Fog formation

Fog Formation A vapor or vapor-gas mixture may be cooled below the dew point. If cooled sufficiently, droplets of condensate may form in the bulk vapor stream. The droplet formation process is known as homogeneous nucleation and significant nucleation will result in a noticeable fog in the vapor. Fog droplets 

Fogging may be prevented or reduced by reducing the temperature difference or increasing the inlet superheat. Inlet gases can be filtered to remove foreign nuclei. The gas stream can be seeded with condensation nuclei to produce drops that will be captured by conventional demisters. The vent can be heated to reduce fog. Most of these methods are costly or impractical an alternative to preventing fog is to allow it to form and then remove it with special demisters which first coalesce the droplets before removal. 

Fog can form when a vapor mixture cools below its saturation temperature, in a similar manner to atmospheric fog formation. In condensers, it takes place when the heat transfer rate from the cold surface exceeds the mass transfer rate to the surface. Fog droplets range in size from 0.1 to 40 p,m (78, 380). Below about 3 to 5 p.m, the droplets tend not to collect on the cool surface but remain with the vapor and pass down the vent line in partial condensers. Reported measurements (381) suggest that in some cases, 20 to 40 percent of the mixture can be in the form of fog droplets. 

Fog is difficult to separate from the vapor. Fog droplets containing corrosive, reactive, or noxious materials may settle and create problems downstream (242). If vented to the atmosphere, they may create an emission problem. Other problems are reduced condensate yield and possibly a somewhat reduced heat transfer rate (78). 

Fog formation is favored by conditions which slow the mass transfer rates and enhance the heat transfer rate. Factors which favor fog formation by slowing the mass transfer rates are a high ratio of noncondensables to condensable vapor, and a high molecular weight (low diffusivity). Factors which favor fog formation by speeding the heat transfer are a high temperature difference between the vapor and interface and low initial superheat. 

In order for fog to form, a nuclei must first be formed. Homogeneous nucleation is difficult because of the energy barrier associated with the creation of an interface. The rate of nucleation is a very steep function of the supersaturation ratio. Generally, below a certain critical supersaturation ratio nucleation is slow enough to be ignored, and above this, it will be substantial. The critical supersaturation ratio can be predicted from Amelin s equation (78, 242, 380). Procedures for fog formation prediction are outlined in the cited references. 

The following procedures have been recommended (78, 242, 380) for minimizing fog formation  

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If condensation requires gas stream cooling of more than 40—50°C, the rate of heat transfer may appreciably exceed the rate of mass transfer and a condensate fog may form. Fog seldom occurs in direct-contact condensers because of the close proximity of the bulk of the gas to the cold-Hquid droplets. When fog formation is unavoidable, it may be removed with a high efficiency mist collector designed for 0.5—5-p.m droplets. Collectors using Brownian diffusion are usually quite economical. If atmospheric condensation and a visible plume are to be avoided, the condenser must cool the gas sufftciendy to preclude further condensation in the atmosphere. 

Growth on Foreign Nuclei As noted above, foreign nuclei are often present in abundance and permit fog formation at much lower supersaturation. For example,... 

While generally fog formation is a nuisance, it can occasionally be useful because of the high surface area generated by the fine drops. An example is insecticide application. 

As mentioned in the previous section, the increased number of nuclei in polluted urban atmospheres can cause dense persistent fogs due to the many small droplets formed. Fog formation is very dependent on humidity and, in some situations, humidity is increased by release of moisture from industrial processes. Low atmospheric moisture content can also occur, especicilly in urban areas two causes are lack of vegetation and rapid runoff of rainwater through storm sewers. Also, slightly higher temperatures in urban areas lower the relative humidity. 

Schotte, W., 1988, Thermodynamic Model for HF Fog Formation , letter to C. A. Soczek, dated August 31, 1988, E.I. DuPont de Nemours Company, duPont Experimental Station, Engineering Department, Wilmington, DE. 

Steinmeyer, D. E., Special Problems in Process Heat Transfer (Fog Formation in Partial Condensers), AIChE 71 Nat l. Meeting, Dallas, Feb. (1972). 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

Fogging is formation of small water droplets (visible condensation) on the surface of a polymer film. Undesirable effects may result from fog formation, such as reduction of clarity and dripping. Incorporation of antifogging agents eliminates the reduction of transparency by migration to the surface and increases the polymer surface critical wetting tension. This results in... 

