Abstract:
Fog is dispelled from a site by passing fog-laden air into a drying unit where it is contacted with an aqueous solution of calcium chloride under conditions which effectuate absorption of the water particles and some water from the air effective to increase the temperature of the air and dry it to a predetermined relative humidity range, then discharging the dried heated air from the unit into fog-laden air at the site to effectuate vaporization of suspended water particles and associated cooling of the discharged air without development of thermals of the discharged air sufficient to create substantial circulation of fog-laden air into the site.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to processes of weather control or modification, and more particularly, to methods and apparatus for dispelling fog. 
     2. Description of Related Art 
     There is a need for a method of dispelling fog at definable sites, such as airports or racetracks, in order that events such as flight arrivals and departures at airports or racing programs at race tracks can occur as scheduled. Although there has been substantial effort directed to meeting this need, the methods that have been developed still have not sufficed, for reasons including environmental pollution and cost. 
     Fog is a weather condition in which moisture particles are suspended in saturated air near the ground at levels of between 0.1 and 0.5 grams per cubic meter. Control or dispersement of that fog requires the evaporation or removal of these suspended particles. Various fog-dissipation methods have been tried in the past. 
     Heating fog-laden air evaporates the suspended water particles by increasing the air temperature, adding heat of vaporization, and increasing the amount of moisture that the air can hold. This process creates thermals of warm air which rise from the site, circulating cool, fog-laden air into the site. Heating air to dissipate fog was used during World War II in Great Britain when airplane engines were run along the runway. Barrels of burning fuel oil were also used along runways to add heat to the air and evaporate the suspended water particles. Both of these concepts added air pollutants, had high operating costs, and did not accomplish the desired result unless operated continuously. 
     Helicopter downwash has been applied to clear fogs and clouds with small scale success, but it has not proved practical for large-scale operations. 
     Subcooling the air removes suspended liquid and vapor by cooling and collecting the moisture in suspension, dropping the air temperature, and condensing moisture vapor from the air. Air is subcooled using a mechanical cooling system which circulates a cold liquid through a coil. Both the latent and sensible heat are removed from the air as it is circulated over the coil. After the moisture and sensible heat have been removed, the cooled dried air is reheated to the surrounding temperature so that it may absorb the suspended moisture from the wet air in the discharge area of the fan system. This process is expensive due to the mechanical removal of both sensible and latent heat and the addition of sensible heat back to the air. Large quantities of high-cost, limited-supply electricity are used in this process. The initial cost and maintenance costs are also high. 
     Hydroscopic particles can be seeded from aircraft to evaporate fog droplets and drop the resultant dilute solution droplets to the ground. This method has been tested in many places in the world with small-scale success, but since the material is thrown away every time, the cost is high and environmental pollution becomes severe. Examples of U.S. Pat. Nos. involving use of chemicals to dispel fog include 2,934,275; 3,274,035; 3,378,201; 3,434,661; 3,608,810; 3,608,820; 3,730,432; 3,802,624; 3,899,129; 4,600,147; and 4,653,690. U.S. Pat. No. 2,934,275 discloses a process of dispelling fog by forming a mixture of an aqueous solution of chloride salts of calcium, magnesium or zinc with thickening agents of starches, sugars or proteins into a mist having particles smaller than 1/2 mm in diameter; forming a normally liquid chlorinated aliphatic hydrocarbon into a mist having particles smaller than 1/2 mm in diameter; and commingling the mist with the fog to be treated. Calcium chloride is a chemical desiccant. Chemical dessicants act as defoliates and are environmentally harmful to plant life, in practical effect prohibiting their utility as an airborne treatment. 
     Calcium chloride has been used to dry city gas; for example, see the Chemical Engineers Handbook, Third Edition, John H. Perry, Ph.D., Ed., McGraw-Hill Book Co., Inc., at topic &#34;Drying of Gases,&#34; pp. 877-880. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, a method is proved for dispelling fog from a site such as an airport. Fog-laden air containing suspended water particles at the site is moved into a chamber or housing through an inlet to the chamber and in the chamber is passed into contact with an aqueous solution of calcium chloride under conditions effective for the solution of calcium chloride to absorb the suspended water particles from the fog-laden air and increase the temperature of the air a controlled extent so that the air is heated and dried to a predetermined relative humidity range. The heated dried air is then discharged from the chamber through at least one outlet into fog-laden air at the site under conditions effective to vaporize the suspended water particles in that fog and cool the discharged air without the development of thermals of rising discharged air that are sufficient to create substantial circulation of fog-laden air from outside the site into the site. 
