degree of humidity for different dry- and wet-bulb readings of the psychrometer. STEAM Steam is the vapor of water formed at any temperature at or above the boiling point of the water. It is a certain vaporized or gaseous state of water. Water vaporizing below its boiling point forms vapor but not steam. Thus, while all steam is vapor, correctly speaking, all vapor is not steam. Steam in its natural state or when saturating a given space, has a temperature corresponding to the pressure it supports. This will be more clearly understood by taking an example of a given volume of steam in contact with the water from which it was formed. For instance, the steam in a steam boiler, at a pressure of 65 lb. gage (sea level) or, say 80 lb. absolute, has a temperature of 312 deg. F. But any increase of pressure will be accompanied with a corresponding increase in its temperature, so that, at a pressure of 155 lb. absolute, the temperature of the steam will have increased to 361 deg. F. Again, assuming a given volume of steam in contact with the water from which it was formed, such steam can neither be compressed nor expanded without a corresponding change taking place in its temperature. For example, for the same temperature, any increase of pressure would cause some of the steam to condense, while a decrease of pressure would cause more steam to form, as the water would vaporize under the decreased pressure. Thus, the space above the water is always saturated by a weight of steam corresponding to the temperature, which is fixed for any given pressure. Saturated Steam.-Saturated steam may be defined as steam in contact with water. From the foregoing, it will be understood that saturated steam is in its natural state, having a temperature corresponding to its pressure. Saturated steam may be either dry or wet, according as it does or does not hold any entrained water. The density of dry saturated steam is always the same for the same temperature. Superheated Steam.-When steam is not in contact with water, any addition of heat causes an increase in both temperature and pressure, the pressure increasing with the absolute temperature. The steam is no longer saturated, and is said to be "superheated." Superheated steam is always dry. Unlike saturated steam, superheated steam follows the laws of a perfect gas. For a constant volume, its pressure increases with the absolute temperature; and, for a constant temperature, the pressure increases inversely as its volume. Steam is superheated, therefore, whenever its temperature exceeds that of saturated steam, for any given pressure. 70 80 90 100 110 120 130 140 150 160 170 180 190 200 FIG. 10. Steam Tables.-The table, in the following pages, gives the temperature, specific volume, heat of the liquid above 32 deg. F., latent heat of evaporation, and the total heat in the steam, for different absolute pressures, as taken from Marks & Davis Steam Tables, which are the generally accepted values, today. The diagram, Fig. 10, was compiled by J. T. Beard, Jr. from the same source, and will be found convenient for use in connection with the tables. SECTION III MINE GASES GEOLOGICAL CONDITIONS-COMMON MINE GASES-HYDROCARBON GASES-PROPERTIES AND BEHAVIOR OF MINE Gas, Oil and Water. The strata of the earth's crust form a great natural reservoir for gas, oil and water. These collect in the formations, in the order of their relative densities. As illustrated in the Fig. 11, which represents an ideal geo logical section, the subterraneous water collects in the lower permeable strata, the oil next above, while the gas is found higher on the anticline. This condition is only true, however, in a general way, depending on the nature of the strata and their power to absorb and hold these elements. Water, and oil to a less extent, find their way by gravity to a "hard-pan" or stratum impervious to them; while gas drains to the surface and escapes, unless confined by an overlying stratum of clay or cil, from the overlying rocks into the synclinal basins, creates enormous pressures, which are exerted more or less equally on the water, oil and gas. |