PART II.
THE FORCES OF AIR AND STEAM.
QUESTION 7. _What is meant by the pressure of the air?_
_Answer._ It is the pressure exerted by the weight of the air on every point with which it is in contact. The globe of the earth is surrounded by a layer of air about 50 miles thick, and, like every other substance, the air possesses weight, and hence presses upon every object with which it is in contact.
QUESTION 8. _How can it be shown that the air possesses weight?_
_Answer._ By weighing a flask when it is filled with air, and again when the air is exhausted from it. In the latter condition the weight of the flask will be found to be sensibly less than it was when full of air, showing that the air which the flask contained when it was first weighed increased its weight.
QUESTION 9. _Why do we not feel this pressure on our bodies?_
_Answer._ Because the air surrounds us on all sides, and presses just as much in one direction as it does in another, so that the pressures in different directions just balance each other, or are _in equilibrium_; but if you disturb this balance, for example, by sucking the air from a tube closed at one end, it will cling to your tongue; or if you take a thick piece of leather under ordinary conditions it will not adhere to anything, but if it be thoroughly wet and pressed hard against the surface of a smooth stone, so as to force out the air from under it, the stone, as nearly all school-boys know, can be lifted up if a string is attached to the leather; or if the air be sucked out of a tube, one end of which is inserted in a liquid, the latter will be forced up the tube. These phenomena are due to the pressure of the atmosphere in the first case on one side of the person’s tongue, pressing it against the mouth of the tube; in the second, to the same pressure on the top of the leather, causing it to adhere to the stone; and in the last, to the weight of the air pressing on the surface of the liquid, forcing it into the vacuum in the tube.
[Illustration: _Fig. 7._
Scale ¹⁄₄.]
QUESTION 10. _What is the amount of the pressure of the atmosphere, and how is it measured?_
_Answer._ It is usually measured by the pressure on one square inch of surface, which at the earth’s surface is 15 pounds.[3] If, for example, we have a cylinder, _A_, fig. 7, with an air-tight piston, _B_, fitted to it whose area is just one square inch, if we exhaust the air through the tube _C_ from the cylinder above the piston, the air will press against the under side of the piston so that, if no power is required to overcome its friction in the cylinder, the pressure of the air will raise a weight of 15 pounds. The pressure of the air varies, however, as you ascend or descend from the surface of the earth, because as you go up on a mountain or in a balloon the layer of air above you becomes thinner, and, therefore, its weight and consequent pressure are diminished; and as you descend, as in a deep mine, the layer is thicker, and its pressure consequently greater.
[3] In common practice it is generally taken at 15 lbs. per square inch, but the average atmospheric pressure is more accurately 14.7 pounds.
QUESTION 11. _What is steam?_
_Answer._ Steam is water changed by means of heat into a gas. At every temperature there is formed from water, on its surface, vapor of which the clouds are formed at all seasons of the year. This change of water into vapor, or evaporation of water, takes place at low temperatures only on its surface, however. But if we heat water in a vessel to a temperature of 212 degrees Fahrenheit, then the inner particles of the mass of water (lying on the heating surface of the vessel) are changed into steam, and rise to the surface in bubbles, which is the phenomenon we call _boiling_. It must not be imagined, however, that the visible cloud which escapes from a kettle or the exhaust-pipe of a steam engine is true steam. It is rather small particles of water, into which the steam has condensed through contact with the cold air. True steam is invisible, as we may observe near the mouth of a kettle or the exhaust-pipe of an engine from which we know it is escaping.
QUESTION 12. _If water is heated in an open vessel what occurs?_
_Answer._ It continues for some time to increase in temperature, and the evaporation becomes more and more rapid. At length bubbles of vapor break out and reach the surface, and the process of boiling or ebullition has begun. When this takes place the temperature of the water ceases to rise, and it remains stationary until all the water has boiled away, the only difference being that if the supply of heat be very great the process is very rapid, and if the supply of heat be small the process is very slow. The point at which ebullition commences is called the _boiling-point_.
QUESTION 13. _On what does the boiling-point depend?_
_Answer._ Chiefly on the pressure on the surface of the water, but to some extent upon the purity of the water. Thus, boiling, which takes place at 212 degrees under the ordinary atmospheric pressure, in lighter air, as on high mountains, takes place at a much lower temperature than on lowlands, and so water boils in a glass tube from which the air has been exhausted by the warmth of the hand, that is, at 92 degrees.
