in the inside of the cylinders of an engine, to show accurately how the steam is distributed, the use of the steam-engine indicator is necessary. The indicator consists essentially of a small steam-cylinder, whose under side is connected by pipes to the main cylinder of the engine under inspection. Inside the indicator cylinder is a nicely fitting piston, whose upper movement is resisted by a spring of known strength. The piston-rod passes up through the top of the indicator cylinder; and its extremity is connected with mechanism for operating a pencil, and marking on a card a diagram whose lines coincide with the movement of the indicatur piston.

Fig. 39 gives perspective and sectional views of the Tabor indicator, an instrument well adapted for application to locomotives. The card to be marked is fastened in the paper drum attached to the indicator.This drum receives a circular motion from a cord which is operated by the cross-head of the locomotive, and the connection is so arranged that the drum will begin to move round just as the main piston begins its stroke. The circular motion of the drum is continued till the piston reaches the end of its stroke, when the drum reverses its movement, and returns to the exact point from which it started. Now the indicator cylinder being in communication with the main cylinder, when the latter begins to take steam, the pressure will be applied to the indicator piston, which was pushed upward, at the same time transmitting its movement to the pencil. The indicator piston will rise and fall in accordance with the steam pressure in the cylinder; and the circular movement of the drum coinciding with the cross-head movement, the pencil will describe a diagram which represents the pressure inside the main cylinder at the various points of the stroke.

Fig. 40 is a very good diagram taken from a locomotive cutting off at about 37 per cent of the stroke and running at 150 revolutions per minute. A is the atmospheric line traced before steam is admitted to the indicator. V is the vacuum line traced according to measurement, 14.7 pounds below the atmospheric line. D E is the admission-line, D being the point where the valve opens to admit steam. E F is the steam-line, beginning at the point of change in direction of the admission-line. The steam-line in this diagram drops down before the point of cut-off is reached, through the steam admission not being rapid enough to keep it up. F G is the expansion-line traced after the steam is cut off. At the point G the exhaust takes place, and the exhaust-line is from G to the end of the stroke. H I is the line of counter-pressure, and is high or low according to the quantity of steam left in the cylinder by the exhaust. The use of small nozzles always causes a high counter-pressure line. The compression line begins at I, the point where the value closes, and runs up to D, the pressure rising as the steam left in the cylinder, after the valve closes, gets pressed by the piston into small space.

For an exhaustive and easily understood treatise on the indicator, our readers are referred to Hemenway's "Indicator Practice and Steam-engine Economy," published by John Wiley and Sons, New York.

Suppose the steam in our boiler is raised to 165 pounds absolute pressure, and we apply it under different conditions to do work in the cylinder Z Z shown in Fig. 41, which is 16 inches diameter and has a stroke of 24 inches. The diagram above the cylinder represents the action of steam in the cylinder. The vertical lines represent the steam at different points of the. piston's stroke. If the cylinder were filled with steam at boiler pressure during the entire stroke of the piston, the diagram of work would resemble the rectangle A C E B Using the steam in this way is impracticable, but an approximation to it is possible, and it will serve to illustrate the subject. Ignoring the quantity needed to fill the clearance spaces, the steam from one pound of water, which is called a pound of steam, would just be sufficient to fill the cylinder once.

Instead of permitting the steam to follow the piston unimpeded during the whole stroke, we will cut it off at 6 inches or one-quarter stroke, as shown in the illustration Fig. 41, where the valve Y is closing the port y, just as the piston X has moved one quarter the stroke. The piston will now be pushed the remainder of the stroke by the expansive force of the steam, the latter falling in pressure as the space to be filled increases, and obeying what is called Mariotte's law, the pressure varying inversely as the volume. By the time the piston has moved to half-stroke, the steam is filling twice the space it was in when cut-off took place, and accordingly its pressure has fallen to the point b which represents 82.5 pounds to the square inch. At the end of the stroke when release takes place, the pressure has fallen to 41.25 pounds. We find by calculation that the average pressure on the piston when the steam was cut off at quarter-stroke was 98.42 pounds to the square inch. In this case just one quarter the quantity of steam was drawn from the boiler that was taken when steam followed full stroke, yet with the small quantity of steam, the average pressure on the piston was considerably more than half of what it was when four times the volume of steam was used.

The description of the action of the steam does not represent with any degree of accuracy what actually takes place; but it gives the facts closely enough to indicate how steam can be saved or wasted.

All engineers who have given the economical use of steam intelligent study agree that the proper way to use steam in a cylinder is to get it in as near boiler pressure as possible, so that the greatest possible ratio of expansion may be obtained while doing the necessary work. Where this practice is not followed, the steam is used wastefully. Locomotives that are run with the throttle partly closed, when, by notching the links back it could be used full open, are throwing away part of the fuel-saving advantages that high pressure offers.

For this practice the engineers are not in every case to blame, for many locomotives are constructed with valve motion so imperfectly designed that the engines will not run freely when they are linked close up. With the small nozzles made necessary to force the steam-making in small boilers, the back cylinder pressure is so great that the high compression, resulting from an early valve-closure, prevents the engine from running at the speed required.