Fog formation. In the condensation of a vapour from a non-condensable gas, if the bulk temperature of the gas falls below the dew point of the vapour, liquid can condense out directly as a mist or fog. This condition is undesirable, as liquid droplets may be carried out of the condenser. Fog formation in cooler-condensers is discussed by Colburn and Edison (1941) and Lo Pinto (1982). Steinmeyer (1972) gives criteria for the prediction of fog formation. Demisting pads can be used to separate entrained liquid droplets. 

Other applications of microparticles include spray drying, stack gas scrubbing, particle and droplet combustion, catalytic conversion of gases, fog formation, and nucleation. The removal of SO2 formed in the combustion of high-sulfur coal can be accomplished by adding limestone to coal in a fluidized bed combustor. The formation of CaO leads to the reaction... 

Direct development by sulfite ion does not take place, probably because the rate of reduction of silver ions by the sulfite is much smaller than the rate of solution of the silver halide (see Section YI of this chapter). The writer has obtained physical development, however, with a solution of silver nitrate and sodium sulfite. A specially hardened gelatin film which was able to withstand the action of the solution at 70° for 40 minutes was used. The developing solution contained 1.7 g. silver nitrate and 13 g. sodium sulfite per liter. Fog formation was relatively high, as might be expected. 

The relative rate of fog formation compared to image development increases with increasing pH of the hydroxylamine solution. This is to be expected from analogy with the studies of the reduction of silver chloride and silver bromide precipitates, where the change in nitrogen yield shows that the uncatalyzed reaction becomes more and more prominent as the pH is increased. 

Data obtained by Shiberstoff (63) on the development of photographic film in solutions of conventional developers offer further evidence in confirmation of the latter point. Shiberstoff obtained a general increase in selectivity with decreasing temperature of development for all agents tested. He calculated selectivity in terms of the ratio of the rate of image development to fog formation, using as the rate of the former the reciprocal of the time required to attain a density of 1.5 for a fixed... 

FIGURE 8.20 Schematic of role of atmospheric aerosols in fog formation and evaporation (adapted from Munger et at., f 983). 

On the humidity chart of Figure 2.5, temperatures are plotted as abscissas and humidities as ordinates. Any point on the plot represents a specific mixture of air and water. The curve marked 100% humidity refers to saturated air and is a function of air temperature. Any point to the left of the saturation curve represents a mixture of saturated air and liquid water (this portion of the plot is useful in determining fog formation). Any point to the right of the saturation curve represents undersaturated air. Any point on the temperature axis represents bone-dry air. The curves between the two limits (saturated line and the temperature axis) represent mixtures of air and water of definite percentage humidities. Linear interpolation between the saturation curve and the temperature axis locates lines of constant percentage humidity. 

Figure 6.12 Portion of psychrometric chart illustrating fog formation.

Figure 6.13 Fog formation is assisted by wake formation and hourly variations in the ambient air humidity.

Under certain conditions, the exhaust air of conventional mechanical draft cooling towers may form a fog plume, causing visibility and icing problems to highways and equipment. In cases where this cannot be tolerated, a combination wet/dry cooling tower is shown to be effective fog plume control method. The paper describes the basic phenomena of cooling tower fog formation. The operation and performance characteristics of the wet/dry tower are discussed as well as a method of select wet/dry design criteria. 11 refs, cited. 

The effect of condensation upon transfer rates with application to flue-gas washing plants and cooling towers are discussed. Theoretical models were developed for determining the rate of heat and mass transfer under conditions where fog formation prevails. Derived relationships are functions of the vapor and liquid equilibria and local heat and mass transfer of driving forces. They were used for a numerical study of the amount of fog formation as a function of the operational variables of a flue-gas washing plant in which the inlet gas temperature is typically... 

C with a water content of 0.075 kg/kg. Although heat and mass transfer rates were relatively insensitive to the choice of the model, the amount of fog formation was not. The models neglect the effects of condensation within the boundary layer, thus underestimating fog formation by a factor of up to three. The amount of fog formed in flue-gas washing plants increased up to a maximum value with decreasing feed-water temperature over a narrow band of liquid-to-gas ratios. 

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