Enthalpy of humid air

Using numerical values mentioned above, the enthalpy of humid air hi. (Eq. (4.87)) can be written as... 

The enthalpy of humid air responding to the point 14 C on the saturation curve is... 

The remaining curves on the psychrometric chart are almost vertical and convex to the left, with labeled values (on Figure 8.4-1) of -0.05, -0.1, -0.2, and so on. (The units of these numbers are kJ/kg DA). These curves are used to determine the enthalpy of humid air that is not saturated. The procedure is as follows (a) locate the point on the chart corresponding to air at its specified condition (b) interpolate to estimate the enthalpy deviation at this point ... 

Use the psychrometric chart to estimate (1) the absolute humidity, wet-bulb temperature, humid volume, dew point, and specific enthalpy of humid air at 41°C and 10% relative humidity, and (2) the amount of water in 150 m of air at these conditions. 

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 ... 

Example 4 Evaporative Cooling Air at 95 F dry-bulb temperature and 70 F wet-bulb temperature contacts a water spray, where its relative humidity is increased to 90 percent. The spray water is recirculated makeup water enters at 70 F. Determine exit dry-bulb temperature, wet-bulb temperature, change in enthalpy of the air, and quantity of moisture added per pound of dry air. 

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. 

Next we draw the saturation curve in the hj -x coordinate system. Vapor pressures can be calculated with Eqs. (4.106) and (4.108) or taken directly from the tables. The humidity x corresponding to the saturation pressure pi,(t) is calculated with Eq. (4.83) noting that p = 0.875 bar. The enthalpy of humid saturated air is calculated with Eq. (4.94) ... 

In the cooling tower the temperature of the liquid falls and the temperature and humidity of the air rise, and its action is thus similar to that of an air humidifier. The limiting temperature to which the water can be cooled is the wet-bulb temperature corresponding to the condition of the air at inlet. The enthalpy of the air stream does not remain... 

Define the dry-bulb temperature, wet-bulb temperature, and humid volume of humid air. Given values of any two of the variables plotted on the psychrometric chart (dry-buib and wet-bulb temperatures, absolute and relative humidity, dew point, humid volume), determine the remaining variable values and the specific enthalpy of the humid air. Use the psychrometric chart to carry out material and energy balance calculations on a heating, cooling, humidification, or dehumidification process involving air and water at 1 atm. 

The psychrometric chart (oT humidity chart) conlam values of a number of process variables for air-water vapor systems at 1 atm. The values listed on the chart include dry-bulb temperature (the temperature measured by common temperature-measurement instruments), moisture content or absolute humidity (mass ratio of water vapor to dry air), relative humidity, humid volume (volume per mass of dry air), wet-bulb temperature (the temperature reading on a thermometer with a water-saturated wick around the bulb immersed in a flowing stream of humid air), and enthalpy per mass of dry air. If you know the values of any two of these variables for humid air at or near 1 atm, you can use the chart to determine the values of the other four, which can greatly simplify material and energy balance calculations. 

For practical purposes, the properties of humid air are recorded on psychrometric (or humidity) charts such as those of Figures 9.1 and 9.2, but tabulated data and equations also are available for greater accuracy. A computer version is available (Wiley Professional Software, Wiley, New York). The terminal properties of a particular adiabatic humification of air are located on the same saturation line, one of those sloping upwards to the left on the charts. For example, all of these points are on the same saturation line (7)//) = (250,0.008), (170, 0.026) and (100,0.043) the saturation enthalpy is 72 Btu/lb dry, but the individual enthalpies are less by the amounts 2.5, 1.2, and 0, respectively. 

The relative humidity of mine incoming air flow in hottest summer days is 57%, and hj (the enthalpy of incoming air flow) is 89.81 kJ/kg. 

Mg. 2.5 Enthalpy-loading diagram of humid air at 1 bar according to Mollier... 

The enthalpy of saturated air is plotted versus 7] on an // versus T plot as shown in Fig. 10.5-3. This enthalpy is calculated with Eq. (9.3-8) using the saturation humidity from the humidity chart for a given temperature, with 0°C (273 K) as a base temperature. Calculated values are tabulated in Table 10.5-1. 

Based on these definitions and the data provided, we compute the enthalpy of the air at the inlet and outlet of the column (Figure 3.20). Alternatively, we can use the functions defined in the exercise for mixtures of air streams to estimate absolute humidity and enthalpy. 

First, we determine the equilibrium line, the enthalpy of saturated air, based on the atmospheric conditions, assuming 760 mmHg of pressure (in Excel file A2 = 760 mmHg). The key point here is that we need an equation that predicts the equilibrium line (Equations 3.20 and 3.21) so that we can compute the interphase points. The computation of the equilibrium line is performed as given by Equation 3.34. Thus, for a certain temperature and the corresponding vapor pressure, we compute the saturating moisture and the enthalpy of the humid air ... 

In column C, we define the saturated vapor pressure given by the Antoine correlation (Equation 3.21) for all the temperature points. In column D, we calculate the absolute humidity (Equation 3.20) and finally in column E the enthalpy of saturated air (Equation 3.22). Now, as in a previous example, we define an equation that may fit our data. In this case, a polynomial will do the work. In cells G1 to Kl, we define the coefficients of the polynomial ... 

With the humid heat in hand, we are in a position to formulate the enthalpy of an air-water mixture. With T+chosen as the datum temperature for both components and adding sensible and latent heats we obtain... 

The exact solution of the equations that describe both the heat and mass transfer processes is rather complicated, although both must be considered as just described. However, for the most common case of an air/water system the ratio of the heat transfer coefficient to the product of the mass transfer coefficient and gas-phase humid heat capacity is unity. In this case, humidity and temperature potentials are combined into enthalpy driving forces that control the rate of transfer [3]. Equation 6-7 represents the operating line of a countercurrent flow water cooling tower. The equilibrium curve is a plot of the enthalpy of saturated air (in Btu/lb BDG) against the temperature of water in thermal equilibrium with the air. 

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