Thermodynamics

Chapter 14 | 727EXAMPLE 14–4The Use of the Psychrometric ChartConsider a room that contains air at 1 atm, 35°C, and 40 percent relativehumidity. Using the psychrometric chart, determine (a) the specific humidity,(b) the enthalpy, (c) the wet-bulb temperature, (d ) the dew-point temperature,and (e) the specific volume of the air.Solution The relative humidity of air in a room is given. The specific humidity,enthalpy, wet-bulb temperature, dew-point temperature, and specific volumeof the air are to be determined using the psychrometric chart.Analysis At a given total pressure, the state of atmospheric air is completelyspecified by two independent properties such as the dry-bulb temperatureand the relative humidity. Other properties are determined by directly readingtheir values at the specified state.(a) The specific humidity is determined by drawing a horizontal line from thespecified state to the right until it intersects with the v axis, as shown inFig. 14–16. At the intersection point we readv 0.0142 kg H 2 O/kg dry air(b) The enthalpy of air per unit mass of dry air is determined by drawing aline parallel to the h constant lines from the specific state until it intersectsthe enthalpy scale, givingh 71.5 kJ/kg dry air(c) The wet-bulb temperature is determined by drawing a line parallel to theT wb constant lines from the specified state until it intersects the saturationline, givingT wb 24°C(d ) The dew-point temperature is determined by drawing a horizontal line fromthe specified state to the left until it intersects the saturation line, givingT dp 19.4°C(e) The specific volume per unit mass of dry air is determined by noting thedistances between the specified state and the v constant lines on both sidesof the point. The specific volume is determined by visual interpolation to bev 0.893 m 3 /kg dry airDiscussion Values read from the psychrometric chart inevitably involve readingerrors, and thus are of limited accuracy.T dphT wbT = 35°Cφ = 40%FIGURE 14–16Schematic for Example 14–4.vω14–6 HUMAN COMFORT AND AIR-CONDITIONINGHuman beings have an inherent weakness—they want to feel comfortable.They want to live in an environment that is neither hot nor cold, neitherhumid nor dry. However, comfort does not come easily since the desires ofthe human body and the weather usually are not quite compatible. Achievingcomfort requires a constant struggle against the factors that cause discomfort,such as high or low temperatures and high or low humidity. As engineers, itis our duty to help people feel comfortable. (Besides, it keeps us employed.)