     The concentration and temperature of the aqueous solution of calcium chloride and the volume of flow of air through the chamber is controlled to regulate the dryness and temperature of the discharged air to the predetermined relative humidity range. 
     The chamber may include a plurality of ducts associated with the chamber outlet, each duct having at least one outlet and being organized for distribution of dried air at the site where fog is to be dispelled. By controlling one or more of the (i) concentration and (ii) temperature of the aqueous solution of calcium chloride, the (iii) flow volume of air through the chamber, and the (iv) distribution of dried air through the ducts, vaporization of suspended water particles and cooling of the discharged air is essentially horizontally effectuated to dispel site fog in horizontal strata without development of thermals of rising discharge air sufficient to create substantial circulation of fog-laden air from outside the site into the site. Substantial circulation occurs when the discharge of the drying unit vertically ascends through the surrounding air to such an extent that it induces an influx of cooler heavier fog-laden air from outside the site equal to the discharge flow of the drying unit. 
     According to the scope of the drying requirements of a particular site and conditions employed, the manner of contacting the aqueous solution of calcium chloride with the fog-laden air suitably may be by sprays or tower packings to ensure large surface exposure and low pressure drop. 
     Apparatus for dispelling the site fog preferably comprises a chamber having a inlet or outlet and a filter media disposed in the chamber between the inlet and outlet. Sprayers are operatively associated with the chamber for spraying an aqueous solution of calcium chloride onto the filter media, and provision is made for collecting solution of calcium chloride draining from the media and recirculating it back over the media. An air mover, such as a large-volume, low-static fan, is operatively associated with the chamber to move the fog-laden air into the chamber, through the media, and out the chamber outlet as dried discharge air. Ducting, preferably inflatable, is arranged with the outlet of the chamber for distributing the dried air according to the needs of the site. In the usual application, the apparatus will include a reservoir for the solution of calcium chloride within the recirculation circuit. In smaller applications, suitably the solution of calcium chloride that drains down from the media and is collected in the base of the chamber is recirculated over the media until it absorbs approximately its weight in water. The dilute solution of calcium chloride may be then pumped from the reservoir and replaced with a concentrated solution of calcium chloride. Particularly where site conditions call for plurality of treatment units, the replacement process may use transport of the solution of calcium chloride to and from a central concentrator system. Suitable transport may be lined or fiberglass tank trucks or a piping system. Liquid volume may be monitored to determine when the solution of calcium chloride should be changed. 
     The recirculation liquid may be heated at a central concentrator to reduce the dilution of the liquid resulting from removal of moisture from the fog-laden air by the solution of calcium chloride. 
     In large-scale permanent installations, the central concentrator may be included in the solution of calcium chloride recirculation circuit, and after-coolers for cooling the recirculation liquid to a temperature within a predetermined range at or slightly above the temperature of the fog-laden air may be provided in the recirculation circuit after the liquid is heated and before the liquid is recirculated onto the media. 
     In accordance with this invention, about 0.1 to 0.5 gram per cubic meter of suspended particulate moisture and about 5 grams per cubic meter of moisture vapor is condensed and absorbed by the solution of calcium chloride. Temperature elevation of the treated air results from the heat of vaporization given up by the moisture-laden air as the moisture condenses and is absorbed. The heat of the discharged dry air evaporates suspended water particles in the foggy air at the site, removing heat of vaporization from the discharged air and cooling it to surrounding site temperatures. Each cubic meter of foggy air that passes through the system and is dried is effective to vaporize suspended water particles in and thereby clear about 50 cubic meters of foggy air at the site. 
