Percent humidity

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...

A saturated aqueous solution in contact with an excess of a definite solid phase at a given temperature will maintain constant humidity in an enclosed space. Table 11.4 gives a number of salts suitable for this purpose. The aqueous tension (vapor pressure, in millimeters of Hg) of a solution at a given temperature is found by multiplying the decimal fraction of the humidity by the aqueous tension at 100 percent humidity for the specific temperature. For example, the aqueous tension of a saturated solution of NaCl at 20°C is 0.757 X 17.54 = 13.28 mmHg and at 80°C it is 0.764 X 355.1 = 271.3 mmHg. 

TABLE 11.5 Concentrations of Solutions of H2SO4, NaOH, and CaCl2 Giving Specified Vapor Pressures and Percent Humidities at 25°C... 

Percent humidity Aqueous tension, mmHg H2SO4 NaOH CaClj ... 

Dryer inlet and exit air temperatures. Percent humidity of the dryer inlet air. Particle size of the atomized droplet. Dryer feed temperature. 

Bob yawns. Let s go for a walk as we finish our discussion. They take the elevator to the eighth floor, which is kept at a constant 90 percent humidity to make some of the alien artists feel at home. The air smells of machine oil and wine. 

Fig. 12.1. Force-elongation curves of natural and manufactured staple fibers at standard conditions of 70°F and 65 percent humidity.

C In one of the hotter regions of the country a home owner decides to keep her home at an average temperature of 80°F and 40 percent humidity. On a typical day the outside conditions are as follows dry-bulb temperature = 95°F and wet-bulb = 85°F. The city water supply is at 70°F and scarce. She therefore decides to use an electric refrigeration unit to cool the air entering the ventilating ducts. Summary of conditions ... 

Find the humidity. In Fig. 19.9, the humidity is the ordinate (along the right side of the graph) of the point on the saturation line (the 100 percent humidity line) that corresponds to the dew point, the latter being read from the abscissa along the bottom. In the present case, the humidity is found to be 0.011 lb water per pound of dry air (0.011 kg water per kilogram of dry air). 

Find the adiabatic-saturation temperature. Find the adiabatic-cooling line (these are the straight lines having negative slope) that passes through point A, interpolating a line if necessary, and read the abscissa of the point (point B) where this line intersects the 100 percent humidity line. This abscissa is the adiabatic-saturation temperature. In the present case, it is 80°F (300 K). 

Low humidity is a familiar problem to indoor gardeners. During the heating season, indoor humidity may only be 10 percent. Most tropical species require 40-60 percent humidity to thrive. The easiest way to increase humidity is to run a whole-house or portable humidifier. Not quite as efficient, but still worthwhile, is the practice of keeping a pan of water near a heat source. Grouping plants together also improves indoor humidity because leaf transpiration from a cluster of plants increases the air moisture in the immediate area. To further humidify a plant grouping, set plants on trays filled with l"of pebbles and A" of water. 

Many manufactured products, as well as solvents and raw materials are analyzed for their water content (percent humidity). Of all the available methods, the Karl Fischer titration is perhaps the most widely used, accounting more than 500 000 determinations performed daily world-wide. 

Figure 23.1 is a psychrometric chart for the air-water system. It shows the relationship between the temperature (abscissa) and absolute humidity (ordinale, in g water per kg dry air) of humid air from 0°C to 130°C at one atmosphere absolute pressure. Line as representing percent humidity and adiabatic saturation are drawn according to the thermodynamic definitions of these terms. Equations for the adiabatic saturation and wet-bulb temperature lines on the chart are as follows (Geankoplis 1983) ... 

Relative humidity is dehned as the ratio of the partial pressure of the vapor to the vapor pressure of the liquid at the gas temperature. It is usually expressed on a percentage basis, so 100 percent humidity means saturated gas and 0 percent humidity means vapor-free gas. By definition... 

BOUND AND UNBOUND WATER. If an equilibrium curve like those in Fig. 24.3 is continued to its intersection with the axis for 100 percent humidity, the moisture content so defined is the minimum moisture this material can carry and still exert... 

Consider, for instance, curve 2 for worsted yarns. This intersects the curve for 100 percent humidity at 26 percent moisture consequently, any sample of wool that contains less than 26 percent water contains only bound water. Any moisture that a sample may contain above 26 percent is unbound water. If the sample contains 30 percent water, for example, 4 percent of this water is unbound and 26 percent bound. Assume, now, that this sample is to be dried with air of 30 percent relative humidity. Curve 2 shows that the lowest moisture content that can be reached under these conditions is 9 percent. This, then, is the equilibrium-moisture content for this particular set of conditions. If a sample containing 30 percent total moisture is to be dried with air at 30 percent relative humidity, it contains 21 percent free water and 9 percent equilibrium moisture. Any amount up to a concentration of 26 percent is still bound water, but most of this can be evaporated into the air and hence is free water. Thus water can be both bound and free at the same time—partially bound to the solid but free to be evaporated. The distinction between bound and unbound water depends on the material itself, while the distinction between free and equilibrium moisture depends on the drying conditions. 

DEW POINT - Temperature at which vapor (at 100 percent humidity) begins to condense and deposit as liquid. 

It has been observed from the Table 13.4 that percent moisture absorbance of neat UPE and its composites increases with an increase in percent humidity level from 20-80%. Also, moisture absorbance has been found to increase with increase in percent fiber loading. This could be due to the presence of more -OH groups on the polymeric backbone at high loading, which causes more moisture absorption by UPE matrix-based composite materials. Further, among raw, mercerized and benzoylated fibers-reinforced UPE matrix-based composites, benzoylated fibers-reinforced ones have been found to have the highest moisture resistance, followed by mercerized and raw fiber. This trend matches with the trend obtained in the case of the water absorbance study and can be accounted for by the same explanation as given earlier in the water absorbance section. 

The average yearly relative humidity at the Hanford Site for the period from 1946 to 1980 was 54.4 percent. Humidity is higher in winter than in summer. The monthly averages range from 32.2 percent for July to 80 percent in December. The lowest monthly average (21.9 percent) occurred in July 1959 and the highest monthly average (90.5 percent) occurred in December 1950 (DOE-RL 1990). 

Keeping a consistent room temperature is essential for maintaining accurate positioning of the machine. A stable temperature throughout the PCB process assists in decreasing the amount of potential growth and contraction of the substrate materials. The drill room should be kept at a constant temperature and humidity, typically 72 2°F (22 +1.1°C) and 45 to 60 percent humidity. 

Figure 3.25 shows the equipment and psychrometric elements of a direct evaporative cooler. Its greatest application is in hot, arid climates. For example, the 100°F (38°C), 15-percent relative humidity (RH) outdoor air in Arizona could be cooled to 70°F (21°C), 82-percent RH with an 88-percent efficient unit. Efficiency is the quotient of the dry-bulb conditions shown at (2), (3), and (4) in Fig. 3.25fc. Note that the discharge air from a direct evaporative cooler is near 100-percent humidity and that condensation will result if the air is in contact with surfaces below its dew point. The discharge dew point in the above example is 64°F (18°C).

Mass transfer between liquids and gases depends on the vapor pressure of the components as functions of temperature. Thus appropriate selection of operating temperature and pressure allows the reverse (desorption or stripping, and dehumidification) to be performed. The purpose of absorption and stripping operations is to remove and recover the maximum amount of a particular component from a feed stream. It is most efficiently accomplished in multiple stages, as in tray or packed columns. Humidification and dehumidification arc similar in principle, but are directed toward control of an environment short of equilibrium (e.g., <100 percent humidity) for them, a single stage is ordinarily sufficient. 

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