Humidity of saturated air

Humidity of saturated air Mo. This is the humidity of air when it is saturated with water vapour. The air then is in equilibrium with water at the given temperature and pressure. 

The following terms are employed in discussing drying humidity and humidity of saturated air, relative... 

On Fig. 23.2 temperatures are plotted as abscissas and humidities as ordinates. Any point on the chart represents a definite mixture of air and water. The curved line marked 100 percent gives the humidity of saturated air as a function of air temperature. By using the vapor pressure of water, the coordinates of points on this line are found from Eq. (23.3). Any point above and to the left of the saturation line represents a mixture of saturated air and liquid water. This region is important only in checking fog formation. Any point below the saturation line represents undersaturated air, and a point on the temperature axis represents dry air. The curved lines between the saturation line and the temperature axis marked in even percents represent mixtures of air and water of definite percentage humidities. As shown by Eq. (23.5), linear interpolation between the saturation line and the temperature axis can be used to locate the lines of constant percentage humidity. 

Figure 3 is the humidity chart diagram in molar quantities where enthalpy deviations are not given. Enthalpy may also be calculated from the enthalpy of saturated air and of dry air using % saturation ... 

Adiabatic-Saturation Temperature, or Constant-Enthalpy Lines If a stream of air is intimately mixed with a quantity of water at a temperature t, in an adiabatic system, the temperature of the air will drop and its humidity will increase. If t, is such that the air leaving the system is in equihbrium with the water, t, will be the adiabatic-saturation temperature, and the line relating the temperature and humidity of the air is the adiabatic-saturation line. The equation for the adiabatic-saturation line is... 

Dewpoint means the temperature of saturated water vapor that has the same vapor pressure as the humid air in question. When the total pressure is constant, the constant vapor pressure means the same as the humidity x. In other words, dewpoint is the temperature of saturated air that has the same humidity as the air being considered. 

Acid Dew Point For fossil fuels, the acid dew point temperature is that temperature at which the actual mixed acid vapor pressure equals the mixed acid vapor saturation pressure. The mixed acid dew point can be approximated by the sulfuric acid dew point (Fig. 24-56). It can be described as a function of the SO3 and water content of the flue gas (Huijbregts). These concentrations result from the sulfur, hydrogen, and free water content of the fuel the relative humidity of the air and the amount of excess air used. Using the equation of Ver-hoff, where T is degrees K and P is mm Hg (see OUces, A.G.) ... 

Suppose we take a sample of bone-dry air at some temperature, Ti, and directly contact it with water until it becomes saturated at the same temperature. The water vapor that enters into the air contains with it its latent heat of vaporization. The vapor pressure of water out of the liquid will be greater than it is in the saturated air, causing vaporization to occur and subsequently increasing the humidity of the air-water-vapor mixture. The process of vaporization ends when the vapor pressure of the water in the air becomes equal to that of the liquid. At this condition the air is saturated. During the air saturation process, isothermal conditions for the water can be maintained if heat is supplied to replace the heat lost from it to the gas as latent heat of vaporization. Thus, heat transfer during the saturation of a gas with a liquid can be accomplished without a temperature differential (although this is rarely encountered). This type of heat transfer phenomenon, better known as diffusional heat transfer, is different from conduction, convection or radiation. 

The water content of concrete and cement mortars depends on the temperature and relative humidity of the air and fluctuates between 1% and less at 20°C and 60% relative humidity up to 10% in air saturated with humidity.395 In case of permanently high humidity, penetrating wetness from outside, a huge part of the pore system can be filled with water.400... 

The evaporation of moisture into a warm air stream, with the latter providing the latent heat of evaporation, is a common drying mechanism although it is not easily adapted to the recovery of the liquid. In the evaporation from a liquid surface which, with the passage of air, falls to the wet bulb temperature corresponding to the temperature and humidity of the air, the rate at which water vapor is transferred from the saturated layer at the surface to the drying stream is described by Eq. (15),... 

The dew point (T p) is the temperature to which a particular mixture of air and water vapor must be cooled to become saturated with respect to water vapor. If the mixture is cooled below the dew point, then the system becomes supersaturated and it will separate into a two-phase system of saturated air and liquid water. Many of the best humidity meters are actually dew point detectors. 

As the water from the wick evaporates, the wick cools down and continues to cool until the rate of energy transferred to the wick by the air blowing on it equals the rate of loss of energy caused by the water evaporating from the wick. We say that the temperature of the bulb with the wet wick at equilibrium is the wet-bulb temperature. (Of course, if water continues to evaporate, it eventually will all disappear, and the wick temperature will rise.) The equilibrium temperature for the process described above will lie on the 100% relative humidity curve (saturated-air curve). 

Water flow in the vadose zone also is much more difficult to measure than is water flow in the saturated zone. Piezometers placed in unsaturated porous media do not fill with water, so head is less readily measured than it is in saturated media, and thus the direction and magnitude of the hydraulic gradient are more difficult to determine. For values of ifj between 0 and — 1 atm, tensiometers may be employed to directly measure pore water pressure. For very low values of i/j, other techniques may be used, such as the indirect assessment of if from measurements of the humidity of the air in the pore spaces. 

Some typieal moisture content values for green wood are noted in Table 3.1. These values are considerably greater than the fibre saturation point. Absorbed water at the surface will evaporate and the lumber will dry provided the surrounding atmosphere is not totally humid. Indeed the absorbed water in the lumens cannot remain there in equilibrium with the atmosphere unless the relative humidity of the air is in excess of 99% (Table 3.3). If the wood is left under cover - keeping the rain off - it will eventually dry to a moisture content that will vary according to the temperature and humidity of the air (Figure 3.2). This moisture content will be below the fibre saturation point so all the absorbed water and some of the adsorbed water will have evaporated. If an even lower moisture content is required it is necessary to use a kiln to lower the relative humidity and raise the temperature (Figure 3.2). 

Relative humidity of air is defined as the vapour pressure of water vapour in a given volume of air divided by the saturated vapour pressure. The absolute humidity at saturation (in g m ) and the saturated vapour pressure of water are shown in Table 8.1. Thus a cubic metre of saturated air can hold 17 g of water vapour at 20°C, 130 g at 60°C and 600 g at 100°C. In the wider drying literature, absolute humidity is normally given as a mass ratio of water vapour to the mass of dry air (kg kg dry basis), since the flow rates of dry air and dry solids through any process are normally constant. 

Moist warm air from the stacks is drawn through the evaporator coils and is cooled until the absolute humidity of the air corresponds to the absolute humidity at saturation (Table 8.1), the dew point. On further cooling moisture condenses on the cold evaporator coils. This water is collected and drained from the kiln. The cold air then passes through the warm condenser coils where it is reheated. The dry warm air then passes through the timber stacks again. 

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