Water vapor contents

Low Density Gases. A fan may have to operate on low density gas because of temperature, altitude, gas composition (high water vapor content of the gas can be a cause of low density), reduced process pressure, or a combination of such causes. To develop a required pressure, the fan has to operate at a considerably higher speed than it would at atmospheric pressure, and hence it must operate much closer to top wheel speed. Bearing life is shorter, and the fan tends to vibrate more or can be overstressed more easily by a slight wheel unbalance. Abrasion of the blades from dust particles is more severe. Therefore, a sturdier fan is needed for low density gas service. 

Many commercial gases are generated by burning hydrocarbons (qv) eg, natural gas or propanes, in air (see Gas, natural Liquified petroleum gas). The combustion process, especially the amount of air used, determines the gas composition. For a given fuel-to-air ratio, the gas composition can be used to determine the water vapor content required to achieve a desired equiUbrium carbon content of the austenite (see Combustiontechnology). 

Basically, an air-conditioning system consists of a fan unit which forces a mixture of fresh outdoor air and room air through a series of devices which 2LCt upon the air to clean it, to increase or decrease its temperature, and to increase or decrease its water-vapor content or humidity. 

The real atmosphere is more than a dry mixture of permanent gases. It has other constituents—vapor of both water and organic liquids, and particulate matter held in suspension. Above their temperature of condensation, vapor molecules act just like permanent gas molecules in the air. The predominant vapor in the air is water vapor. Below its condensation temperature, if the air is saturated, water changes from vapor to liquid. We are all familiar with this phenomenon because it appears as fog or mist in the air and as condensed liquid water on windows and other cold surfaces exposed to air. The quantity of water vapor in the air varies greatly from almost complete dryness to supersaturation, i.e., between 0% and 4% by weight. If Table 2-1 is compiled on a wet air basis at a time when the water vapor concentration is 31,200 parts by volume per million parts by volume of wet air (Table 2-2), the concentration of condensable organic vapors is seen to be so low compared to that of water vapor that for all practical purposes the difference between wet air and dry air is its water vapor content. 

Humidity The water vapor content present in atmospheric air. 

In the lower part of the atmosphere, the water vapor content of the atmosphere varies widely. On a volume basis, the normal range is 1 to 3 percent, though it can vary from as little as 0.1 percent to as much as 5 percent. 

The relatively high hydrogen content of gas, contained in the methane, leads to a water vapor content of approximately 18 per cent by volume in the flue products. The analysis of the other constituent gases is affected by whether or not the water vapor is included. 

NOTE Economizers are, in fact, fitted to some gas-fired, packaged FT boilers. The economizer is designed to recover some of the enthalpy lost as a result of the increased water vapor content in the exit gases from burning natural gas. 

The large value of Le results in a very strong dependence of vapor pressure on temperature. As a result, the water vapor content of the air is extremely variable, from parts per million by volume in the coldest parts of the atmosphere to several percent in the warmest and wettest... 

Water stability is a major challenge that has to be overcome before metal organic framework can be used in removing carbon dioxide from flue gas. The core structure of MOF reacts with water vapor content in the flue gas leading to severe distortion of the structure and even failure. As a consequence, the physical structure of MOF is changed, e.g., reduction of porosity and surface area, etc. that decreases the capacity and selectivity for C02. Complete dehydration of flue gas increases the cost of separation. It is therefore essential for MOFs to exhibit stability in the presence of water up to certain extent [91]. 

Mixing dry and water vapor-saturated air in defined proportions also can be used to generate constant relative humidity. Control of flow rates and the water vapor content of the dry and saturated air are essential [27,28],... 

At sea level, Pj is approximately 1 atm, but exhibits some temporal and spatial variability. For example, the annual mean pressure in the northern hemisphere is 0.969 atm and in the southern hemisphere is 0.974 atm, with monthly averages varying by as much as 0.0001 atm, i.e., about 1 mbar (1 atm = 1013.25 mbar). These fluctuations are caused by spatial and temporal variations in atmospheric temperature and water vapor content associated with weather, and seasonal and longer-term climate shifts. Pj is also affected by diurnal atmospheric tides, and it decreases with increasing altitude above sea level. Some gases, such CO2 and O2, exhibit seasonal variability that is caused in part by seasonal variability in plant and animal activity (see Figures 25.4 and 6.7). 

The warming climate is likely to induce changes in the hydrological cycle that will lead to further climate change. Increased heating should increase the rate of evaporation and, hence, the amount of water vapor, which is a GHG. The IPCC s Fourth Assessment Report, published in 2007, finds that the average atmospheric water vapor content has increased since at least the 1980s over land and ocean as well as in the upper troposphere. ... 

In the conditions described by the curve ( dew point temperature ), dew or clouds are formed, i.e. the water vapor condenses. If the conditions shown in the part above the curve are achieved, the condensed water vapor falls in the form of precipitation. In the conditions below the curve, water vapor enrichment or a cooling-off may occur without causing any formation of dew or precipitation. Since the air temperature is substantially determined by relief and altitude, these factors also have an impact on the maximum possible water vapor content in the air. In principle, precipitation is formed when air masses cool down, the consequences of which can be either dynamic (orographical and frontal induced precipitation) or thermal (convective induced precipitation) (Fig. 2, [6]). 

