Modern Machine-Shop Practice Part 247

The total heat of steam is the sensible heat, or that shown by the thermometer, added to the latent heat; hence the heat necessary to evaporate water into steam at a temperature of 212 (which corresponds to a pressure of 14.7 lbs. per square inch) is 212 + 966, which is 1178, and these, therefore, are the number of degrees that must be imparted by the coal to the water, in order to form steam at a temperature of 212.

WATER.

Water is at its greatest density when at a temperature of 39.1 Fahrenheit, that is to say, it occupies its least s.p.a.ce and weighs the most per given quant.i.ty (as per cubic inch) when at that temperature.

At a lower temperature water expands, its freezing point being 32 Fahrenheit, below which it forms ice. The weight of a cubic foot of water when at its maximum density (39.1) is 62.382 lbs. Water also expands as its temperature is increased above 39.1; thus, while it is heated from 39.1 to 212, its volume increases from 1 to 1.04332. The expansion for each degree of heat added to its temperature increases from 0 at 40 Fahrenheit to .0043 at 212.

The rate of expansion of water at a temperature above 212 is unknown.

STEAM.

At every temperature above freezing point water pa.s.ses from the liquid into a gaseous state, the gas being termed steam. While water is below its boiling point its evaporation occurs at its surface only; but when its ma.s.s is heated to boiling point, and additional heat is imparted to it, evaporation occurs from the water lying against the surface from which it receives the heat, and an ebullition is caused by the vaporized water pa.s.sing through the ma.s.s, the ebullition being what is known as boiling.

The temperature at which water boils depends upon the pressure acting upon its surface, the boiling point being at a lower temperature in proportion as the pressure is reduced; thus water at the top of a mountain, where the pressure of the atmosphere is less than at the sea level, would boil at a lower temperature than 212, which is the boiling point when the atmospheric pressure is 14.7 lbs., which it is a.s.sumed to be at the sea level. Conversely, the boiling point is raised in proportion as the pressure upon its surface is raised, whether that pressure consists of air or of steam. As, however, the pressure is increased, the boiling point is at a higher temperature. So long as the steam is in contact with the water both are at the same temperature, as denoted by the thermometer (although they do not contain the same quant.i.ty of heat, as will be show presently), and the steam is termed _saturated_ steam.

The pressure of saturated steam cannot be either increased or diminished without either increasing or diminishing its temperature, hence there is a definite relation of pressure to temperature, which enables the pressure to be known from the temperature, or conversely, the temperature to be known from the pressure. But if the steam be separated from the water and heated, it may be what is termed _superheated_, which is that it may be surcharged with heat or contain more heat than saturated steam at the same pressure. Such additional heat, however, is latent.

The pressure of steam is the lbs. of force it exerts upon a given area, as upon a square inch. In non-condensing engines the effective pressure of the steam is its pressure above that of the atmosphere, because the exhaust side of the piston being exposed to the atmosphere receives the atmospheric pressure, which must be overcome by a corresponding pressure of steam on the steam side of the piston, and this pressure is not, therefore, available for producing work or power in the engine.

In condensing engines, however, the exhaust side of the piston is (as nearly as practicable), relieved of the atmospheric pressure, and a.s.suming a perfect vacuum to be formed, the whole of the steam pressure is exerted to propel the piston, in which case the steam pressure is termed the _absolute_ pressure.

In considering the weight or density or the expansion of steam, its _absolute_ and not its effective pressure must obviously be taken.

What is termed dry steam is _saturated_ steam that does not contain what may be termed entrained water, which is water held in suspension in the steam, which may be caused by the surface of the water through which the steam is allowed to rise being too small in proportion to the volume of steam formed, in which case the rapid pa.s.sage of the steam through the water causes it to carry up water with it and hold it in suspension, this action being termed _foaming_ or _priming_.

Suppose, for example, that a boiler be filled with water up to the bottom of the steam dome, then all the steam formed would require to find exit from the water within the area of the dome, and the violence of the ebullition would cause foaming. Obviously, then, to obtain dry steam there must be provided a sufficient area of water surface for the steam to pa.s.s through.

