Thermodynamics

726 | Thermodynamics(c) The enthalpy of air per unit mass of dry air is determined from Eq. 14–12:h 1 h a1 v 1 h v1 c p T 1 v 1 h g1 11.005 kJ>kg # °C2125°C2 10.00653212546.5 kJ>kg2 41.8 kJ/kg dry airDiscussion The previous property calculations can be performed easily usingEES or other programs with built-in psychrometric functions.Saturation line, φ = 100%v = const.φ = const.Twb = const.h = const.Dry-bulb temperatureSpecific humidity, ωFIGURE 14–14Schematic for a psychrometric chart.Saturation line15°CT db = 15°C15°CT dp = 15°CT wb = 15°CFIGURE 14–15For saturated air, the dry-bulb,wet-bulb, and dew-pointtemperatures are identical.14–5 THE PSYCHROMETRIC CHARTThe state of the atmospheric air at a specified pressure is completely specifiedby two independent intensive properties. The rest of the properties canbe calculated easily from the previous relations. The sizing of a typical airconditioningsystem involves numerous such calculations, which may eventuallyget on the nerves of even the most patient engineers. Therefore, thereis clear motivation to computerize calculations or to do these calculationsonce and to present the data in the form of easily readable charts. Suchcharts are called psychrometric charts, and they are used extensively inair-conditioning applications. A psychrometric chart for a pressure of 1 atm(101.325 kPa or 14.696 psia) is given in Fig. A–31 in SI units and in Fig.A–31E in English units. Psychrometric charts at other pressures (for use atconsiderably higher elevations than sea level) are also available.The basic features of the psychrometric chart are illustrated in Fig. 14–14.The dry-bulb temperatures are shown on the horizontal axis, and the specifichumidity is shown on the vertical axis. (Some charts also show thevapor pressure on the vertical axis since at a fixed total pressure P there is aone-to-one correspondence between the specific humidity v and the vaporpressure P v , as can be seen from Eq. 14–8.) On the left end of the chart,there is a curve (called the saturation line) instead of a straight line. All thesaturated air states are located on this curve. Therefore, it is also the curveof 100 percent relative humidity. Other constant relative-humidity curveshave the same general shape.Lines of constant wet-bulb temperature have a downhill appearance to theright. Lines of constant specific volume (in m 3 /kg dry air) look similar, exceptthey are steeper. Lines of constant enthalpy (in kJ/kg dry air) lie very nearlyparallel to the lines of constant wet-bulb temperature. Therefore, the constantwet-bulb-temperaturelines are used as constant-enthalpy lines in some charts.For saturated air, the dry-bulb, wet-bulb, and dew-point temperatures areidentical (Fig. 14–15). Therefore, the dew-point temperature of atmosphericair at any point on the chart can be determined by drawing a horizontal line (aline of v constant or P v constant) from the point to the saturated curve.The temperature value at the intersection point is the dew-point temperature.The psychrometric chart also serves as a valuable aid in visualizing the airconditioningprocesses. An ordinary heating or cooling process, for example,appears as a horizontal line on this chart if no humidification or dehumidificationis involved (that is, v constant). Any deviation from a horizontal lineindicates that moisture is added or removed from the air during the process.