Psychrometric charts

Using mass transfer rate, the heat transfer at equlibrium can be written as  

Equation 11.7 shows how wet bulb temperature is dependent on humidity and temperature. 

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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 at atmospheric pressure, 101.3 kPa (1 atm)(3). To convert kj to kcal, divide by 4.184.

Eigure 3 shows the winter and summer comfort zones plotted on the coordinates of the ASHRAE psychrometric chart. These zones should provide acceptable conditions for room occupants wearing typical indoor clothing who are at or near sedentary activity. Eigure 3 appHes generally to altitudes from sea level to 2150 m and to the common case for indoor thermal environments where the temperature of the surfaces (/) approximately equals air temperature (/ and the air velocity is less than 0.25 m/s. A wide range of environmental appHcations is covered by ASHRAE Comfort Standard 55 (5). Offices, homes, schools, shops, theaters, and many other appHcations are covered by this specification. 

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). 

Before drying can begin, a wet material must be heated to such a temperature that the vapor pressure of the contained Hquid exceeds the partial pressure of vapor already present in the surrounding atmosphere. The effect of a dryer s atmospheric vapor content and temperature on performance can be studied by constmction of a psychrometric chart for the particular gas and vapor. Figure 2 is a standard chart for water vapor in air (6). 

Heating, Refrigerating and Air-Conditioning Engineers, New York, 1967. Quinn, Humidity The Neglected Parameter, Test Eng. (July 1968). Treybal, Mass-Ttansfet- Operations, 3d ed. McGraw-Hill, New York, 1980. Wexler, Humidity and Moisture, vol. I, Reinhold, New York, 1965. Zimmerman and Lavine, Psychrometric Charts and Tables, Industrial Research Service, 2d ed., Dover, N.H., 1964. 

Values of andt, are given by the saturation curve of the psychrometric chart, such as Fig. 12-2. By trial and error, = 72.1 F, or the adiabatic-saturation temperature is 0.1 F higher than the wet-bulb temperature. 

Examples Illustrating Use of Psychrometric Charts In these examples the following nomenclature is used ... 

Solution. Reading directly from the psychrometric chart (Fig. 12-2),... 

FIG. 12-2 Psychrometric chart—medium temperatures. Barometric pressure, 29.92 iuHg. To convert British thermal units per pound dry air-degree Fahrenheit to joules per Idlogram-kelvin, multiply by 4186.8 and to convert cubic feet per pound to cubic meters per kilogram, multiply by 0.0624. 

Solution. Figure 12-8 shows the path on a psychrometric chart. The leaving dry-bulb temperature is obtained directly from Fig. 12-2 as 72.2 F. Since the spray water enters at the wet-bulb temperature of 70 F and there is no heat added to or removed from it, this is by definition an adiabatic process and there will be no change in wet-bulb temperature. The only change in enthalpy is that from the heat content of the makeup water. This can be demonstrated as follows ... 

FIG. 12-5 Revised form of high-temperature psychrometric chart for air and combustion products, based on pound-moles of water vapor and dry gases. [Hatta, Chem. Metall. Eng., 37, 64 (1930).]... 

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

Use of Psychrometric Charts at Pressures Other Than Atmospheric The psychrometric charts shown as Figs. 12-1 through 12-4 and the data of Table 12-1 are based on a system pressure of 1 atm (29.92 inHg). For other system pressures, these data must be corrected for the effect of pressure. Additive corrections to be apphed to the atmospheric values of absolute humidity and enthalpy are given in Table 12-2. 

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

Psychrometric charts plot wet bulb and dry bulb data for air-water vapor mixtures at atmospheric pressure. These charts are quite useful for... 

Figure B-2. Psychrometric chart atmospheric air, mixture pressure (barometric) 29.921 in. Hg. (From Short, Kent and Walls, Pressure-Enthalpy Otarts for Selected Engineering Substances. Gulf Publishing Company, Houston, TX, 1970.)...

Propagating stall, 237 Proximity sensors, 350 Proximity transducers, 350 Pseudocriticals, 20 Psychrometric chart, 20,... 

A comparison of wet and dry bulb readings allows the relative humidity to be determined from a psychrometric chart. The wet bulb temperature is always lower than the dry bulb value except when the air is already saturated with water - 100% relative humidity. This is when the wet and dry bulb temperatures are the same. Tlie air will no longer accept water and the lack of evaporation does not allow the wetted bulb to reject heat into the air by evaporation. This situation would be... 

Figure 9.14 shows the processes of humidification and dehumidification in a skeleton psychrometric chart, while Table 9.5 explains the cycle. 

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

Constant volume lines Lines on a steam or psychrometric chart passing through the state points representing an equal volume of steam or air (dry or wet). 

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

Refer to psychrometric chart. Figures 9-124A and B for basic considerations in establishing tower conditions. 

Figure 9-124A. Psychrometric chart, reference barometric pressure of 29.92 in. Hg. Used by permission of Westinghouse Eiectric Co., Sturte-vant Div.

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. 

When two samples of air are brought together the condition of the mixture may be arrived at arithmetically by adding the heat flow of each and dividing by the total mass flow and similarly for the moisture flow. Alternatively, plot the condition of each onto a psychrometric chart. The mixed condition lies on a straight line between the two in a position proportional to the two quantities. 

Where the pan is in the air stream the condition downstream of the pan has increased moisture content (kg/kg) found from the airflow and moisture input. On a psychrometric chart, this will lie on a line of sensible-to-total heat ratio of 0.3. Thus, the psychrometric plot shows a steep rise in moisture content with a small rise in dry bulb temperature. The latter is a disadvantage when cooling is... 

Make no excuse for checking whether the sun shines on the thermostats or someone has washed the floor. Another common occurrence is that while the room is not being controlled as specified, the plant appears to be operating correctly. Here a series of simultaneous readings of air flows and wet and dry bulb temperatures should be taken around the whole of the air circuit - on and off coil, condition after fan, condition at inlet grilles, etc. Referring these to a psychrometric chart will usually indicate where the fault or leak lies. 

On this chart, the wet bulb temperatures appear as diagonal lines, coinciding with the dry bulb at the saturation line. If measurements are taken with the two thermometers of the sling psychrometer, the condition can be plotted on the psychrometric chart by taking the intersection of the dry bulb temperature, as read on the vertical line, with the wet bulb temperature, read down the diagonal wet bulb line. 

Figure 23.5 Basic CIBSE psychrometric chart (Courtesy of the Chartered Institution of Building Services Engineers)...

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