But water so entrained is evaporated into steam, if the steam is wire drawn, that is, allowed to expand and reduce in pressure.

THE EXPANSION OF STEAM.

A cubic inch of water, when evaporated into steam at a pressure of 14.7 lbs. per square inch, occupies as steam a s.p.a.ce or volume of 1644 cubic inches, and its weight will be equal to that of the water from which it was evaporated.

If additional heat be imparted (after its evaporation into steam), such additional heat becomes latent and does not cause an increase of sensible temperature or of pressure.

The weight of a given volume of steam, therefore, bears a definite and constant relation to the pressure and sensible temperature of the steam, so that the pressure or the sensible temperature being known, the weight of a given volume, as say a cubic foot, may be known therefrom. Or the weight of a cubic foot of steam being known, its sensible temperature and pressure may be known therefrom.

This would not be the case if steam expanded by heat. Suppose, for example, we have a cubic foot of steam at any absolute pressure, as say 15 pounds per square inch, a cubic foot weighing .0387 of a lb., and its sensible temperature will be 213. Now it is evident that the weight will remain the same whatever the amount of heat that may be imparted to the steam. Now if the steam were maintained within the cubic foot of s.p.a.ce, and was capable of expansion by the absorption of additional heat, its pressure would increase and its weight remaining the same, there would be no definite relation between the weight and the temperature and pressure.

But if the cubic foot of steam were allowed to expand so as to occupy more s.p.a.ce, then additional heat is necessary to prevent its condensation.

The relation between the temperature, pressure, and weight of steam is not quite proportional to the volume, because steam is not a perfect gas, and does not, therefore, strictly follow Mariotte's law.

A perfect gas is one that during expansion or compression follows the law laid down by Boyle and Mariotte, this law being that, if maintained at a constant temperature, the volume is inversely proportional to the pressure.

For example, the quant.i.ty of gas that, if confined in a cubic inch of s.p.a.ce, would give a pressure of 80 lbs. per square inch, would give a pressure twice as great (or 160 lbs. per inch of area), if confined in one-half the s.p.a.ce, that is, if compressed into one-half of a cubic inch. Conversely, if the cubic inch was allowed to expand until its pressure was 40 lbs., it would occupy 2 cubic inches of s.p.a.ce, a.s.suming, of course, that the temperature remains the same. Since, however, if a gas be compressed, its temperature is increased by reason of the friction of the particles moving one upon the other, the law of Mariotte may be better explained as follows:

Suppose we have three vessels, A, B, and C, filled with a fluid which is a perfect gas, the temperatures being equal. Let the pressure be: A 40, B 80, and C 160 lbs. per square inch, then 2 cubic inches of the fluid in B will weigh the same as 4 cubic inches in A, because that in B is at twice the pressure of that in A, and the 2 cubic inches in B will weigh the same as 1 cubic inch in C, because its pressure is one-half that of C, or, what is the same thing, whatever number of cubic inches of the fluid in C it takes to weigh a pound, it will take twice as many in B, and four times as many in A to weigh one pound.

But steam is not a perfect gas, as is evidenced by the fact that its volume does not increase in a ratio inverse to its pressure. For example, if a cubic inch of water be evaporated into steam at a pressure of 14.7 lbs. per square inch, its volume will be 1644 cubic inches, and its temperature 212 Fahrenheit.

But if the cubic inch of water be evaporated into steam at twice the pressure, which is 29.4 lbs per square inch, its volume will be 838 inches.

The volume then is not inversely as the pressure, although the actual quant.i.ty and weight remain the same, as is proven by the fact that if the steam at either pressure were condensed it would pa.s.s back into the cubic inch of water from which it was generated.

This may be accounted for in the difference in the boiling point of the water in the two pressures, or in other words, by the difference in the temperatures; thus the boiling point of the water at a pressure of 14.7 lbs. is 212, while that for the pressure of 29.4 is increased about 38.4 degrees, and the steam is at the higher pressure expanded by these 38.4 degrees of heat, which adds to its pressure, although not affecting its actual quant.i.ty or weight.