From whatever cause it originates, the practice of running with the throttle partly closed causes much waste of fuel. A few examples will be given:
The diagram shown in Fig. 42 was taken from a locomotive running at 192 revolutions per minute. The boiler pressure was 145 pounds, and the initial pressure on this card is 136 pounds. This high cylinder pressure was obtained by keeping the throttle-valve full open. The driving-wheels were 68 inches diameter, and the engine was running close on forty miles an hour and was developing, with 18 x 24-inch cylinders, sufficient power to haul a train weighing 300 tons at the rate of fifty miles an hour. Steam was cut off at about seven inches of the stroke, expanded down to 25 pounds above the atmospheric line, and showed an average back pressure of 4 pounds. The work was done using at the rate of 21.5 pounds per horse-power per hour—very economical work.

Diagram Fig. 43 shows about the same power as the other one; but it was taken with the steam partly throttled, and cutting off at 10½ inches. In this case it will be noted that the initial pressure is only 102 pounds, that the terminal pressure is 31 pounds above the atmosphere, and that the counter-pressure is 7 pounds. In this case the work is done by using steam at the rate of 25.8 pounds per horse-power per hour, which is 16.6 per cent more steam than was used with the other way of working. There was no reason whatever for working the engine in this manner, except the careless practice that some runners get into.

A still worse case is shown by the diagram Fig. 44. Here the engine, which was running at 176 revolutions per minute, was worked cutting off at half stroke, and the average steam pressure kept down by throttling. Consequently the initial pressure is low, the terminal pressure and the back pressure high. This condition of working calls for the use of a large volume of steam to perform the work. The initial pressure is 109 pounds, the terminal pressure 45 pounds, and the back pressure 11 pounds. The engine while working this way used steam at the rate of 32 pounds per horse-power per hour, or 33 per cent more than was used in the first case. These are examples taken from the ordinary working of locomotives. They are no mere theories. They are the record of accurate measurements, and are as trustworthy as the indications of the steam-gauge. Using 33 per cent more steam than what is absolutely necessary is just throwing away one-third of the coal put into the fire-box.

To put the matter in a more concrete form: If the engine from which diagram Fig. 42 was taken was running 33.3 miles to the ton of coal, only 27.7 miles to the ton would be made when using the steam shown in diagram Fig. 43 and only 22.3 miles when diagram Fig. 44 was the record of the steam consumed.

There are some disadvantages to working with wide extremes of pressure in a cylinder. The temperature tends to change with changes of pressure, and this leads to loss through condensation of the steam in the cylinder. In the working of the simple engine we have been dealing with, where steam of 165 pounds absolute pressure was used, the steam enters the cylinder at about 365 degrees Fah., and escapes close to atmospheric pressure at a temperature of about 220 degrees. The metal of the cylinder inclines to maintain an even temperature at some average point between the high admission and the low exhaust temperatures. When the steam enters the cylinder it goes into a comparatively cool chamber, and the metal of the cylinder walls and heads draws some heat from the incoming steam. The portion of the steam robbed of its heat becomes spray, and helps to dampen the steam that continues to pass into the cylinder. As the events of the stroke go on, and release of pressure takes place after the opening of the exhaust port, the steam which became condensed in the beginning of the stroke is ready to flash back into steam under the release of pressure. If this happens as the steam is passing into the exhaust port, it draws heat from the cylinder metal to aid in the act of vaporization, the whole of this heat being carried up the chimney. The heat thus carried away from the cylinder metal has to be returned by the incoming steam of next stroke, and causes the initial condensation spoken of. Compression helps to prevent condensation by heating the cylinder at the end where steam is about to enter.

Another disadvantage of the locomotive cylinder is that the opportunities for using the steam expansively are very limited.

To provide a remedy for the losses due to cylinder condensation, and to provide better means of using the steam expansively, compound locomotives have been brought into use. A compound locomotive, while expanding the steam more than can be done with a simple engine, has a much more even temperature throughout the two strokes in which the steam is used. If there is condensation and revaporization of steam in the high-pressure cylinder, it passes into the low-pressure cylinder and is there used to do useful work. In a compound engine the work is more evenly distributed throughout the stroke than in a simple engine, consequently the strains and shocks given to the machinery are less. This ought to make the compound a durable machine.

The most common compound locomotive has two cylinders, high pressure on one side and low pressure on the other. The capacity of the low-pressure cylinder is about twice that of the other. Steam passes from the boiler into the high-pressure steam-chest, and is, by the slide-valve, admitted to the cylinder in the ordinary way. When the exhaust port of this cylinder opens, the steam passes into a receiver which carries it to the low-pressure cylinder steam-chest. From thence it is admitted into the cylinder by the slide-valve at the proper time. The initial pressure of the low-pressure cylinder is always less than the terminal pressure of the high-pressure cylinder. In other words, the high-pressure cylinder has at least as much back pressure as the low-pressure cylinder has positive pressure.

An intercepting valve or other mechanism is generally provided with compound locomotives to admit steam of low pressure direct from the boiler to the low-pressure cylinder to help in starting.

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