QUESTION 14. _What is the pressure of steam which escapes from boiling water in an open vessel?_
_Answer._ It is exactly equal to the pressure of the atmosphere in which it is boiled. Ordinarily this is 15 lbs., and the boiling-point 212 degrees; but if we go up on a mountain where the atmospheric pressure is only 10 lbs. per square inch, the water will then boil at a temperature of 193.3 degrees, and the steam which escapes will have the same pressure as the atmosphere, or 10 lbs. per square inch. On the other hand, if we could go down into a mine where the atmospheric pressure was 20 lbs. per square inch, the water would not boil until it was heated to 228 degrees, and the pressure of the escaping steam would then be 20 lbs. per square inch.
QUESTION 15. _If water is boiled in an enclosed vessel like a covered tea-kettle or a steam boiler, what occurs?_
_Answer._ The steam rises and fills the space above the water, and, if it cannot escape, increases in pressure. The temperature of both the water and the steam rises with the pressure, and will continue to do so as long as the heat is increased, or until the steam can escape or the vessel is exploded. The boiling point also rises as the steam pressure increases.
QUESTION 16. _Is there any pressure which corresponds to the temperature of steam and water?_
_Answer._ Yes. There is a fixed pressure for every temperature, when steam is in contact with water, and its pressure cannot be increased or diminished without at the same time heating or cooling the water, and the higher the temperature of the water the greater will be the corresponding steam pressure. Thus water at 212 degrees produces steam with a pressure equal to that of the atmosphere; at 240 degrees the steam will have a pressure of 25 lbs., or 10 lbs. more than the atmospheric pressure; at 281 degrees a pressure of 50 lbs.; and at 328 degrees, 100 lbs. As this relation of pressure to temperature is fixed, if we know the one we can tell the other. This is true, however, only where the steam is in contact with water, when it is called _saturated steam_. If it is separated from water it may be heated to a higher temperature, and is then called _superheated steam_.
QUESTION 17. _How is the pressure of steam measured?_
_Answer._ In the same way as that of the atmosphere,--that is, by the force exerted on one square inch of surface. Thus if steam is admitted into the cylinder _A_, fig. 8, under the piston _B_, whose area is equal to one square inch of surface, supposing, as we did before, that no power is required to overcome its friction in the cylinder, if the steam thus admitted would just balance the atmosphere, its pressure would be equal to 15 lbs. If, besides overcoming the pressure of the atmosphere, it would raise a weight of 15 lbs., then its pressure per square inch would be equal to 30 lbs. When the atmospheric pressure is _included_ with that of the steam, we call it the _absolute steam pressure_. In ordinary engines, however, the steam must always overcome the pressure of the atmosphere, and therefore the only part of the pressure which is effective is that above, or by which it exceeds, the atmospheric pressure. For example, although the steam admitted under the piston in fig. 8 has an absolute pressure of 30 lbs. per square inch, yet it will only raise a weight of 15 lbs., because it must first overcome the pressure of the air on the other side of the piston. The pressure of the steam used in most stationary and in locomotive engines is, therefore, measured by its pressure above the atmosphere. That is, if steam introduced under the piston in fig. 8 will raise a weight of only 15 lbs., we say it has a pressure of 15 lbs. per square inch; if it will raise 50 lbs., its pressure is said to be 50 lbs. per square inch, and so on. The pressure of the atmosphere is disregarded, and all steam-gauges used on locomotives are graduated in that way. In speaking of steam pressure in future, therefore, unless otherwise specified, we shall mean _effective_ and not _absolute_ pressure.
[Illustration: _Fig. 8._
Scale ¹⁄₄.]
QUESTION 18. _What is meant by the expansion of steam?_
_Answer._ In all gases a repulsion is exerted between the various particles, so that any gas, however small in quantity, will always fill the vessel in which it is held. Steam possesses this same property, and if placed in any vessel the particles in endeavoring to separate from each other will exert a force on all its sides. This force we call the steam pressure. To illustrate this we will suppose that the cylinder _A_ in fig. 8 is half filled with steam of 30 lbs. pressure. If now the supply of steam is shut off, the steam in the cylinder will expand so as to push the piston upward, but with a somewhat diminishing force, the nature of which we will explain hereafter.
