Humidity charts

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 ... 

Fig. 3. Humidity chart for the ait—water system, molal quantities. To convert kj to Btu, divide by 1.054 to convert cm to fT, multiply by 35.31 X 10 ...

Fig. 3. Humidity chart illustrating changes in air temperature and humidity in adiabatic direct-heat (convection) dryers. AB is an adiabatic saturation line.

FIG. 12-4 Humidity chart for air-water vapor mixtures. To convert British thermal units per pound to joules per Idlo-gram, multiply by 2326 to convert British thermal units per pound dry air-degree Fahrenheit to joules per Idlogram-kelvin, multiply hy 4186.8 and to convert cubic feet per pound to cubic meters per kilogram, multiply by 0.0624. 

A given humidity chart is precise only at the pressure for which it is evaluated. Most air-water-vapor charts are based on a pressure of 1 atm. Humidities read from these charts for given values of wet- and diy-bulb temperature apply only at an atmospheric pressure of 760 mmHg. If the total pressure is different from 760 mmHg, the humidity at a given wet-bulb and dry-bulb temperature must be corrected according to the following relationship. 

Humidity charts for other solvent vapors may be prepared in an analogous manner. There is one important difference involved, however, in that the wet-bulb temperature differs considerably from the adiabatic-saturation temperatures for vapors other than water. 

Figures 12-37 to 12-39 show humidity charts for carbon tetrachloride, oenzene, and toluene. The lines on these charts have been calculated in the manner outlined for air-water vapor except for the wet-bulb-temperature lines. The determination of these hnes depends on data for the psychrometric ratio /j Z/c, as indicated by Eq. (12-22). For the charts shown, the wet-bulb-temperature hnes are based on the following equation ...

Equation (12-48) can be solved numerically or graphically. Figure 12-47 indicates how H, and may be determined graphically on a humidity chart by the point of intersection on the saturation-humidity curve of a straight line of slope h(CjXh passing through point (H, tj. 

Figure 28 shows the key features of the humidity chart. The chart consists of the following four parameters plotted as ordinates against temperature on the abscissas (1) Humidity H, as pounds of water per pound of dry air, for air of various relative humidities (2) Specific volume, as cubic feet of dry air per pound of dry air (3) Saturated volume in units of cubic feet of saturated mixture per pound of dry air and (4) latent heat of vaporization (r) in units of Btu per pound of water vaporized. The chart also shows plotted hiunid heat (s) as abscissa versus the humidity (H) as ordinates, and adiabatic humidification curves (i.e., humidity versus temperature). Figure 28 represents mixtures of dry air and water vapor, whereby the total pressure of the mixture is taken as normal barometric. Defining the actual pressure of the water vapor in the mixture as p (in units of mm of mercury), the pressure of the dry air is simply 760 - p. The molal ratio of water vapor to air is p/(760-p), and hence the mass ratio is ... 

Two cases may be considered to illustrate the use of enthalpy-humidity charts. These are the mixing of two streams of humid gas and the addition of liquid or vapour to a gas. 

Two methods of changing the humidity and temperature of a gas from Aidj. JP x i to B(()2. J 2) may be traced on the humidity chart as shown in Figure 13.11. The first method consists of saturating the air by water artificially maintained at the dew point of air of humidity (line AC) and then heating at constant humidity to 82 (line CB). In the second method, the air is heated (line AD) so that its adiabatic saturation temperature corresponds with the dew point of air of humidity JP2- It is then saturated by water at the adiabatic saturation temperature (line DC) and heated at constant humidity to 82 (line CB). In this second method, an additional operation — the preliminary heating—is carried out on the air, hut the water temperature automatically adjusts itself to the required value. 

It is supposed that water is to be cooled at a mass rate L per unit area from a temperature 0L2 to Ql - The air will be assumed to have a temperature 6G, a humidity Jf ], and an enthalpy Hoi (which can be calculated from the temperature and humidity), at the inlet point at the bottom of the tower, and its mass flow per unit area will be taken as G. The change in the condition of the liquid and gas phases will now be followed on an enthalpy-temperature diagram (Figure 13.16). The enthalpy-temperature curve PQ for saturated air is plotted either using calculated data or from the humidity chart (Figure 13.4). The region below this line relates to unsaturated air and the region above it to supersaturated air. If it is assumed that the air in contact with the liquid surface... 

From the humidity chart, Figure 13.4 in Volume 1, air at 294 K and of 40 per cent relative humidity has a humidity of 0.006 kg/kg. This remains unchanged on heating to 366 K. At the dryer inlet, the wet bulb temperature of the air is 306 K. In the dryer, the cooling takes place along the adiabatic cooling line until 60 per cent relative humidity is reached. 

The water removed by vaporisation is generally carried away by air or hot gases, and the ability of these gases to pick up the water is determined by their temperature and humidity. In designing dryers using air, the properties of the air-water system are essential, and these are detailed in Volume 1, Chapter 13, where the development of the humidity chart is described. For the air-water system, the following definitions are of importance ... 

If air of humidity M is passed over heating coils so that its temperature rises to 9, this operation may be represented by the line AB on the humidity chart shown in Figure 16.8. This air then passes over the wet material and leaves at, say 90 per cent relative humidity,... 

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... 

The humidity chart or psychrometric chart is a convenient plot for representing the properties of mixtures of a permanent gas and a condensable vapor. There are a number of forms in which this chart has been presented. One form, proposed by Grosvenor [5], is shown in Figure 2.5 for the air-water system at 1 atm. 

On the humidity chart of Figure 2.5, temperatures are plotted as abscissas and humidities as ordinates. Any point on the plot represents a specific mixture of air and water. The curve marked 100% humidity refers to saturated air and is a function of air temperature. Any point to the left of the saturation curve represents a mixture of saturated air and liquid water (this portion of the plot is useful in determining fog formation). Any point to the right of the saturation curve represents undersaturated air. Any point on the temperature axis represents bone-dry air. The curves between the two limits (saturated line and the temperature axis) represent mixtures of air and water of definite percentage humidities. Linear interpolation between the saturation curve and the temperature axis locates lines of constant percentage humidity. 

The use of these relationships in constructing and applying humidity charts is best illustrated by examining a simplified case, that of adiabatic cooling or humidification. Figure 5.4 illustrates this process between air and water that is recycled through the cooling tower. In this operation air is both cooled and... 

Reference TDI contains an enthalpy table for ammonia at different pressures. Reference TD2 contains a series of tables in an appendix from which the specific heats of the reaction-gas mixture were calculated. Humidity charts were also useful. Reference TD3 is valuable for its steam tables, while Ref. TD4 contains both thermodynamic and chemical equilibria data for nitric acid. The final reference, Robertson and Crowe (Ref. TD5), contains formulae and tables for the sizing and choice of an air-feed compressor. 

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