Elsa Garmire's diagram for the fog nozzles.
A Technical description of the Fog
by Elsa Garmire
A cloud bank enshrouds the roof of the Pavilion, an ever-changing fog which is generated by an atomizer system. This is the largest water fog system ever built. Its principle of cloud generation is the same as that of natural fog. A jet-spray nozzle system introduces tiny water droplets which partially evaporate to increase the humidity of the air surrounding the cloud to 100%. At this humidity level the tiny fog droplets remain indefinitely.
The 2520 jet-spray nozzles are capable of spraying forty tons of water per hour into the air generating a cloud 150 feet in diameter, six feet thick around the dome. The nozzles are arranged symmetrically along the exterior ridges and the trough lines of the building's panels, one nozzle every foot. Not all the nozzles need run at once. If they were all running on a good day, the system could produce fifty acres per minute of very dense fog to a thickness of six feet! In order not to fog in all of Expo, readings of several anemometers (wind gauges), humidity gauges, and thermometers are monitored in the control room. The operator has nine possible combinations of fog nozzles he can choose from to reduce or increase the amount of fog.
Thus, as with natural clouds, the character of the cloud depends entirely upon the weather. As wind and humidity conditions vary, the shape and quality of the cloud changes. Actually, the fog produced here is much more dense than usually experienced in nature. Visibility in the middle of such a fog is from three to ten feet. The visitor can walk through the cloud on the plaza and experience its peculiar wetness and low visibility.
The nozzles which produce this cloud are specially designed to operate at 500 pounds per square inch pressure. They have a hundredth of an inch diameter brass exit hole embedded in a pin of stainless steel. Water droplets less than a thousandth of an inch in diameter are ejected three to six feet out from the nozzle. Stainless steel is required in the pin so that the powerful water pressure does not wear it out. The number of fog nozzles required to maintain the 150-foot cloud depends on the wind and humidity.
When the system is first turned on, the individual droplets evaporate, raising the local humidity to saturation level. From that point on, only replacement of fog blown away by wind is required. A portion of the water ejected goes to maintaining the high humidity and a portion into droplets to replenish the visible cloud.
On a calm dry day when the wind is normally 1-2 mph (one meter per second), the influx of dry air into the cloud is 12,000 cubic meters per second. One-third of the nozzles would normally be running, generating 38,000 gms/sec of water, of which half would exist as droplets. That means that the typical water droplet ejected from the nozzle will evaporate to half its size before reaching equilibrium. The liquid water content of such a fog is 2 grams per cubic meter.
On a windier day, with 10 mph wind velocity, all nozzles are activated to produce 11,500 gms/sec of water. Two-thirds of the water volume evaporates to maintain the humidity and the rest produces droplets, yielding a fog with 0.8 gm/cm liquid water content. This is half as dense as the previous example.
In movies, where fog is needed for outdoor sets, fog machines which produce tiny oil droplets are used. The droplets do not evaporate and are small enough to remain in the air for a long while, but eventually the surroundings become coated with an oil film. Dry ice on water produces a ground-hugging fog, or billowing "smoke" and is often used in theatricals. Very cold liquid air condenses moisture from the air to produce a smoky effect, but is expensive to use.
Water-vapor fogs similar to those used here can provide a means of air-conditioning outdoor areas. For example, when introduced into orange groves on cold nights, an artificial fog keeps the oranges from freezing.
Notes and Comments on Clouds and Fog
by Thomas R. Mee
The dictionary defines fog as a visible mass of particles of water or ice in the form of a mist or haze suspended in air. A more scientific definition might be: Sunlight scattered from a colloidal suspension of small particles in the atmosphere; particles with diameters 10 to 20 times the wavelength of green light which causes the mass of particles to appear as a white hazy substance. This is because the human eye cannot distinguish the individual particles and sees only the scattered light that has been bounced many, many times from one particle to another.
These particles are mostly liquid water...but not all water. To form cloud droplets in nature, it is necessary to start with very small solid particles, commonly referred to as condensation nuclei, and then if conditions are proper, some of the gaseous water molecules that make up a part of the earth's atmosphere will condense onto the nuclei to form liquid droplets. Whether or not a droplet will form and remain without evaporating depends very critically upon the chemistry of the nuclei present and the relative humidity of the atmosphere.
Relative humidity is not an absolute measure of how much water vapor is in the air but is a measure of the amount of water vapor actually present compared to the amount which would be present if the atmosphere were totally saturated. The absolute amount of water vapor that can be present depends upon the temperature of the air. If the temperature is 35°C (about 95° F), then a saturated atmosphere will contain about 39 grams of water vapor per cubic meter of air. At 0° C (32° F)the amount of water vapor at saturation would be only 4 grams per cubic meter.
