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 space and weighs the most per given quantity (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 passes 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 mass 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 passing through the mass, 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 assumed 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 quantity 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.