Water vapor relative humidity

Humidity is defined as the concentration of water molecules in the atmosphere. In practice, parameters important in the measurement of humidity are partial pressure of water, mixing ratio, specific humidity, absolute humidity, mole fraction of water vapor, relative humidity (r. h.), and dew-point temperature. Of these measurements, relative humidity, which is the ratio of the actual water pressure to the saturated pressure, is widely used. The concentration of water molecules in the air is low and, moreover, the effects of water are very complicated not only chemically but also physically. The measurement of humidity is difficult compared to the measurement of temperature. In many industries from the electronic industry to agriculture, there is a demand for humidity control. For example, a dry atmosphere is required for the pro-... 

Moisture Condensation (Ref 21)i Air normally contains moisture in the form of invisible water vapor. Relative humidity, expressed as a percentage, provides a measure of the amount of water vapor actually in the air compared to the total amount of water vapor that the air can hold. As the temperature of air increases, its capacity to hold water vapor increases. 

The moisture content of a given component is in turn determined by the existing vapor pressure of water (or relative humidity) and is described by an isotherm for that component. Furthermore, it seems plausible to assume that the deterioration of the food is a function of the moisture content of the particular sensitive component (or components) that is involved in the deteriorative reaction. The over-all moisture content is thus of little significance. 

Reaction with water vapor or humid air (60-95% relative humidity) in the temperature range 20-500 °C results in the formation of a WO3 layer. No lower oxides or hydrates could be found by ESCA. The thickness of the oxide layer increases with increasing humidity. The reaction rate depends on temperature and I20]/[H2] partial pressure ratio and is more rapid than with liquid water. Water molecules are adsorbed at the tungsten surface and dissociate. The oxygen atoms difiuse into the tungsten metal, forming at first a solid solution and then the oxide compound, while, hydrogen escapes as element. 

Since saturated NaCl brine has a vapor pressure approximately 75% of that of water, a relative humidity greater than 75% will cause condensation onto the salt. This is known as the critical humidity. KCl solutions have higher vapor pressures and therefore higher critical humidity. Because KCl also has a higher temperature coefficient of solubility, the concentration of saturated solutions increases more rapidly with temperature. The critical humidity therefore is not so nearly constant as is the case with NaCl, and it decreases with increasing temperature. At most ambient temperatures, the critical humidity of KCl is 85% or higher. 

As mentioned in Section IX-2A, binary systems are more complicated since the composition of the nuclei differ from that of the bulk. In the case of sulfuric acid and water vapor mixtures only some 10 ° molecules of sulfuric acid are needed for water oplet nucleation that may occur at less than 100% relative humidity [38]. A rather different effect is that of passivation of water nuclei by long-chain alcohols [66] (which would inhibit condensation note Section IV-6). A recent theoretical treatment by Bar-Ziv and Safran [67] of the effect of surface active monolayers, such as alcohols, on surface nucleation of ice shows the link between the inhibition of subcooling (enhanced nucleation) and the strength of the interaction between the monolayer and water. 

Relative Humidity (rh). Relative humidity is the ratio of the mole fraction of water vapor present in the air to the mole fraction of water vapor present in saturated air at the same temperature and barometric pressure it approximately equals the ratio of the partial pressure (or density) of the water vapor in the air to the saturation pressure (or density) of water vapor at the same temperature. 

Sohd sorbent materials have the abiUty to adsorb water vapor until an equiUbrium condition is attained. The total weight of water that can be adsorbed in a particular material is a function of the temperature of the material and of the relative humidity of the air (see Adsorption). To regenerate the sorbent, its temperature must be raised or the relative humidity lowered. The sohd sorbents most commonly used are siUca (qv), alumina (see Aluminum compounds), and molecular sieves (qv). 

Anhydrous sodium tripolyphosphate is slow to hydrate in contact with the atmosphere under normal ambient conditions and generally remains free-flowing. If the relative humidity is below a critical relative humidity, which is different for both anhydrous forms of STP and dependent on temperature, hydration does not take place. For prolonged storage at room temperature, relative humidities above ca 60% in the air result in water absorption. For shorter periods, high levels of humidity can be tolerated. However, even at higher humidities, the amount of water absorbed is small. The heats evolved from vapor hydration of STP-I and -II have been estimated at 343 and 334 kj /mol (82.0 and 79.9 kcal/mol), respectively (25). 

The humidity term and such derivatives as relative humidity and molal humid volume were developed for the air—water system. Use is generally restricted to that system. These terms have also been used for other vapor—noncondensable gas phases. 

