Dew Point. Relative Humidity

Example 8 Determination of Air Properties For a barometric pressure of 25.92 inHg (Ap = —4), a dry-bulb temperature of 90 F, and a wet-bulb temperature of 70 F determine the following absolute humidity, enthalpy, dew point, relative humidity, and specific volume. 

The psychromelric chart provides a quick means for convening from one humidity format to another because dew point, relative humidity, ambient temperature, and wet bulb temperature can he conveniently related to each other on a single sheet of paper. The psychromelric chan has... 

The AirQUIS model runs with input from MM5 (Berge et al. 2002) in 1-h time resolution. The transferred 2D surface parameters are precipitation, total cloud cover, mixing height and surface temperature. 2D parameters from lowest model level are the air temperature, dew point, relative humidity, and vertical temperature... 

For a process system that involves a single condensable component, a vapor-liquid phase change, and specified or requested values of feed or product stream properties (temperature, pressure, dew point, relative saturation or humidity, degrees of superheat, etc.), draw and label the flowchart, carry out the degree-of-freedom analysis, and perform the required calculations. 

The dew point of humid air at a given point on the psychrometric chart can easily be determined. For example, locate the point on Figure 8.4-1 corresponding to air at 29 C and 20% relative humidity. Cooling this air at constant pressure ( = 1 atm) corresponds to moving horizontally (at constant absolute humidity) to the saturation curve. Tjp is the temperature at the intersection, or 4 C. (Verify this statement.)... 

The calculations of absolute humidity, relative humidity, wet-bulb temperature, dew-point temperature, humid volume, humid heat, and humid enthalpy are based on information found in Pakowski and Mujumdar (2006) and Moyers and Baldwin (1997). 

Table 11.4 Solutions for Maintaining Constant Humidity Table 11.5 Concentration of Solutions of H2SO4, NaOH, and CaCi2 Giving Specified Vapor Pressures and Percent Humidities at 25°C Table 11.6 Relative Humidity from Wet and Dry Bulb Thermometer Readings Table 11.7 Relative Humidity from Dew Point Readings...

TABLE 11.7 Relative Humidity from Dew Point Readings... 

Moisture measurements are important in the process industries because moisture can foul products, poison reactions, damage equipment, or cause explosions. Moisture measurements include both absolute-moisture methods and relative-humidity methods. The absolute methods are those that provide a primaiy output that can be directly calibrated in terms of dew-point temperature, molar concentration, or weight concentration. Loss of weight on heating is the most familiar of these methods. The relative-humidity methods are those that provide a primaiy output that can be more direc tly calibrated in terms of percentage of saturation of moisture. 

Example 3 Air Heating Air is heated by a steam coil from 30 F dry-bulb temperature and 80 percent relative humidity to 75 F dry-bulb temperature. Find the relative humidity, wet-bulb temperature, and dew point of the heated air. Determine the quantity of heat added per pound of dry air. 

Relative humidity = 15 percent Wet-bulb temperature = 51.5 F Dew point = 25.2 F... 

Relative humidity and dew point can be determined for other than atmospheric pressure from the partial pressure of water in the mixture and from the vapor pressure of water vapor. The partial pressure of water is calculated, if ideal-gas behavior is assumed, as... 

Humans are sensitive to moisture and can reliably describe the humidity of the environment using word scales as demonstrated in Fig. 5.12. The subject s humidity judgments appear to be functions of the air s dew point, a measure of absolute humidity, and are relatively unaffected by the ambient temperature. Further, people are also good at perceiving skin moisture, as illustrated in Fig. 5.13, where perceived skin wettedness is seen to correlate well with measured skin wettedness. 

Psychrometry has to do with the properties of the air-water vapor mixtures found in the atmosphere. Psychrometry tables, published by the US Weather Bureau, give detailed data about vapor pressure, relative humidity and dew point at the sea-level barometer of 30 in Hg, and at certain other barometric pressures. These tables are based on relative readings of dry bulb and wet bulb atmospheric temperatures as determined simultaneously by a sling psychrometer. The dry bulb reads ambient temperature while the wet bulb reads a lower temperature influenced by evaporation from a wetted wick surrounding the bulb of a parallel thermometer. 

This is defined as the percentage ratio of the water vapour pressure in the atmosphere compared to that which would saturate the atmosphere at the same temperature. Alternatively, the difference in temperature between the ambient atmosphere and that to which it would have to be cooled before moisture condensed from it, is also used as a measure of moisture content. This difference in temperature is called the dew point depression. The actual temperature at which condensation takes place is known as the dew point. The relative humidity is then expressed as ... 

Fig. 2.13 Dew point depression below ambient temperature as a function of the relative humidity of the ambient atmosphere over a range of temperature...

The formation of dew and fog are consequences of this variation in relative humidity. Warm air at high relative humidity may cool below the temperature at which its partial pressure of H2O equals the vapor pressure. When air temperature falls below this temperature, called the dew point, some H2 O must condense from the atmosphere. Example shows how to work with vapor pressure variations with temperature, and our Chemistry and the Environment Box explores how variations in other trace gases affect climate. 

C05-0037. A weather report gives the current temperature as 18 °C and sets the dew point at 10 °C. Using data from Table 5A, determine the partial pressure of water vapor in the atmosphere and calculate the relative humidity. 

The practical measurement of water activity nowadays is usually performed by dew point determination or a change of electrical properties of the material depending on the relative humidity. 

The inlet air had a temperature of 29 °C and a dew point of 16 °C (a relative humidity of about 47%). The outlet air temperature was rising to about 45 °C, while the outlet dew point went down to less than 5 °C. 

Acid Dew Point For fossil fuels, the acid dew point temperature is that temperature at which the actual mixed acid vapor pressure equals the mixed acid vapor saturation pressure. The mixed acid dew point can be approximated by the sulfuric acid dew point (Fig. 24-56). It can be described as a function of the SO3 and water content of the flue gas (Huijbregts). These concentrations result from the sulfur, hydrogen, and free water content of the fuel the relative humidity of the air and the amount of excess air used. Using the equation of Ver-hoff, where T is degrees K and P is mm Hg (see OUces, A.G.) ... 

Dew point - 1) The temperature of the air at which dew begins to be deposited. 2) The temperature at which a given sample of air will have a relative humidity of 100%. 

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