Vapor free gas

Another way to express vapor concentration is by molal saturation, which is the ratio of the moles of vapor to the moles of vapor-free gas. 

DEFINITIONS. In humidification operations, especially as applied to the system air-water, a number of rather special definitions are in common use. The usual basis for engineering calculations is a unit mass of vapor-free gas, where vapor means the gaseous form of the component that is also present as liquid and gas is the component present only in gaseous form. In this discussion a basis of a unit mass of vapor-free gas is used. In the gas phase the vapor will be referred to as component A and the fixed gas as component B. Because the properties of a gas-vapor mixture vary with total pressure, the pressure must be fixed. Unless otherwise specified, a total pressure of 1 atm is assumed. Also, it is assumed that mixtures of gas and vapor follow the ideal-gas laws. 

Humidity yf is the mass of vapor carried by a unit mass of vapor-free gas. So defined, humidity depends only on the partial pressure of the vapor in the mixture when the total pressure is fixed. If the partial pressure of the vapor is... 

Relative humidity is dehned as the ratio of the partial pressure of the vapor to the vapor pressure of the liquid at the gas temperature. It is usually expressed on a percentage basis, so 100 percent humidity means saturated gas and 0 percent humidity means vapor-free gas. By definition... 

Hamid volume Vu is the total volume of a unit mass of vapor-free gas plus whatever vapor it may contain at 1 atm and the gas temperature. From the gas laws, tijj in fps units is related to humidity and temperatures by the equation... 

Total enthalpy Hy is the enthalpy of a unit mass of gas plus whatever vapor it may contain. To calculate H, two reference states must be chosen, one for gas and one for vapor. Let Tq be the datum temperature chosen for both components, and base the enthalpy of component A on liquid A at Tq. Let the temperature of the gas be T and the humidity yf. The total enthalpy is the sum of three items the sensible heat of the vapor, the latent heat of the liquid at Tq, and the sensible heat of the vapor-free gas. Then... 

EQUATIONS FOR GAS-LIQUID CONTACTS. Consider the countercurrent gas-liquid contactor shown diagrammatically in Fig. 23.9. Gas at humidity and temperature T j, enters the bottom of the contactor and leaves at the top with a humidity and temperature Liquid enters the top at temperature and leaves at the bottom at temperature The mass velocity of the gas is Gy, the mass of vapor-free gas per unit area of tower cross section per hour. The mass velocities of the liquid at inlet and outlet are, respectively, G a and G t,. Let dZ be the height of a small section of the tower at distance Z from the bottom of the contact zone. Let the mass velocity of the liquid at height Z be G,., the temperatures of gas and liquid be Ty and T, respectively, and the humidity be j/f. At the interface between the gas and the liquid phases, let the temperature be 7j and the humidity be 3 . The cross section of the tower is S, and the height of the contact section is Zj-. Assume that the liquid is warmer than the gas, so the conditions at height Z are those shown in Fig. 23.8a. The following equations can be written over the small volume S dZ. 

Humidity, mass of vapor per unit mass of vapor-free gas at top of contractor at bottom of contactor at gas-liquid interface saturation humidity saturation humidity at wet-bulb temperature Percentage humidity,... 

The result is clean, vapor-free gas. Venturi, radial flow, packed bed and Dynawave scrubbers are used. 

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