How does a fog or cloud get started? If something causes air to cool, the relative humidity will increase. The reason for the cooling may be that the air mass gets pushed up a mountain slope and cools because of expansion, or the nighttime radiation from the earth might cool the surface and hence cool the air near the surface. In either case, the result is that the air becomes cooler, but the water vapor content remains the same. The relative humidity rises until the air becomes temporarily supersaturated and water vapor will begin to condense onto the nuclei present.
In nature, ground clouds (fog) rarely contain more than about 0.5 gram per cubic meter of liquid water. The amount of water condensed onto tlie nuclei is so small that the droplets formed remain very tiny, and ground clouds rarely "rain out". The typical average diameter of a droplet that small is about 0.3 centimeters per second, or about .006 miles per hour. This is so slow, that the droplets tend to remain suspended indefinitely, usually until the air warms up and the relative humidity goes down, at which point, the droplets evaporate.
In designing the cloud for the Pavilion, a number of restraintshad to be taken into account. Because of the exhibits and the many people that would be in the area, no chemicals were allowed in the fog system. This meant that all of the simple techniques involving nuclei seeding and smoke making had to be ruled out. It was decided early in the design phase that only pure water would be used to create the cloud.
Two possible methods were considered for making pure water fog. One involved the atomization of water into droplets small enough to remain suspended in the air and the other involved forming small droplets by boiling the water and thus generating steam and then allowing that steam to recondense into fog drolets. The steam method was eliminated because of the large power requirements for boiling water. A quick calculation showed that approximately 500 times more power would be required to generate a cloud by the steam method as by the atomizing method.
To obtain an estimate of how much water would be required to make a dense cloud, two considerations have to be taken into account. The first is the size distribution of the water droplets. It is undesirable to have droplets larger than about 60 microns because of their high fall velocity, producing a light drizzly rain. The other consideration, is light scattering; for a cloud with a given liquid water content, scattering increases if the drop size is made small. We tested a number of atomizing techniques and chose one that would produce a size distribution of droplets ranging from 2 microns to about 50 microns diameter. Most of the droplets produced are less than 30 microns.
The desired volume of fog worked out to be about 8000 cubic meters. The fog had to be controlled so that it would not spread too far from the generating site under calm wind conditions, and would not be dissipated by the mixing in of dry air under the anticipated wind conditions. A check of the climatology of the region showed that if we could design a system that would work in wind speeds up to about 5 meters per second, it should work suitably for at least 90 of the time the Fair would be open.
The climate during most of the 6 months of the Fair can best be described as hot and humid, resembling that of New Orleans or Houston. Wind speeds are typically low in the early morning and evening hours and pick up to velocities of 2 to 3 meters per second during the early and mid-afternoon hours.
Earlier in this discussion, it was pointed out that to make a stable fog, enough water vapor has to first be pumped into the atmosphere to bring the humidity to the saturation point. Once that has been achieved, additional droplets pumped into the atmosphere will not evaporate until fresh dry air has mixed into the system.
Wind has two effects on the stability of the fog. One is to cause the fog to drift away from the generation site, but by far the more important effect in high winds is the resulting turbulence that causes dry air to rapidly mix with the moist air in the cloud. Thus, fundamental design parameters for a controllable fog system are the flux of dry air that mixes into the cloud and the humidity of that air. High wind velocities and very low humidities require that high rates of fog generation be used. When the wind speed is low or calm and humidity is high, low fog generating rates must be used to assure that the cloud does not grow too large.
The final design consisted of nozzles that would produce droplets ranging in size from.2 to 40 microns diameter. Approximately 2500 of these were designed into the fog system and arranged as shown in Figure ApA. To account for the different conditions of wind speed and humidity, 9 separate systems of pumps and nozzles were used in the patterns shown, plus one additional system for making ground fog.
The system is designed to be operated either manually or automatically. In the automatic mode, the system will actually react to its environment by creating more or less fog as conditions of wind and humidity demand it. The effect is to create a cloud that will swirl and boil around the dome just as mountain clouds surround a remote mountain peak. Under calm conditions, the cloud will drift down the sides of the dome and swirl around the patio areas a ground fog. Whenever desired, the ground fog system in the Float area can be activated to create a low lying fog that Pavilion visitors may walk through for an experience similar to the feel and smell of fog along the seacoast.