The effect of plasticizers and temperature on the permeabiUty of small molecules in a typical vinyUdene chloride copolymer has been studied thoroughly. The oxygen permeabiUty doubles with the addition of about 1.7 parts per hundred resin (phr) of common plasticizers, or a temperature increase of 8°C (91). The effects of temperature and plasticizer on the permeabiUty are shown in Figure 4. The moisture (water) vapor transmission rate (MVTR or WVTR) doubles with the addition of about 3.5 phr of common plasticizers (92). The dependence of the WVTR on temperature is a Htde more comphcated. WVTR is commonly reported at a constant difference in relative humidity and not at a constant partial pressure difference. WVTR is a mixed term that increases with increasing temperature because both the fundamental permeabiUty and the fundamental partial pressure at constant relative humidity increase. Carbon dioxide permeabiUty doubles with the addition of about 1.8 phr of common plasticizers, or a temperature increase of 7°C (93). 

The water-vapor transmission rate (WVTR) is another descriptor of barrier polymers. Strictly, it is not a permeabihty coefficient. The dimensions are quantity times thickness in the numerator and area times a time interval in the denominator. These dimensions do not have a pressure dimension in the denominator as does the permeabihty. Common commercial units for WVTR are (gmil)/(100 in. d). Table 2 contains conversion factors for several common units for WVTR. This text uses the preferred nmol/(m-s). The WVTR describes the rate that water molecules move through a film when one side has a humid environment and the other side is dry. The WVTR is a strong function of temperature because both the water content of the air and the permeabihty are direcdy related to temperature. Eor the WVTR to be useful, the water-vapor pressure difference for the value must be reported. Both these facts are recognized by specifying the relative humidity and temperature for the WVTR value. This enables the user to calculate the water-vapor pressure difference. Eor example, the common conditions are 90% relative humidity (rh) at 37.8°C, which means the pressure difference is 5.89 kPa (44 mm Hg). 

A humidification subsystem controls the temperature, flow rate, and relative humidity of the sweep stream. Air and water can be fed to a Hquid-gas packed contactor to produce the desired moisture level ia the vapor stream. The saturation temperature controls the water loading of the air which can be heated to give exactly the desired relative humidity. 

Ethanol removed by the vapor stream can be recovered by condensation, vapor recompression, or scmbbiag. Ia the first two methods, the coaceatratioa of the recovered ethanol depends on the relative humidity of the sweep stream and the ratio of sweep and permeation rates. In scmbbiag, the rate of water deflvery to the Hquid-gas coatactor affects the ethanol concentration ia the recovered stream. 

The life persistency of a smoke cloud is deterrnined chiefly by wind and convection currents in the air. Ambient temperature also plays a part in the continuance or disappearance of fog oil smokes. Water vapor in the air has an important role in the formation of most chemically generated smokes, and high relative humidity improves the performance of these smokes. The water vapor not only exerts effects through hydrolysis, but it also assists the growth of hygroscopic (deliquescent) smoke particles to an effective size by a process of hydration. Smoke may be generated by mechanical, thermal, or chemical means, or by a combination of these processes (7). 

The drying mechanisms of desiccants may be classified as foUows Class 1 chemical reaction, which forms either a new compound or a hydrate Class 2 physical absorption with constant relative humidity or vapor pressure (solid + water + saturated solution) Class 3 physical absorption with variable relative humidity or vapor pressure (soHd or liquid + water + diluted solution) and Class 4 physical adsorption. 

Because calcium chloride has a number of hydrates, the one that is in equiUbrium with a saturated solution is a function of the temperature. In this case, the sohd is dissolved as it absorbs water to form the saturated solution, and three phases are present soHd, saturated solution, and vapor. Systems having these three phases, or two soHds and a vapor phase, have a constant vapor pressure at a given temperature. Therefore, Class 2 drying agents can be used to maintain a constant relative humidity. 

Percent saturation is the ratio of the partial pressure of a condensable vapor ia a gas to the vapor pressure of the Hquid at the same temperature, expressed as a percentage. For water vapor ia air this is called percent relative humidity. 

At any given relative humidity, x is constant. In a typical siHca gel—air—water vapor system at 5—50°C, pjp = 1.79 x), where... 

Percentage relative humidity is defined as the partial pressure of water vapor in air divided by the vapor pressure of water at the given temperature. Thus RH = lOOp/p,. 

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... 

The relative humidity is obtained by dividing the calculated partial pressure by the vapor pressure of water at the diy-bulb temperature. Thus ... 

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