Water vapor in saturated air

Table 5.27 Compressibility of Water Table 5.28 Mass of Water Vapor In Saturated Air Table 5.29 Van der Waals Constants for Gases Table 5.30 Triple Points of Various M aterlals 5.9.1 Some Physical Chemistry Equations for Gases...

Humidity, relative The ratio of the mole fraction of water vapor in moist air to the mole fraction of water vapor in saturated air at the same temperature and pressure. 

Atmospheric air can be viewed as a mixture of dry air and water vapor, and the atmospheric pressure is the sum of the pressure of dry air and the pressure of water vapor, which is called the vapor pressure P . Air can hold a certain amount of moisture only, and the ratio of the actual amount of moisture in the air at a given temperature to the maximum amount air can hold at that temperature is called the relative humidity (f>. The relative humidity ranges from 0 for dry air to 100 percent for saturated air (air that cannot hold any more moisture). The partial pressure of water vapor in saturated air is called the saturation pressure Table 14 9 lists the saturation pressure at various teraperaturest... 

Relative Humidity (rh). Relative humidity is the ratio of the mole fraction of water vapor present in the air to the mole fraction of water vapor present in saturated air at the same temperature and barometric pressure it approximately equals the ratio of the partial pressure (or density) of the water vapor in the air to the saturation pressure (or density) of water vapor at the same temperature. 

Saturation Temperature. The temperature at which the water vapor in moist air is in equiUbrium with Hquid water. 

When the partial pressure p of water vapor in the air at a given temperature equals the vapor pressure of water p, at the same temperature, the air is saturated and the absolute humidity is designated the saturation humidity H,. 

The real atmosphere is more than a dry mixture of permanent gases. It has other constituents—vapor of both water and organic liquids, and particulate matter held in suspension. Above their temperature of condensation, vapor molecules act just like permanent gas molecules in the air. The predominant vapor in the air is water vapor. Below its condensation temperature, if the air is saturated, water changes from vapor to liquid. We are all familiar with this phenomenon because it appears as fog or mist in the air and as condensed liquid water on windows and other cold surfaces exposed to air. The quantity of water vapor in the air varies greatly from almost complete dryness to supersaturation, i.e., between 0% and 4% by weight. If Table 2-1 is compiled on a wet air basis at a time when the water vapor concentration is 31,200 parts by volume per million parts by volume of wet air (Table 2-2), the concentration of condensable organic vapors is seen to be so low compared to that of water vapor that for all practical purposes the difference between wet air and dry air is its water vapor content. 

Relative Humidity ratio of the vapor pressure of water in air compared to the saturated vapor pressure of pure water at the same temperature, measure of the amount of water vapor in an air mass expressed as percent of how much water vapor that air mass can hold Relative Mass mass measured with respect to a standard, atomic masses based on C-12 standard... 

Isotopic fractionation resulting from evaporation from standing water bodies can be described in terms of equilibrium and nonequilibrium fractionation effects. Equilibrium fractionation occurs when the isotopic composition of the evaporated water or lake evaporate is in thermodynamic equilibrium with the lake water (23). Equilibrium fractionation, however, can occur only when the water vapor in the air mass above the lake is 100% saturated. The process of equilibrium isotopic fractionation is described by Raleigh fractionation. The isotopic composition of water vapor in equilibrium with liquid water at any time is given by... 

The principle of the saturated sail dew point sensor is based on the relationship that the vapor pressure of water is reduced in the presence of a salt. When water vapor in the air condenses on a soluble salt, it forms a saturated layer on the surface of the salt. This saturated layer has a lower... 

Water vapor in the air can be regarded as low-temperature, low-pressure steam the amount of water in the air (its relative humidity) is dictated by the air temperature, with hotter air holding more water vapor before the saturation point is reached. 

An uninsulated, chilled water pipe carrying water at 2°C passes through a hot, humid factory area where the temperature is 35°C and the relative humidity is 80 percent because of steam-operated equipment in the factory. If the pipe is 5 cm in diameter and the exposed length is 7.5 m, estimate the condensate which will drip off the pipe. For this estimate assume that the pipe is exposed to saturated vapor at the partial pressure of the water vapor in the air. 

Properties The saturation pressure of water at IS C is 1.705 kPa (Table A-9). Analysis The air at the water surface is saturated. Therefore, the partial pressure of water vapor in the air at the lake surface simply is the saturation pressure of water at IS C,... 

SOLUTION The liquid layer on the inner surface of a circular pipe is dried by blowing air through it, The mass transfer coefficient is to be determined. Assumptions 1 The low mass flux model and thus the analogy between heat and mass transfer is applicable since the mass fraction of vapor in the air is low (about 2 percent for saturated air at 300 K). 2 The flow is fully developed, Properties Because of lov/ mass flux conditions, v/e can use dry air properties for the mixture at the specified temperature of 300 K and l atm, for which v = 1.58 X 10 mVs (Table A-15). The mass diffusivity of water vapor in the air at 300 K is determined from Eq. 14-15 to be... 