QUESTION 19. _What is meant by the volume of steam?_
_Answer._ It means the space which the steam occupies.
QUESTION 20. _What is the proportion which exists between the volume and the pressure of steam?_
_Answer._ If the temperatures remain the same they are INVERSELY PROPORTIONAL TO EACH OTHER; that is, the one increases in the same proportion as the other diminishes. If we admit steam of 30 lbs. pressure per square inch into the cylinder _A_, fig. 8, and then cut off the supply by closing the cock _C_ and allow the steam in the cylinder to expand to double its volume by pushing the piston to the end of the cylinder, the steam pressure will then be only 15 lbs.; if it should expand to three times its volume its pressure would be only one-third, or 10 lbs. per square inch. This method for calculating the pressure of steam after it has expanded is correct only for the _absolute_ and not for the _effective_ pressures of steam. In order to ascertain the effective pressures of steam after expansion, it is only necessary to make the calculation with the absolute pressure and deduct the atmospheric pressure from the result. If, after being thus expanded, the piston be pushed down again so as to compress the steam into its original space, its pressure will again be 30 lbs., providing no heat has been lost in any way.
QUESTION 21. _With a cylinder of any given stroke_[4] _how can we determine approximately the pressure of the steam after expansion for any given point of cut-off?_[5]
[4] The _stroke_ of a piston is the distance it moves in the cylinder, and in ordinary engines is always twice the length of the crank measured from center to center of the shaft and crank-pin.
[5] The steam is said to be _cut off_ when the steam-port by which steam is admitted to the cylinder is closed by the valve.
_Answer._ BY MULTIPLYING THE ABSOLUTE PRESSURE PER SQUARE INCH OF THE STEAM IN THE CYLINDER BEFORE IT IS CUT OFF, BY THE DISTANCE FROM THE BEGINNING OF THE STROKE AT WHICH IT IS CUT OFF, AND DIVIDING THE PRODUCT BY THE WHOLE LENGTH OF THE STROKE. Thus, if we have a cylinder whose piston has a stroke of 24 inches, if we cut off the steam at 8 inches, and have an ABSOLUTE pressure of 90 lbs. in the cylinder, the calculation is as follows:
90 × 8 ------ = 30 lbs. final pressure. 24
If we cut off at 10, 12 and 15 inches, the final pressure would be 37¹⁄₂, 50 and 56¹⁄₄ lbs., respectively. To get the effective pressure deduct the atmospheric pressure from this result.
QUESTION 22. _What is the proportion between the volume of steam and that of the water from which it is formed?_
_Answer._ At the pressure of the atmosphere (15 lbs.) each cubic inch of water will make 1,610 cubic inches of steam. At double that pressure, or 30 lbs. absolute pressure, it will make a little more than half as much, or 838 cubic inches; at four times, or 60 lbs. absolute pressure, 437 cubic inches, or a little more than a fourth as much as at the pressure of the atmosphere.
QUESTION 23. _Why is it that the quantity of steam at high pressures is somewhat greater than in inverse proportion to the pressure?_
_Answer._ Because the boiling-point of water, as has already been explained, is higher as the pressure increases, and therefore the temperature of the steam produced at such pressure is also higher than at lower pressures; and as all gases are expanded by heat, therefore the volume of steam at the higher pressures is somewhat greater than in inverse proportion to its pressure, on account of being somewhat expanded by its high temperature. To make this plain, if we take a cubic inch of water and convert it into steam of atmospheric pressure, its volume will be 1,610 times that of the water, and its temperature 212 degrees.[6] If we convert this quantity of water into steam with a pressure double that of the atmosphere, the volume of the steam will be 838 times that of the water and its temperature will be 250.4 degrees. If the volume of the steam were exactly _inversely proportional_ to the pressure, the cubic inch of water at double the atmospheric pressure would make only 805 cubic inches of steam; but as the boiling-point at that pressure is 38.4 degrees higher, the steam is expanded 33 cubic inches by the increase of its heat due to the higher boiling-point.
[6] More accurately, 213.1 degrees, if we call the atmospheric pressure 15 lbs., as we have.
A table in the appendix gives the pressure, temperature and volume of saturated steam up to 300 lbs. absolute pressure.
