Humidity total enthalpy

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... 

An enthalpy balance can be written over this process. Pump work is neglected, and the enthalpy balance is based on temperature 7 as a datum. Then the enthalpy of the makeup liquid is zero, and the total enthalpy of the entering gas equals that of the leaving gas. Since the latter is a% datum temperature, its enthalpy is simply where is the saturation humidity, and is the latent heat, both at %, From Eq. (23.9) the total enthalpy of the entering gas is Cj T — T ) + Aj, and the enthalpy balance is... 

Differentiating the definition of the total enthalpy of the mixture, equation (8-3), and substituting the definition of the humid heat, equation (8-8) we have... 

Cg = humid heat for humidity H in units of kj / (kg-K) and = latent heat of vaporization at / in kj /kg. The slope of the constant-enthalpy adiabatic cooling line is —C j which is the relationship between temperature and humidity of gas passing through a totally adiabatic direct-heat dryer. The humid heat of a gas—vapor mixture per unit weight of dry gas includes the specific heat of the vapor... 

Comparing Examples 2a and 2b we notice that the total air pressure has effects on the humidity x, partial density of dry air p total pressure or pressure of humid air, and enthalpy h. Knowing the total pressure is therefore essential in calculations of the thermodynamic properties of humid air. 

Thus the total mass flows tn= m, + m,) differ in different cases. Water vapor flow th, is obtained by multiplying the dry air mass flow by the corresponding humidity x (Eq. 4.93). As a basic quantity in humid air mass and energy balance calculations, we use dry air mass flow m and the effect of humidity on the energy balance is noted in the enthalpy h, (Eq. 4.87). 

The shape of the humidity-enthalpy line for saturated air is such that the proportion of the total area of the diagram representing saturated, as opposed to supersaturated, air is small when rectangular axes are used. In order to enable greater accuracy to be obtained in the use of the diagram, oblique axes are normally used, as in Figure 13.5, so... 

Considering the countercurrent flow of water and air in a tower of height z (Figure 13.15), the mass rate of flow of air per unit cross-section G is constant throughout the whole height of the tower and, because only a small proportion of the total supply of water is normally evaporated (1 -5 per cent), the liquid rate per unit area ll can be taken as constant. The temperature, enthalpy, and humidity will be denoted by the symbols 6, H, and Tf respectively, suffixes G, L, 1, 2, and / being used to denote conditions in the gas and liquid, at the bottom and top of the column, and of the air in contact with the water. 

Consider a countercurrent operation in which cold water contacts hot air. There are two unknowns the outlet air humidity and the water loading to the tower, Even though the outlet-air temperature may be known, we cannot determine its enthalpy unless its humidity is known. Also. without the outlet humidity, the total heat load, qt, cannot be determined as well as the water,... 

The relationships among the psychrometric states, loads, and sensible heat ratio are shown graphically in Fig. 3. For a given pair of entering and leaving air states, sensible, latent, and total loads are proportional to the differences in temperature, humidity ratio, and enthalpy, respectively, as shown in the figure. The SHR is defined by the slope of the line connecting the two points. 

The concentration of vapour at the foot of the tank is estimated by assuming thermodynamic equilibrium. Given total liquid flow rates and air entrainment rates (and the temperatures of both) the fihal temperature and vapour concentration can be calculated straight fonvardly. Examples of results of such a calculation for a winter grade petrol are given in Annex 2. Water vapour condensation should be included in the enthalpy balance but only makes a substantial difference if the humidity and ambient temperatures are high. 

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