![]() ![]() Sensible Heat Gain (SHG) or Loss (SHL) is the heat gained or lost by a substance (solid, liquid or gas, in this case air and water vapor mixture) without a change in state from solid to liquid or liquid to gas. Both the heat content (H) and moisture content (W) were removed by cooling the outdoor air. All other psychrometric parameters (DB, WB, DP, W and H) of the indoor air is less than that of the outdoor air. Note that the outdoor RH is 40% but the indoor RH is higher at 50%. ![]() Figure Figureįigure ? shows typical summer outdoor air conditions for a location such as Chicago and the desirable indoor air design conditions in summer. For HVAC design and for all everyday practical purposes, Relative Humidity (RH) and Degree of Saturation (DS) can be considered to be almost the same. Figureĭegree of Saturation (DS) is the ratio of moisture content (or humidity ratio) of the air at a given DB temp to the moisture content (humidity ratio) of the saturated air at the same DB temp. However, it is very close to the ratio of moisture contents and the difference is about 2%. RH is a ratio of water vapor pressures and not a ratio of moisture contents. Relative Humidity (RH) is the ratio of the actual water vapor pressure at a given DB temp to the water vapor pressure of the air if the air was saturated at the same DB temp. This maximum moisture content increases with increasing temperature. Saturated Air contains the maximum possible moisture (water vapor or steam) that the air can hold at a given DB temp. It is the moisture content of the air and it is indicative of the latent heat content of the air. Humidity Ratio (W) is the weight (lbs) of water vapor per unit weight (1 lb) of dry air. It is the best indicator of comfort conditions. DP temp is indicative of the moisture content and latent heat in the air. WB temp is indicative of the total heat (sum of both sensible heat and latent heat) content of the air.ĭew Point (DP) temperature is the temperature at which condensation of moisture begins when the air is cooled by removing sensible heat only. The evaporation of the moisture surrounding the bulb cools the air. Wet Bulb (WB) temperature is the temperature of air as registered by a thermometer whose temperature sensing bulb is covered by a material (cloth) soaked in water and exposed to a rapidly moving current of air. DB temp is indicative of the sensible heat content of the air. If dbt and % Relative Humidity (RH) are known, then also it is possible to locate the point on the chart to read other properties of air.Dry Bulb (DB) temperature is the temperature of air as registered by an ordinary (mercury) thermometer. In majority of situations, dbt and wbt of the air are measured to find the location of the point on the psychrometric chart. After locating the point of quality, many other properties can be easily obtained from the chart. It is necessary to know to parameters to locate the quality of air on psychrometric chart. The actual psychrometric chart is depicted in Fig. under different conditions using various psychrometric relations. The constructional procedure is of psychrometric chart is based on the calculations for relative humidity, enthalpy, specific humidity etc. ASHRAE has also developed psychrometric charts for other temperatures and barometric pressures. Psychrometric charts are readily available for standard barometric pressure of 101.325 kPa at sea level and for normal temperatures (0-50 oC). Fig.26.4 shows the schematic of a psychrometric chart. The left hand side of the psychrometric chart is bounded by the saturation line. Standard psychrometric charts are bounded by the dry-bulb temperature line (abscissa) and the vapour pressure or humidity ratio (ordinate). It saves lot of time and labour required for calculation of different properties of air and such values can be directly read from the chart. Using the chart, it is easy to obtained psychrometric properties of air and it helps to decide the various processes to be followed to achieve required quality of air. Psychrometric chart shows the inter-relation of all the important properties of air. ![]()
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