The amount of this expansion may be estimated as follows:

Taking the 1644 cubic inches, and supposing the steam to be a perfect gas, we divide it by 2 to obtain half the volume, 1644 2 = 822.

If then we subtract this 822, which is the volume of the steam if it acted as a perfect gas from the 838 it actually occupies, we get 16 (838-822 = 16), which is the number of cubic inches of expansion due to the increase in the boiling temperature.

THE CONVERSION OF HEAT INTO WORK.

When steam performs work a certain portion of the heat it contains is converted into work, the steam simply being a medium of conveying the heat into the cylinder in which the motion of the piston converts this proportion of heat into work. It has been proven that a given quant.i.ty of heat will pa.s.s into a given quant.i.ty of work, and conversely that a given quant.i.ty of work is convertible into a given quant.i.ty of heat, and it has also been proven that so much heat is convertible into so much work, independent of the temperature of the heat during its conversion into work, power, or energy, all three of these words being used to imply pressure, force, or weight in motion.

The accepted measurement of the conversion of heat into work is known as _Joule's equivalent_; _Joule_ having determined that the amount of power exerted in raising 772 lbs. one foot is the equivalent of the amount of heat that is required to raise the temperature of 1 lb. of water when at or near its freezing point (that is, at a temperature of 32) one degree.

This is called the _mechanical equivalent of heat_, being merely the quant.i.ty of heat necessary to do a certain amount of work, but having no relation to the time in which that work was done.

The conversion of heat into work and of work into heat may be demonstrated as follows: Suppose a cylinder to be so situated that heat can neither be transferred to it or from it, and that saturated steam be admitted under the piston so as to fill one-half of the cylinder at a pressure of 50 lbs.

Suppose then that we raise the piston from an independent application of power, the steam simply expanding to fill the s.p.a.ce given by the piston, but not exerting its force to move the piston.

Now suppose the experiment is repeated, permitting the force of the steam to lift the piston, and the temperature of the steam will be less in the second than it was in the first, proving that in the second experiment a certain portion of the heat in the steam was converted into the work of raising the piston.

If we desire to reconvert the work into heat, we may force the piston back again to its original position, and its temperature will be restored to what it was before we allowed it to raise the piston. It is here, of course, a.s.sumed that there is no friction in moving the piston in the cylinder.

The apparent or external work performed by steam in expanding and moving a piston against a given resistance is measurable by multiplying the amount of the resistance against which the piston moved by the distance it moved through, thus:

Suppose a piston weighs 100 lbs. and had resting upon it a weight of 50 lbs., and that it be raised by the expansive action of steam a distance of a foot, then, since the total resistance it moved against would be (supposing it to move frictionless in the cylinder) 150 lbs., and since the amount of motion was 1 foot, the external or apparent amount of work performed by the steam will be 150 foot lbs., or 150 lbs. moved 1 foot.

But in expanding, the steam has performed a certain amount of what is called _internal_ work, that is to say, its particles or atoms have done work in expanding, and this work has been done at the expense of some of the heat in the steam, so that the loss of heat due to the motion of the piston is the amount of heat converted into work in moving the piston against the piston resistance, added to that converted into the internal work due to the expansion of the steam.

It is because of this internal work that the steam in expanding does not strictly follow Mariotte's law.

The mechanical theory of heat is, that the atoms of which bodies are composed are at absolute rest when at a temperature of 461.2 below the zero of Fahrenheit, which is supposed to be absolute cold, and at any degree of temperature above this the atoms are in motion; the extent and force of their motion determines what we know as the temperature of the body.

Atoms are capable of transmitting their motion to adjoining atoms of the same or of other bodies, losing, of course, the amount of motion they transmit, and it is in this way that heat is conveyed from one to another part of the same body, or from one body to another, this being known as the heat of _conduction_.

But heat may be conveyed by means of what is known as _radiation_, and also by _convection_.

Thus, the air surrounding a heated body becomes heated, and by reason of its expansion it then becomes lighter and rises, a fresh supply of cooler air taking its place, becoming in turn heated, and again giving place to cooler air; the heat thus conveyed away by the fluid or air is conveyed by what is termed _convection_.