Method and apparatus for dispelling fog

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.

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 "Drying of Gases," 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.sub.a, and at a temperature of 
about 10.degree. 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.degree. 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.

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'. Filter media 23, 23' are angled away from each 
other at their narrow ends 24 and 24' nearest inlet 18 so one corner of 
each of ends 24 and 24' adjoins the sides 14 and 15 of chamber 11 for the 
full depth of the filter media 23 and 23'. Filter media 23 and 23' are 
joined at their other narrow ends 25 and 25', 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' to reach outlet 19. 
Supported above filter media 23 and 23' are sprayers 26 and 26' 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' 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' 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' onto filter media 23 and 23' 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', 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' 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' 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.sub.id) and temperature (T.sub.i) at the inlet of the device, and the 
dew point (T.sub.od) and temperature (T.sub.o) as well as the flowrate 
(F.sub.o) at the outlet of the adapter, were measured for different 
flowrates (in cubic feet per minute or cfm). The flowrate (F.sub.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.sub.i) and outlet (RH.sub.o) were calculated from their corresponding 
dew points and temperatures. The results are listed in Table 1. 
TABLE 1 
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RESULTS UNDER THE CONDITIONS OF TEST IN 
WHICH TEMPERATURES, DEW POINT AND RELATIVE 
HUMIDITY OF ROOM AIR WERE, RESPECTIVELY, 
23.degree. C., 0.degree. C. and 22% 
F.sub.O RH.sub.i RH.sub.o 
(cfm) T.sub.i (.degree.C.) 
T.sub.id (.degree.C.) 
(%) T.sub.O (.degree.C.) 
T.sub.Od (.degree.C.) 
(%) 
______________________________________ 
337 13 11 88 20 4 35 
477 13 11 88 20 5 37 
640 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.sub.r) of the air was estimated by 
dividing this volume with the flowrate. The flow velocity (V.sub.f) at the 
filter was obtained by dividing the flowrate with the effective filter 
cross-sectional area of 4 ft.sup.2. 
Describing the vapor pressure of the saturated solution at 20.degree. C. 
(outlet temperature) as e'.sub.s, the vapor pressure at the inlet as 
e.sub.i and the vapor pressure at the outlet as e.sub.o, the relaxation 
time T may be defined by the following equation: 
##EQU1## 
From the results given in Table 1, V.sub.f, t.sub.r, T and (e.sub.o 
-e'.sub.s)/(e.sub.i -e'.sub.s) were calculated for different flowrates and 
are listed in Table 2. 
TABLE 2 
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EXPERIMENTALLY DETERMINED VALUES OF 
V.sub.f, t.sub.r, T AND (e.sub.o - e'.sub.s)/(e.sub.i - e'.sub.s) 
V.sub.o (cfm) 
V.sub.f (ft/min) 
t.sub.r (s) 
T(s) (e.sub.o - e'.sub.s)/(e.sub.i 
______________________________________ 
- e'.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.sup.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.