     At a barometric pressure of about 101.325kP a , and at a temperature of about 10° C., fog-laden air or air in a foggy condition contains about 9.5 to about 9.9 grams per cubic meter of water, of which about 0.1 to 0.5 grams per cubic meter is water in excess of saturation capacity of the air at that temperature and pressure. At these conditions air dried and discharged from the treating unit in accordance with this invention will have a water content of about 4.4 grams per cubic meter and a relative humidity near 50%, about 47%. At about the same barometric pressure and at a temperature of 20° C., the saturation capacity of air is about 17.3 grams per cubic meter, and after removal of 0.1 to 0.5 grams per cubic meter of suspended particulate water and about 5 grams per cubic meter of water vapor, the dried air discharged from a treating unit has a water content of about 12.3 grams per cubic meter and a relative humidity of 71%. 
     If, for example, the volume of air to be cleared is one hundred million cubic meters and has a suspended particulate moisture of 0.1 grams per cubic meter of fog, about 10 tons of suspended particulate moisture must be evaporated to clear the fog. At least an equal tonnage of the solution of calcium chloride is required, and with a conventional safety factor of 4, preferably 40 tons of a solution of calcium chloride is employed in the process. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a drying unit used for treating fog in accordance with this invention; and 
     FIG. 2 is a side elevational view of the unit shown in FIG. 1. 
     FIG. 3 is a schematic view illustrating ducting leading from the unit of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, a device generally indicated by reference numeral 10 and constructed to dispel fog in accordance with this invention is schematically illustrated. The unit includes a chamber 11 consisting of a top panel 12, bottom panel 13, side panels 14 and 15, and end panels 16 and 17. End panel 16 does not close the chamber in a major area below top 12, defining an opening or inlet 18. End panel 17 does not close the chamber in a major area below top 12, defining an opening or outlet 19. A large-volume, low-static fan 20 is mounted for rotation in outlet 19 within a fan shroud (not illustrated). Mounted horizontally between side panels 14 and 15 at the upper margin of end 16 (lower margin of inlet 18) is a perforated horizontal support member 21. Mounted vertically between side panels 14 and 15 and joining the horizontal support 21 remote from inlet 18 is a vertical partition 22. 
     Interposed between the inlet 18 and outlet 19 upon a perforated horizontal support 21 is a filter media which includes two identical filter structures 23 and 23&#39;. Filter media 23, 23&#39; are angled away from each other at their narrow ends 24 and 24&#39; nearest inlet 18 so one corner of each of ends 24 and 24&#39; adjoins the sides 14 and 15 of chamber 11 for the full depth of the filter media 23 and 23&#39;. Filter media 23 and 23&#39; are joined at their other narrow ends 25 and 25&#39;, remote from inlet 18. This orientation maximizes the surface area facing foggy air admitted by inlet 18 and requires all admitted air to pass through the filter media 23 and 23&#39; to reach outlet 19. 
     Supported above filter media 23 and 23&#39; are sprayers 26 and 26&#39; comprising tubing 27 provided with numerous apertures 28 along the tubing length through which liquid in the tubing is sprayed down upon the media to thoroughly wet the media through its full extent with an aqueous solution of calcium chloride and provide a high surface area of solution of calcium chloride for contact with foggy air admitted by inlet 18. Below perforated horizontal support 21, a collector 37 flows the solution of calcium chloride draining from media 23 and 23&#39; into the reservoir 29 defined by end 16, sides 14 and 15, vertical partition 22, and bottom panel 13. A pump 30 recirculates liquid from reservoir 29 to sprayers 26 and 26&#39; and provides spray pressure. The pump 30 and motor for fan 20 are powered by an energy source (not illustrated). 
     In the operation of device 10, reservoir 29 is charged with an aqueous solution of calcium chloride, and pump 30 is engaged to circulate liquid through tubing 27 to sprayers 26 and 26&#39; onto filter media 23 and 23&#39; to thoroughly wet the filter media through their entire extent. Fan 20 is then energized. Fog-laden air at the site of chamber 11 is moved by the draw of the fan into unit 10 through inlet 18, and under the further draw of the fan is passed in contact across the filter media 23 and 23&#39;, wetted with the solution of calcium chloride for absorption of the water particles and an effective amount of the water vapor from the fog-laden air to increase the temperature of the air a controlled extent, thereby heating and drying the air to a predetermined relative humidity range. Solution of calcium chloride draining from media 23 and 23&#39; is collected by collector 37 and flowed into reservoir 29, where pump 30 recirculates it back through tubing 27 and out sprayers 26 and 26&#39; onto the media. The dried air heated by the heat of vaporization received from the water vapor is then discharged from chamber 11 through outlet 19 under the influence of fan 20. Referring to FIG. 3, the heated, dried fog-free air from unit 10 is distributed at the site of the fog by ducting, suitably a plurality of ducts 38, 39, duct 38 having a plurality of outlets 38a, 38b, 38c, 38d, duct 39 having a plurality of outlets 39a, 39b, 39c,  39d. 
