Humidity dew point

The operating ranges of different humidity sensors. Most humidity/dew point detectors can make measurements at higher values of humidity and temperature, but are limited at low temperatures and at low concentrations. Most are also limited to a maximum operatng temperature of about 200°F (95°C). 

Instruments are used in the chemical industry to measure process variables, such as temperature, pressure, density, viscosity, specific heat, conductivity, pH, humidity, dew point, liquid level, flow rate, chemical composition, and moisture content. By use of instruments having varying degrees of complexity, the values of these variables can be recorded continuously and controlled within narrow limits. 

Define the dry-bulb temperature, wet-bulb temperature, and humid volume of humid air. Given values of any two of the variables plotted on the psychrometric chart (dry-buib and wet-bulb temperatures, absolute and relative humidity, dew point, humid volume), determine the remaining variable values and the specific enthalpy of the humid air. Use the psychrometric chart to carry out material and energy balance calculations on a heating, cooling, humidification, or dehumidification process involving air and water at 1 atm. 

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... 

Dew-point Temperature (DPT). DPT is the temperature at which the condensation of water vapor in a space begins for a given state of humidity and pressure as the temperature is reduced. It is the temperature corresponding to saturation (100% rh) for a given absolute humidity at constant pressure. 

Dehumidification. Dehumidification may be accompHshed in several ways (see Drying). Moderate changes in humidity can be made by exposing the air stream to a surface whose temperature is below the dew point of the air. The air is cooled and releases a portion of its moisture. Closed cycle air conditioning systems normally effect dehumidification also. The cooled air may require reheating to attain the desired dry-bulb temperature if there is insufficient sensible load in the space. 

Some industrial processes produce predorninately latent air conditioning loads. Others dictate very low humidities and when the dew point falls below 0°C, free2ing becomes a major concern. Dehydration equipment, using soHd sorbents such as siUca gel and activated alurnina, or Hquid sorbents such as lithium chloride brine and triethylene glycol, may be used. The process is exothermic and may require cooling the exiting air stream to meet space requirements. Heat is also required for reactivation of the sorbent material. 

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... 

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. 

Although the dew-point method may be considered a fundamental technique for determining humidity several uncertainties occur in its use. It is not always possible to measure precisely the temperature of the polished surface or to eliminate gradients across the surface. It is also difficult to detect the appearance or disappearance of fog the usual practice is to take the dew point as the average of the temperatures when fog first appears on cooling and disappears on heating. 

Vibrating conveyors employing direc t contacting of sohds with hot, humid air have also been employed for the agglomeration of fine powders, chiefly for the preparation of agglomerated water-dispersible food products. Control of inlet-air temperature and dew point permits the uniform addition of small quantities of liquids to sohds by con-... 

Environmental conditions under which solvent release from the adhesive on the substrate is produced must be carefully controlled. Humidity is critical because loss of heat due to solvent evaporation may allow attainment of the dew point (the evaporation of the solvent is an endothermic process), and then condensation of water on the adhesive can result. This phenomenon is often called moisture blooming. The presence of water on the adhesive film causes a detrimental effect because the autoadhesion of rubber chains is greatly inhibited. Therefore, humidity must be controlled and avoided by increasing the temperature during solvent evaporation. 

Temperature-. Typieally, gas temperatures up to about 260°C (500°F), with surges to about 290 C (550 F) ean be aceommodated routinely, with the appropriate fabrie material. Spray eoolers or dilution air ean be used to lower the temperature of the pollutant stream. This prevents the temperature limits of the fabrie from being exeeeded. Lowering the temperature, however, inereases the humidity of the pollutant stream. Therefore, the minimum temperature of the pollutant stream must remain above the dew point of any eondensable in the stream. The baghouse and assoeiated duetwork should be insulated and possibly heated if eondensation may oecur. 

Saturated volume is the volume in cubic feet of 1 lb of dry air when it is saturated with water vapor that is, it is the humid volume at saturation, and is determined by the temperature and pressure. The humid volume of air equals the product of its saturated volume at its dew point and the ratio of the absolute temperature of the air to the absolute temperature of its dew point. 

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. 

Low humidity also affects comfort and health. Comfort complaints about dry nose, throat, eyes, and skin occur in low-humidity conditions, typically when the dew point is less than 0 °C. Low humidity can lead to drying of the skin and mucous surfaces. On respiratory surfaces, drying can concentrate mucus to the extent that ciliary clearance and phagocytic activities are re-... 

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 ... 

Moisture precipitation Apart from wetting by sea-spray, moisture may either be deposited on a surface by rainfall or dew formation. For a known ambient humidity the dew point can be calculated, using the expression given previously, from standard tables giving the saturated vapour pressure of... 

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...

In order to obtain a high rate of humidification, the area of contact between the air and the water is made as large as possible by supplying the water in the form of a fine spray alternatively, the interfacial area is increased by using a packed column. Evaporation occurs if the humidity at the surface is greater than that in the bulk of the air that is, if the temperature of the water is above the dew point of the air. 

Two methods of changing the humidity and temperature of a gas from Aidj. JP x i to B(()2. J 2) may be traced on the humidity chart as shown in Figure 13.11. The first method consists of saturating the air by water artificially maintained at the dew point of air of humidity (line AC) and then heating at constant humidity to 82 (line CB). In the second method, the air is heated (line AD) so that its adiabatic saturation temperature corresponds with the dew point of air of humidity JP2- It is then saturated by water at the adiabatic saturation temperature (line DC) and heated at constant humidity to 82 (line CB). In this second method, an additional operation — the preliminary heating—is carried out on the air, hut the water temperature automatically adjusts itself to the required value. 

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