Air Water Vapor Mixtures

Air is a mixture of many gases, predominantly nitrogen and oxygen. Atmospheric air also contains water vapor. The dry-bulb (db) temperature of an air stream is the temperature of the mixture of air and water vapor at rest. Moist air is said to be saturated when it can coexist in equilibrium with an associated condensed moisture phase.

Wet-bulb (wb) temperature is the temperature that will be reached when air and water are mixed with no transfer of heat from the outside. By evaporating, the water cools itself and the surrounding air until saturation is reached and the water can no longer provide additional cooling. While the db temperature indicates the actual temperature of the air, the wb temperature indicates the temperature that can be reached by evaporating enough water into the air to make it fully saturated.

The relationship between db and wb temperature is useful because if both are known, all other properties of the air-water mixture can be read directly from a psychrometric chart. The concept can be understood through a comparison with db temperature determination. Consider the case of two thermometers placed in a moving stream of gas (i.e., air), with the bulb of one covered with a linen, wet with distilled water. The covered thermometer functions as a wb thermometer. Water will evaporate from the wick due to heat transfer to the air at the db temperature and due to radiant heat transfer from the surroundings. The water vapor is diffused from the wick to the surrounding air-water vapor mixture until the evaporation rate reaches equilibrium with the drying capacity of air. At this point, the temperature reached by the wick-thermometer is called the wb temperature.

The wb thermometer will register a lower temperature than the db thermometer as long as evaporation continues. When the wb temperature is less than the db, the air is only partially saturated. The difference between db and wb temperature for a particular state is known as the wb depression. The greater the RH, the smaller the differential between db and wb temperatures, i.e., the wb depression. The maximum depression for a given db temperature will occur when the gas is dry. Zero depression will be observed at saturation, i.e., at 100% RH. Wb and db temperature will be equal because the gas is completely saturated.

When the cooling of an air stream proceeds at constant total pressure, the partial pressures of the constituents remain constant until the saturation state of the vapor is reached and condensation of the vapor occurs. Dewpoint, or condensation temperature, is the temperature at which condensation begins when the moist air mixture is cooled at constant pressure. The dewpoint is equal to the saturation temperature. When air is saturated, it has reached a condition of 100% RH.

Relative humidity (RH or is the ratio of the mole fraction of water vapor in the mixture to the mole fraction of water vapor in saturated air at the same db temperature and barometric pressure. It is also the ratio of the pressure of the vapor in the air to the vapor pressure at saturation corresponding to the temperature. In simpler terms, RH expresses the moisture content of air as a percentage of what it can hold when air is fully saturated. This maximum value increases as the temperature increases, reaching its maximum at 100% RH (saturation).

The humidity ratio (w), or specific humidity, of an air-water mixture is the ratio of the mass of the water vapor to the mass of dry air in the mixture. It is essentially a comparison of the weight of all of the water molecules to the weight of all of the air molecules in a lbm (or kg) of a mixture. Thus, the humidity ratio may be expressed as:

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