Psychrometric properties

The SHR can also be related to the psychrometric properties of the entering and leaving air states. 

The interrelationships between air and water vapor are called psychrometric properties [3]. Changes in these properties are shown by a psychrometric chart. A sample of the chart is given in Figure 24.2. The psychrometric terms given in the chart are defined as follows. 

Psychrometrics. Psychrometrics is the branch of thermodynamics that deals specifically with moist air, a biaary mixture of dry air and water vapor. The properties of moist air are frequentiy presented on psychrometric charts such as that shown ia Figure 2 for the normal air conditioning range at atmospheric pressure. Similar charts exist for temperatures below 0°C and above 50°C as well as for other barometric pressures. AH mass properties ate related to the mass of the dry air. 

Fig. 2. Psychrometric chart. Below 0°C properties and enthalpy deviation lines ate for ice. Courtesy of Carrier Corp. To convert kj to kcal, divide by 4.184.

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particularly active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). Eor these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

FIG. 12-6 Diagram of psychrometric chart showing the properties of moist air. 

FIG. 12-36 Psychrometric chart properties of air and water-vapor mixtures from 20 to 120 C, Catrier Corp. )... 

The specific enthalpy (h ) of the room-air design condition is obtained from tables of the property of air or the psychrometric chart. 

Psychrometric chart A chart constructed so that knowing any two air properties the remaining thermodynamic properties of air at that condition can be determined. 

The study of moist air. The psychrometric chart shows the relationship between the various properties of moist air in graphical form and can be used for the solution of problems. 

A further property which is shown on the psychrometric chart is the specific volume of the mixture, measured in cubic metres per kilogram. This appears as a series of diagonal lines, at intervals of 0.01 ml... 

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... 

At 100T and this enthalpy, other properties are read off the psychrometric chart as... 

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 a psychrometric chart (or humidity chart) several properties of a gas-vapor mixture are cross-plotted, providing a concise compilation of a large quantity of physical property data. The most common of these charts—that for the air-water system at 1 atm—is used extensively in the analysis of humidification, drying, and air-conditioning processes. 

The following paragraphs define and describe the different properties of humid air at 1 atm that appear on the psychrometric chart. Once you know the values of any two of these properties, you can use the chart to determine the values of the others. We will use the abbreviation DA for dry air. 

P. Gandhidasan j I When engineers need to evaluate the values of moist-air properties, they generally refer to psychrometric charts or tables. 

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 study of the properties of the air-water vapor mixture is called psychometry, and data are presented in various forms of psychrometric charts presenting various data. In Fig. 4, humidity is plotted as ordinate and... 

Figure 12-4 shows a psychrometric chart for combustion products in air. The thermodynamic properties of moist air are given in Table 12-1. Figure 12-4 shows a number of useful additional relationships, e.g., specific volume and latent heat variation with temperature. Accurate figures should always be obtained from physical properties tables or by calculation using the formulas given earlier, and these charts should only be used as a quick check for verification. 

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