In theory, since the absolute water vapor content of air reduces as the temperature sinks or the altitude rises, the volume of precipitation should decline. Yet the opposite is the case, particularly in mountainous regions in the temperate latitudes here the annual volumes of precipitation generally tend to rise with the altitude. This is the result, on the one hand, of higher wind speeds at higher altitudes, which cause a relatively large shift in humid air masses. On the other hand, precipitation occurs more frequently and often at a much greater intensity. 

Atmospheric conditions over the sampling site were variable, and several different air masses passed by- On 16 March the weather map of the northern hemisphere shows two high pressures, one over Lake Baikal and the other over South Korea, so that the sampling site was just in between the air was calm, cloudy, and conducive for pollutants to build up. Later the north high pressure moved southwards. By midday of 17 March the sampling site was dominated by high pressure. At that time air flow over the site was mainly from the northwest, the wind speed was 3 meters/second, and the water vapor content was very low. The sky was clear, and vertical dilution was effective. 

The number of reactions catalyzed by zeolites is continually increasing. Synthetic faujasites containing transition metal cations are active for the complete oxidation of H2, CO, C2H4, NH3 (9). Upon interaction of NH3 with 02 over CrY and AgY, N20 and N2 are formed (9). Mahida et al. (21) investigated the oxidation of propylene over Cu2+Y. Depending on the temperature and on the water vapor content in the reaction mixture,... 

Figures 16-18 and 16-19 give the dew-point water-vapor content of a nitrogen-free. natural gas in equilibrium with liquid water.

The percent water vapor content of air at saturation versus representative temperatures and pressure altitudes is given in Table 5. 

TABLE 5. PERCENT WATER VAPOR CONTENT OF AIR AT SATURATION VERSUS REPRESENTATIVE TEMPERATURES AND PRESSURE ALTITUDES... 

Analyses of Hue gases are always repotted on a volume basis, dry, when an Orsai or other type of gas analysis is used. Flue gases are cooled to room temperature and bubbled through water in most gas analyses, so that the gas becomes saturated with water vapor. This would occur even if no water vapor were formed during combustion. Proportionate parts of the water vapor content of the gas will be absorbed with the different constituents of the gas so that the resulting analysis may be safely assumed to be that of dry gas. These percentages may also be expressed as 10.4 moles COi, 2.8 moles Oi. and 86.8 moles N2 each per 100 moles of dry flue gas. 

Percent relative humidity is the best known ami perhaps the most widely used method for expressing the water vapor content of air. Percent relative humidity is defined as the ratio of the prevailing water vapor pressure c . to the water vapor pressure if the air were saturated, t, multiplied by 100 ... 

Basically, perecnl relative humidiiy is an indicator ol the water vapor saturation deficit of ihe gas mixture, rather ihun an indicator of sorption, desorption, comfort, or evaporation. A measurement of RH without a corresponding measurement of dry bulb temperature is not of particular value, since the water vapor content cannot be determined from % RH alone... 

Fie. 4. Calculation of the water vapor content in an electrolytic hygrometer is dependent on precise control of the flow rate This arrangement controls the sample pressure across tile cell, ensuring conect flow regardless of input pressure fluctuations... 

In order to minimize cold enrichment and to achieve a very low carbon monoxide content in the product, a second methanation stage is frequently employed. To achieve sufficient driving force" to make the reaction proceed, the water vapor content is reduced by cooling the gas, rejecting condensate, and reheating to the required reaction temperauire. 

Analogous experiments with mixtures of identical combustion temperature containing various excess amounts of carbon monoxide showed that the flame velocity is proportional to [CO ]1/2, where [CO ] is the carbon monoxide concentration in the reaction zone. From this it follows that the chemical reaction in a flame is first order in carbon monoxide. The role of water in the combustion of carbon monoxide is well known. Analysis of available data shows that the flame velocity is proportional to [H20]1//2, i.e., the reaction is first order in water vapor content. The influence on combustion of such flegmatizers as CC14 may be ascribed to the binding of hydrogen by halogen with the formation of a molecule of HC1, which is dissociable only with difficulty. However, the latest experiments by Kokochashvili in our laboratory show that the influence of the... 

Thus, in a stoichiometric mixture of 2CO + 02 with 2% water content, the flame velocity is 90 cm/sec addition of 1.8% CC14 decreases the flame velocity to 40 cm/sec. At the limit, with a 4.5% CC14 content, the minimum velocity is 4 cm/sec, in good agreement with absolute calculations and with the minimum velocities in other carbon monoxide mixtures. Drozdov showed experimentally that any factor which increases the flame velocity (an increase in the carbon monoxide and water vapor content, an increase in the combustion temperature) simultaneously increases the amount of flegmati-zor (tetrachloride carbon) which is necessary to render the mixture incombustible. 

---