     The device 10 constructed in accordance with the present invention was tested for ability to reduce the humidity of environmental fog air. Relative humidity at the test site was low, so a swamp cooler was applied to the inlet of the test device to produce a humid air. The dew point (T id ) and temperature (T i ) at the inlet of the device, and the dew point (T od ) and temperature (T o ) as well as the flowrate (F o ) at the outlet of the adapter, were measured for different flowrates (in cubic feet per minute or cfm). The flowrate (F o ) at the outlet was determined by a hand-held anemometer, and dew point was determined by a dew point hygrometer. The relative humidities at the inlet (RH i ) and outlet (RH o ) were calculated from their corresponding dew points and temperatures. The results are listed in Table 1. 
     
                       TABLE 1______________________________________RESULTS UNDER THE CONDITIONS OF TEST INWHICH TEMPERATURES, DEW POINT AND RELATIVEHUMIDITY OF ROOM AIR WERE, RESPECTIVELY,23° C., 0° C. and 22%F.sub.O                 RH.sub.i            RH.sub.o(cfm) T.sub.i (°C.)          T.sub.id (°C.)                   (%)   T.sub.O (°C.)                                T.sub.Od (°C.)                                       (%)______________________________________337   13       11       88    20     4      35477   13       11       88    20     5      37640   12.5     11       91    20     5      37______________________________________ 
    
     The effective volume of the filter through which the air flow passed was 4 cubic feet. The residence time (t r ) of the air was estimated by dividing this volume with the flowrate. The flow velocity (V f ) at the filter was obtained by dividing the flowrate with the effective filter cross-sectional area of 4 ft 2 . 
     Describing the vapor pressure of the saturated solution at 20° C. (outlet temperature) as e&#39; s , the vapor pressure at the inlet as e i  and the vapor pressure at the outlet as e o , the relaxation time T may be defined by the following equation: ##EQU1## 
     From the results given in Table 1, V f , t r , T and (e o  -e&#39; s )/(e i  -e&#39; s ) were calculated for different flowrates and are listed in Table 2. 
     
                       TABLE 2______________________________________EXPERIMENTALLY DETERMINED VALUES OFV.sub.f, t.sub.r, T AND (e.sub.o - e&#39;.sub.s)/(e.sub.i - e&#39;.sub.s)V.sub.o (cfm)   V.sub.f (ft/min)             t.sub.r (s)                     T(s) (e.sub.o - e&#39;.sub.s)/(e.sub.i______________________________________                          - e&#39;.sub.s)337      84       0.71    0.31 0.10 (90% efficiency)477     119       0.50    0.32 0.21 (79% efficiency)640     160       0.38    0.24 0.21 (79% efficiency)______________________________________ 
    
     The theoretical relaxation time of the filter for vapor diffusion can be obtained from the fin-fin distance of the filter (2r) and the vapor diffusivity (D) by the equation T=r 2  /D. The holes within the filter employed in the test device have oval shapes. The maximum and average fin-fin distances were about 1 and 1/2 cm, respectively, and their corresponding relaxation times were estimated as 1.1 and 0.27 seconds, respectively. 
     The last column on Table 2 gives the drying efficiency of the device. The agreement of the measured value of relaxation time (fourth column in Table 2) and the calculated value, and the efficiency of air drying by the device (last column in Table 2) being between 80 and 90%, validate the principle of air drying used in the device. 
     Having described the invention, various modifications within the spirit and scope of the invention, as defined by the following claims, will be apparent to those skilled in the art.