![]() The moisture content of air is expressed as the weight of water vapor per unit weight of dry air ( lb H2O/lb air, kg H2O/kg air). Moisture Content and Humidity Ratio is the amount of water vapor by weight in dry air. Specific volume is indicated along the bottom axis of the psychrometric chart with the constant-volume lines slanting upward to the left. Specific Volume represents the space occupied by a unit weight of dry air ( ft 3/lb, m 3/kg). The Dew Point Temperature is represented along the 100% relative humidity line. The Dew Point Temperature can be read in the psychrometric charts by following the horizontal line from the state-point to the saturation line. Above this temperature the moisture stays in the air. Dew Point Temperature - T dpĭew Point is the temperature at which water vapor starts to condense in the air - the temperature at which air becomes completely saturated. The line representing saturated air where the relative humidity - RH is 100% - is the uppermost curved line in the chart. The relative humidity lines in the psychrometric chart are curved lines that move upwards to the right. Note ! The moisture holding capacity of air increases dramatically with temperature! - important for drying processes. In practice the relative humidity will indicate the moisture level of the air compared to the maximum moisture-holding capacity of air at saturation. The moisture-holding capacity of air increases with air temperature. Relative humidity is expressed as a percentage. ![]() Relative humidity can also be expressed as the ratio of water vapor pressure - p w, to the water vapor pressure of saturated air at the same temperature - p w s. Relative Humidity is the ratio of the mass of water vapor contents in the humid air - m w - to the maximum possible mass of vapor - m wmax - at the actual pressure and temperature. On the chart, the Wet-Bulb Temperature lines slopes a little upward to the left, and the temperature is read at the saturation line. The Wet-Bulb Temperature and the Dry-Bulb Temperature will be identical with 100% relative humidity in the air (the air is at the saturation line). Wet-Bulb Temperatures are always lower than dry bulb temperatures with less than 100% relative humidity in the air. Wet Bulb Temperature can be measured with a thermometer that has the bulb covered with a water-moistened bandage with air flowing over the thermometer. Wet-Bulb Temperature is associated with the moisture content of the air. The vertical lines extending upward from this axis are constant-temperature lines. With Dry-Bulb Temperature the sensible heat content in the air can be determined along the bottom axis of the psychrometric chart. People referring to air temperature normally referring to Dry Bulb Temperature.ĭry-Bulb Temperature - T db - can be measured by using a normal thermometer. Example - Psychrometric Chartĭry-Bulb Temperature, usually referred to as the air temperature, is the air property that is most commonly used. With the intersection point located on the chart or diagram other properties can be read directly. With at least two known properties it is possible to characterize the air in the intersection of the property lines - the state-point. Psychrometry is the science of studying thermodynamic properties of moist air and the use of these to analyze humid air conditions and processes.Īir conditioning processes can be determined with psychrometric charts and Mollier diagrams.
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