The constant enthalpy value lines are assumed to be along the constant WB temp lines. The enthalpy values shown on the psychrometric chart are at saturation (100% RH and DB = WB = DP). Sensible Heat Factor (SHF) is the ratio of Sensible Heat removed (applies mainly to cooling processes) to the Total Heat removed. In metric (SI) units the enthalpy of both the dry air and water vapor is measured from 0 oC. The enthalpy of moisture below 32 oF is not a factor since most of it will have condensed and then frozen out as ice from the air. So the enthalpy of dry air below 0 oF is a negative value. In English (IP) units the heat content of the air is measured from 0 oF and the heat content of the water vapor is measured from 32 oF which is the freezing point of water at standard pressure. Total Heat Gain (THG) or Loss (THL) is the sum of the sensible and latent heat gained or lost.Įnthalpy (H) or heat content is the total heat contained in the air and water vapor mixture. Latent Heat Gain (LHG) or Loss (LHL) is the heat gained or lost that changes the state of a substance (in this case the water vapor part only of the air and water vapor mixture) from solid to liquid or liquid to gas (in this case the water vapor part of the air and water vapor mixture). 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. Dry Bulb (DB) temperature is the temperature of air as registered by an ordinary (mercury) thermometer.
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