QUESTION 24. _What is meant by the condensation of steam?_
_Answer._ It is the reconversion of steam into water by cooling it, or depriving it of part of its heat. It has been shown that the temperature of water must be raised to a certain point to generate steam of a given pressure. If the process is reversed, and we deprive the steam of a part of its heat, some of the steam is then at once reconverted into water, or _condensed_, and the pressure of that which remains will be reduced just in proportion as the heat is lost. When the temperature gets below 212 degrees under atmospheric pressure, all the steam will be condensed. As the useful work which steam can do in an engine is due to its pressure, which in turn depends on its temperature, any loss of heat will diminish its effective power. For this reason, all waste of heat from a steam engine should, as far as possible, be prevented.
QUESTION 25. _How is the heat of the steam wasted or lost in an ordinary steam engine?_
_Answer._ It is wasted in three ways: first, by _conduction_; second, by _convection_; and third, by _radiation_.
QUESTION 26. _What is meant by these three terms?_
_Answer._ 1. By _conduction_ is meant that phenomenon which is manifested when we put one end of a metal bar two or three feet long into the fire and heat it. The heat is then gradually conveyed from one particle of the metal to that next to it until finally the end of the bar farthest from the fire becomes so hot that it cannot be touched. The heat is then said to be _conducted_ through the bar. In the same way the metal of the boiler, pipes, cylinders and other parts of the engine becomes heated on one side, and the heat is thus conveyed to the outside of these parts.
2. The air with which they are surrounded then becomes heated, and being then lighter than the cold air, it rises and is again replaced with air which is not heated. In this way the heat is conveyed away by the air, and this phenomena is therefore called _convection_.
3. If an iron plate be placed in front of an ordinary grate fire three or four feet from it and exposed to the rays of heat from the fire, it will soon become so hot that you cannot bear your hand on it. If you place your hand between the iron plate and the fire you will find that only the side of your hand which is exposed to the fire will become hot, showing that the air between the plate and the fire is not nearly so hot as the plate soon becomes, and therefore that the heat is not conveyed to the plate by the air between it and the fire, but by the rays from the fire. This phenomenon is called _radiation_. The same thing occurs from any hot body, as for example a coil of steam pipe for heating a room, a steam boiler or cylinder of an engine.
QUESTION 27. _Is there any difference in the conducting and radiating power of different substances?_
_Answer._ Yes, very great. The difference in the _conducting_ power of wood and iron is shown if we place one end of a bar of each in the fire. The wood will be consumed without warming the bar more than a few inches from the fire, whereas the iron will soon become hot two or three feet from the fire. Owing to the difference in the conducting power of cotton and wool, we wear cotton clothing in summer and woolen in winter, because cotton allows the heat of the body to be conducted away from it, whereas woolen cloth prevents to a great degree this loss of heat. For the same reason, the venders of roasted chestnuts on our streets wrap them in a piece of blanket to keep them hot, that is, to keep the heat in; and in summer we wrap ice in the same way to keep it cold, that is, keep the warmth of the air out. The wool, being a very bad conductor of heat, simply prevents the heat from being transferred from the inside to the outside, and _vice versa_. It is for this reason that steam boilers, pipes and cylinders are nearly always covered with wood, and sometimes with felt.
The difference in the _radiating_ power of various substances can be shown if we take a large thermometer and heat it up to the temperature of boiling water. If this thermometer is hung up in a room having the temperature of melting ice, it will lose heat in two ways,--first by heating the air which surrounds it, that is by _convection_, and also by _radiation_. In order to confine ourselves to the latter process, we will suppose that the chamber is a vacuum. If we first cover the bulb of the thermometer with a thin coating of polished silver, and then ascertain how much heat it radiates in a minute, and then coat it with lamp-black, and repeat the same experiment,--that is to say, allow the thermometer at the boiling-point to cool for one minute in a vacuum chamber at the freezing-point,--it will be found that the thermometer loses much more in a minute when coated with lamp-black than it did when coated with silver, showing that much more heat is radiated from a surface covered with lamp-black than from polished silver. Generally it may be stated that polished metals radiate much less heat than surfaces which are not polished.[7] For this reason, as well as for ornament, locomotive and other boilers and cylinders are usually covered with Russia iron or polished brass.
[7] The account of the above experiment is copied from Balfour Stewart’s very excellent little book, “Lessons in Elementary Physics,” of which, and the same author’s “Elementary Treatise on Heat,” the writer has made frequent use.