Unsaturated Gas-Vapor Mixtures

If the partial pressure of the vapor in a gas-vapor mixture is less than the equilibrium vapor pressure of the liquid at the same temperature, the mixture is unsaturated. This condition is called partial saturation. In this case, to completely define the thermodynamic state of the mixture, the degree of saturation of the mixture must 

Relative saturation (relative humidity for the system air-water), expressed as a percentage, is defined as 100pA/PA, where PA is the vapor pressure at the dry-bulb temperature of the mixture. Therefore, if the relative saturation, temperature, and pressure of a gas-vapor mixture are specified, all of its thermodynamic properties can be calculated. 

Another way of fixing the state of an unsaturated mixture is to specify its dew point. This is the temperature at which a gas-vapor mixture becomes saturated when cooled at constant total pressure. In this case, the partial pressure of the vapor in the unsaturated mixture equals the vapor pressure of the liquid at the specified dew-point temperature. 

An air-water vapor mixture at a dry-bulb temperature of 328 K and a pressure of 1 atm has a relative humidity of 30%. Calculate for this mixture  

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The wet-bult temperature is the steady-state temperature achieved by a small quantity of liquid evaporating into a large quantity of unsaturated gas-vapor mixture. The wet-bulb temperature is essentially a measure of the humidity of a gaseous mixture. We can describe the usefulness of this parameter by examining the behavior of a liquid particulate surrounded by a moving stream of unsaturated gas-vapor mixture. Figure 2.4 illustrates the system under consideration. 

Calculate the concentration (in terms of absolute humidity and molal absolute humidity) and enthalpy per unit mass of dry gas of saturated and unsaturated gas-vapor mixtures. 

The wet-bulb temperature is the steady-state temperature reached by a small amount of liquid evaporating into a large amount of unsaturated gas-vapor mixture. Under properly controlled conditions it can be used to measure the humidity of the mixture. For this purpose a thermometer whose bulb has been covered with a wick kept wet with the liquid is immersed in a rapidly moving stream of the gas mixture. 

Consider a drop of liquid immersed in a rapidly moving stream of unsaturated gas-vapor mixture. If the liquid is initially at a temperature higher than the gas dew point, the vapor pressure of the liquid will be higher at the drop surface than the partial pressure of the vapor in the gas, and the liquid will evaporate and diffuse into the gas. The latent heat required for the evaporation will at first be supplied at the expense of the sensible heat of the liquid drop, which will then cool down. As soon as the liquid temperature drops below the dry-bulb temperature of the gas, heat will flow from the gas to the liquid, at an increasing rate as the temperature difference becomes larger. Eventually the rate of heat transfer from the gas to the liquid will equal the rate of heat required for the evaporation, and the temperature of the liquid will remain constant at some low value, the wet-bulb temperature Tw. The mechanism of the wet-bulb process is essentially the same as that governing the adiabatic saturation process, except that in the wet-bulb process the humidity of the rapidly moving gas is assumed to remain constant. 

As a precaution, therefore, a Roots pump should always be inserted in front of the condenser at low inlet pressures so that the condensation capacity is essentially enhanced. The condensation capacity does not depend only on the vapor pressure, but also on the refrigerant temperature. At low vapor pressures, therefore, effective condensation can be obtained only if the refrigerant temperature is correspondingly low. At vapor pressures below 6.5 mbar, for example, the insertion of a condenser is sensible only if the refrigerant temperature is less than 0 °C. Often at low pressures a gas -vapor mixture with unsaturated water vapor is pumped (for further details, see Section 2.1.5). In general, then, one can dispense with the condenser. 

Enthalpy The (relative) enthalpy of a vapor-gas mixture is the sum of the (relative) enthalpies of the gas and of the vapor content. Imagine unit mass of a gas containing a mass Y of vapor at diy-bulb temperature t. If the mixture is unsaturated, the vapor is in a superheated state and we can calculate the... 

If a stream of air is intimately mixed with a quantity of water in an adiabatic system, the temperature of the air will drop and its humidity will increase. If the equilibration time or the number of transfer units approaches infinity, the air-water mixture will reach saturation. The acUabatic saturation temperature T is given by a heat balance between the initial unsaturated vapor-gas mixture and the final saturated mixture at thermal equilibrium ... 

Unsaturated Vapor-Gas Mixtures Psychrometry in Relation to Drying.8... 

This is the temperature at which a vapor-gas mixture becomes saturated when cooled at a constant total pressure out of contact with a liquid (i.e., at constant absolute humidity). The concept of the dew point is best illustrated by referring to Figure 1.3, a plot of the absolute humidity versus temperature for a fixed pressure and the same gas. If an unsaturated mixture initially at point F is cooled at constant pressure out of contact of Uqnid, the gas saturation increases until the point G is reached, when the gas is fully saturated. The tanperature at which the gas is fully saturated is called the dew point 7],. If the temperature is reduced to an infinitesimal amount below Tj, the vapor will condense and the process follows the saturation curve. 

One of the oldest and best-known methods of determining the humidity of a gas is to measure its wet bulb temperature and its dry bulb temperature. The wet bulb temperature is the steady temperature reached by a small amount of liquid evaporating into a large amount of rapidly moving unsaturated vapor-gas mixture. It is measured by passing the gas rapidly past a thermometer bulb kept wet by a saturated wick and shielded from the effects of radiation. If the gas is unsaturated, some liquid is evaporated from the wick into the gas stream, carrying with it the associated latent heat. This latent heat is taken from within the liquid in the wick, and the wick is cooled. As the temperature of the wick is lowered, sensible heat is transferred by convection from the gas stream and by radiation from the surroundings. At steady state, the net heat flow to the wick is zero and the temperature is constant. 

Dew point This is the temperature at which a vapor-gas mixture becomes saturated when cooled at constant total pressure out of contact with a liquid. For example, if an unsaturated mixture such as that at F (Fig. 7,4) is cooled at constant pressure out of con,tact with liquid, the path of the cooling process follows the line FG, the mixture becoming more nearly saturated as the temperature is lowered, and fully saturated at the dew-point temperature. All mixtures of absolute humidity Y on this figure have the same dew point. If the temperature is reduced only an infinitesimal amount below t p, vapor will condense as a liquid dew. This is used as a method of humidity determination a... 

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