Enthalpy of air-water vapor mixtures

In Figure 2 the lines, volume, m /kg dry air, indicate humid volume, which includes the volume of 1.0 kg of dry gas plus the volume of vapor it carries. Enthalpy at saturation data are accurate only at the saturation temperature and humidity however, for air—water vapor mixtures, the diagonal wet bulb temperature lines are approximately the same as constant-enthalpy adiabatic cooling lines. The latter are based on the relationship ... 

From air-water vapor-mixture tables, the enthalpy hi of the ambient air at 78 F wet-bulb temperature is 41.58 Btu/lb. 

The left-hand scale gives the enthalpy per pound of dry air of a saturated air-water vapor mixture. The enthalpy of the wet air (in Btu/lb dry air) is actually the following ... 

The adiabatic cooling lines are lines of almost constant enthalpy for the entering air-water mixture, and you can use them as such without much error (1 or 2%). However, if you want to correct a saturated enthalpy value for the deviation which exists for a less-than-saturated air-water vapor mixture, you can employ the enthalpy deviation lines which appear on the chart and which can be used as illustrated in the examples below. Any process that is not a wet-bulb process or an adiabatic process with recirculated water can be treated by the usual material and energy balances, taking the basic data for the calculation from the humidity charts. If there is any increase or decrease in the moisture content of the air in a psychrometric process, the small enthalpy effect of the moisture added to the air or lost by the air may be included in the energy balance for the process to make it more exact as illustrated in Examples 4.47 and 4.49. 

In humidification problems, energy balances are always required, therefore it is necessary to calculate the enthalpy of gas-vapor mixtures. The enthalpy of a gas-vapor mixture is the sum of the enthalpies of the gas and of the vapor content. To calculate the enthalpy of the mixture per unit mass of dry gas, H two reference states must be chosen, one for the gas and one for the vapor. Let T() be the datum temperature chosen for both components, and base the enthalpy of component A on liquid at T0 (T0 = 273 K for most air-water problems). Let the temperature of the mixture be T and the absolute humidity Y. The total enthalpy of the mixture is the sum of three items the sensible heat of the vapor, the latent heat of vaporization of the liquid at T(), and the sensible heat of the vapor-free gas. Then... 

Total enthalpy of an air-water vapor mixture. The total enthalpy of 1 kg of air plus its water vapor is J/kg or kJ/kg dry air. If Tq is the datum temperature chosen for both components, the total enthalpy is the sensible heat of the air-water vapor mixture plus the latent heat Aq in J/kg or kJ/kg water vapor of the water vapor at Tq. Note that (T - To)"C = (T- Tq) K and that this enthalpy is referred to liquid water. 

Table 10.5-1. Enthalpies of Saturated Air-Water Vapor Mixtures (P°C Base Temperature)... 

If a warm vapor-gas mixture is contacted with cold liquid so that the humidity of the gas is greater than that at the gas-liquid interface, vapor will diffuse toward the liquid and the gas will be dehumidified. In addition, sensible heat can be transferred as a result of temperature differences within the system. For air-water-vapor mixtures (Le = 1) contacted with cold water, the methods of water cooling apply with only obvious modification. The operating line on the gas-enthalpy-liquid-temperature graph will be above the equilibrium curve, the driving force is H — H, and Eq. (7.54) can be used with this driving force. For all other systems, for which Le 1, the general methods below must be used. 

Before solving the equations, we need system property data, which, in this case, are thermodynamic properties. Equations 3.2.9 and 3.2.11 states that we may obtain vapor pressures for water from steam tables, such as those compiled by Chaar et al. [13]. Equation 3.2.10 also states that we can find the enthalpy of vaporization in the steam tables. We assume that the air-water mixture is ideal to calculate the enthalpy of air, so we can use the mole-fraction average of the pure-component enthalpies. Equations 3.2.12 and 3.2.13 in Table 3.2.1 give the mole fraction average of the inlet and outlet enthalpy. Table 3.2.1 also lists pure component enthalpies for water vapor (Equations 3.2.14 and 3.2.16) and for air (Equa-... 

Simple FORTRAN programs have been prepared for the reader s use that solve linear and nonlinear equations, retrieve the properties of water and steam, and of air-water mixtures, calculate the vapor pressure of pure substances, calculate enthalpy changes from heat capacity equations, and so on. A disk containing these codes will be found in a pocket in the back of the book. (Readers are encouraged to use library codes when available, codes that may be more accurate and robust than the simple codes provided.) As a result, the portions of the book formerly treating... 

Psychrometric charts are plots of humidity, temperature, enthalpy, and other useful parameters of a gas-vapor mixture. They are helpful for rapid estimates of conditions and for visuahzation of process operations such as humidification and drying. They apply to a given system at a given pressure, the most common of course being air-water at atmospheric pressure. There are four types, of which the Grosvenor and Mollier types are most widely used ... 

COOLING EFFECT, TOTAL - Difference between the total enthalpy of the dry air and water vapor mixture entering the cooler per hour and the total enthalpy of the dry air and water vapor mixture leavir the cooier per hour, expressed in watts. 

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