Application of Raoult s law to species i requires a value for Pf at tlie temperature of ap-plieation, and thus is not appropriate for a speeies whose eritieal temperature is less than the temperature of application. If a system of air in eontact with liquid water is presumed at equilibrium, tlien the air is saturated with water. The mole fraction of water vapor in tlie air is usually found from Raoult s law applied to the water with the assimiption that no air dissolves in the liquidphase. Thus, the liquid water is regarded as pure and Raoult s law for the water (species 2) becomes y2 = At 298.15 K (25°C) and atmospherie pressure, this equation yields ... 

The saturation humidity (/ts ) absolute humidity at which the partial pressure of water vapor in the air is equal to the vapor pressure of pure bulk water at a particular temperature. Since there would be no difference in vapor pressure, there would be no concentration gradient and hence no evaporation at the saturation humidity. 

The relative humidity ( ) may be expressed as the ratio of the actual concentration of water vapor in the air to the saturation concentration of water vapor in the air under the... 

Curves showing the relative humidity (ratio of the mass of the water vapor in the air to the maximum mass of water vapor the air can hold at that temperature, i.e., if the air were saturated) of humid air appear on the psychrometric chart. (See Figure 2.) The curve for 100% relative humidity is also referred to as the saturation curve. The abscissa of the humidity chart is air temperature, also known as the dry-bulb temperature (T b). The wet-bulb temperature (7 yb) is another measure of humidity it is the temperature at which a thermometer with a wet wick wrapped around the bulb stabilizes. As water evaporates from the wick to the ambient air, the bulb is cooled the rate of cooling depends on how humid the air is. No evaporation occurs if the air is saturated with water hence WB and Fob are the same. The lower the humidity, the greater the difference between these two temperatures. On a psychrometric chart, constant wet-bulb temperature lines are straight with negative slopes. The value of 7 vb corresponds to the value of the abscissa at the point of intersection of this line with the saturation curve. 

The Kelvin equation can be combined with the relative humidity, RH, if water is involved as the fluid relative humidity indicates how moist the air is. The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percentage of saturation humidity, generally calculated in relation to the saturated vapor density. Relative humidity may be defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density, usually expressed in percent. Relative humidity is also approximately equal (exactly equal when water is assumed as an ideal gas) to the ratio of the actual water vapor pressure to the saturation water vapor pressure, RH = PJP°. The P° values corresponding to each temperature are given in tables which can be found in handbooks. If RH is measured in an experiment, then Pv can be calculated by using the saturation water vapor pressure tables and can be inserted into the Kelvin equation. 

The moisture content of the air is not a discrete system in itself. The amount of water vapor in the air is intricately linked to the temperature of the environment. Therefore, in order to get a proper perspective on the water-wood interaction, it is important to understand the relationship between temperature and relative humidity (RH). RH is a measure of how much water vapor is in the environment relative to the maximum amount of water vapor that the air could possibly hold (saturation) at a certain temperature. 

The dry-bulb temperature is the thermometer reading in the absence of humidity and radiation it is the temperature most often reported by meteorological agencies. The dew point is the temperature at which the water vapor in the air first begins to condense the dew point equals the measured temperature when air is 100% saturated—100% humidity. 

Dew point (dew temperature) (ca. 1833) n. Temperature of the atmosphere at which the saturation vapor pressure equals the actual vapor pressure of the water vapor in the air and dew begins to form. 

Saturated air is air in which the water vapor is in equilibrium with liquid water at the given conditions of pressure and temperature. In this mixture the partial pressure of the water vapor in the air-water mixture is equal to the vapor pressure p s of pure water at the given temperature. Hence, the saturation humidity Hg is... 

The vapor pressure of water, denoted 7i , is the independent pressure exerted by the water vapor in the air, and it is expressed in Pa. The natural tendency for pressures to equalize will cause moisture to migrate from an area of high vapor pressure to an area of low vapor pressure. The saturation vapor pressure varies with temperature. 

When a volume of air at a given temperature holds the maximum amount of water vapor, the air is said to be saturated. Hence, relative humidity, sometimes called humidity ratio or degree of saturation denoted RH, is a dimensionless quantity which corresponds to the water-vapor content of the air relative to its maximum content at saturation at the same dry-bulb temperature. Relative humidity can also be regarded as the ratio of the mole fraction of water vapor in moist air to the mole fraction in moist air at saturation at the same temperature, and pressure. By use of the ideal gas law, this can be expressed as the ratio of the partial pressure of water vapor to the water vapor pressure at the same temperature. Therefore, relative humidity is the ratio of absolute humidity to absolute humidity at saturation, or the ratio of partial water vapor pressure, and vapor pressure of water at a given temperature ... 

Saturation Point Point where the water vapor in the air is at its maximum for a given temperature and pressure point where condensation occurs. 

Let us examine this phenomenon in the laboratory setting. If we were to half-fill a beaker with water and set it on the laboratory bench, it would slowly evaporate, as pictured on the left of Figure 9.9. Eventually, perhaps over a period of days or weeks, the water would totally evaporate. If, however, we covered and sealed the beaker so that no water vapor could escape into the atmosphere, eventually the water vapor would saturate the air space in the beaker and create a condition of 100% humidity. At that point, the rate of evaporation equals the rate of condensation, as pictured on the right of Figure 9.9, and an equilibrium is achieved. This means that the concentration of the water vapor in the air space is constant and the liquid water would never totally evaporate. 

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