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How Fog Machines Work

Since its introduction in 1979, water soluble fog effects have become among the most commonly used special effects in all aspects of entertainment, including stage, film, television, and theme parks. Earlier technology used mineral oil and kerosene based fluids that were heated. These materials were at best unpleasant, at worst, dangerous to use. The use of a fog fluid that was composed of water and glycols was heralded by the industry with an award in technical achievement from the Academy of Motion Picture Arts and Sciences. There are numerous manufacturers of fog systems today, with many variations of effects and control. However, the basics of a fog system are relatively simple: fog fluid is moved into a heat exchanger by a pump. The heat exchanger maintains a high temperature at which the fluid vaporizes in a process known as "flashing". As the fluid is "flashed" it rapidly expands, and that expansion forces the vapor through the nozzle of the machine. When the vapor mixes with cooler air outside the machine, it instantly forms an opaque aerosol - the effect we call fog or smoke.

That's the short version of the technique now used by nearly all fog equipment manufacturers. But there are dozens of differences among the fluids and the machines and the fog effects they produce. Knowing these differences and understanding how they work can be important for designers, technicians and managers.



The Heat Exchanger
The heat exchangerHeat exchangers vary widely in design and materials, and wattage. Good manufacturing techniques will take into account all these factors for optimum output. Essentially, the heat exchanger is a block of metal with a resistance heating element inserted in it to heat the metal. The temperature of the block is maintained by a thermostat. A path is made through the metal for the fluid to travel through.

In the choice of materials for a heat exchanger, it is important to note that different metals have different heat retention characteristics. The most commonly used material is aluminum. The aluminum can be melted and either poured into a mold or extruded, like cookie dough. The block can contain a straight path, the simplest design form, or a spiral path. Aluminum heats quickly, but also gives up its heat quickly. The result can be large bursts of fog for short periods of time.

Less commonly used, is a tooled nickel-steel alloy block. Steel takes longer to heat initially, but has excellent heat retention properties. With a carefully designed spiral path (see drawing) which can require the fluid to travel as much as 9 feet, the steel block optimizes the amount of power used, giving sustainable, heavy output.

Wattage, or the amount of electrical power used to create the heat, is the specification most manufacturers cite to show how powerful their fog machines are. Although it is one indicator, other factors such as material composition and design, are just as critical to the success of accurately producing fog or smoke effects.

The other main factor in heat exchanger design is the control of the temperature by a thermostat. If the temperature in the heat exchanger is not quite hot enough, the fluid still becomes an aerosol, but it is wet and likely to leave a residue on the stage floor and equipment. If the temperature in the heat exchanger is too hot, the fluid can "burn" which may change its chemical composition.

Whether the thermostat is mechanical or electronic, reliability and accuracy are essential. A good thermostat design will not only prevent a machine from getting too hot, but will also insure that if the heat exchanger becomes too cool to "flash" the fluid, that the delivery system (pump) will be disabled to prevent wet fog from exiting the machine.

The Pump
In a fog machine, the pump plays the critical role of delivering fluid to the heat exchanger. The type of pump utilized needs to be carefully matched to the heat exchanger design. If a pump delivers fluid too quickly (and the machine has a properly calibrated thermostat) then the heater will turn off relatively quickly since too much fluid is moving through the metal block. This results in a machine "shutting down" after 15 seconds of fog. The metal block now needs time to reheat, which can take as long as a minute, and can be very inconvenient.

As with the heat exchanger and thermostats, a variety of pumps are used in fog machines, depending on the machine's intended use and price category. A machine designed for heavy duty use in permanent installations may use an industrial strength diaphragm pump that operates from compressed air and is capable of servicing several heat exchangers simultaneously. A peristaltic pump is specially designed for its ability to pump fluid, on command, at a wide variety of speeds. The most commonly used pump is a piston pump, a light duty design similar to the window washer pump in automobiles.

Control
Besides the volume of smoke generated, the most visible differences among fog machines are the types of control available. Some machines offer only on/off switches. Others offer volume control. And still others offer volume control and timers. The latest equipment even offers optional DMX control, allowing you to turn the machines on and off from your lighting control board.

Volume control, or variable output, gives the machine the ability to deliver a wide variety of effects from wisps of smoke to London style pea-soup fog. It is achieved by varying the voltage applied to the pump thereby controlling the pump speed. The peristaltic pump is especially proficient at lower volumes since it uses a variable speed DC motor turning a rotor pushing against a tube. A piston pump is prone to "chatter" when lower voltages are used.

Timers or timed remote controls, such as the Rosco Super Remote, are designed to give "hands free" operation to a fog machine. The timers are usually designed to control "on time" (the amount of time a machine actually makes fog) and "off time" (the amount of time between bursts of fog) as well as volume. DMX interfaces are the latest in control devices for fog machines. By setting the DMX address for a channel at the machine, it can be run on cue by the board operator.

Fog Fluids
Perhaps no component of making fog or smoke is as misunderstood, or as underrated, as the fluids used in the process. Fog fluids used in these processes are made from a series of glycols mixed with water. Glycols are among the most commonly used chemicals in the world and are found in products from food to cosmetics. The choice of which glycols manufacturers use should be made very carefully.

Fog machines and fluids are designed as systems. Specific fluid formulas require specific temperature for optimum aerosolization during the "flashing" process. Manufacturers of fog equipment design their machines to be compatible with their fluids. If a machine is calibrated at too low a temperature for a given fluid, the result can be "wet" fog that can leave a residue. If the temperature is too high, the fluid can "burn" or decompose the fluid, thus changing its chemical composition. This "burning" can create harmful byproducts.

In a recent report, an agency of the US Federal Government, the National Institute of Occupational Safety and Health (NIOSH) recommended "using only fog fluids approved by the manufacturers of the machines". (HETA 90-355-2449)

Using a small range of glycols in different configurations, manufacturers have been able to develop a large variety of fluids delivering different effects. These can range from the usual thick clouds of fog that have a long "hang time" to fluids that disappear quickly. This gives the user great flexibility in designing shows. With the introduction of reliable, easy-to-use equipment, fog and smoke effects have become the most commonly used special effects in the entertainment industry. These machines are extremely versatile and, with a minimum investment in accessories, can replace expensive "single purpose" equipment. The only limitation is the designer's imagination.

Ducting Fog
It is not always possible to place the fog machine near the exact location where the effect needs to occur. The answer is to use ductwork to direct the fog. Flexible ducting can be used, but it should be of heavy construction. Most dryer hose is too lightweight to stand up to the rigors of theatrical use. If needed, rigid PVC pipe can be used.

Whether flexible or rigid ductwork is used there are some points to remember. For most applications the minimum diameter duct used should be 4 inches. When fog is constricted, it tends to recondense into fluid reducing the output and creating an unwanted residue. Likewise, recondensation can occur if the ductwork is placed directly over the front of the fog machine. The aerosol is created when the "flashed" vapor mixes with fresh air. If flexible hose is used, an openwork frame should be used to keep an air space in front of the machine (see below). If rigid ductwork is used, simply keep the front of the machine 3-4 inches from the duct. If the fog needs to travel long distances in ductwork, a squirrel cage fan can be added. The fog should not come into contact with the fan blades. The air should be added through a "Y" connection.

Air With Fog
The output of fog machines tends to be fairly directional, so the use of a fan can help disperse the fog. The fan should be placed behind the machine so that the air can mix with the fog. Experiment by varying the output of the machine and the speed of the fan to achieve the desired effect. Note: Never direct the fog at a fan since the aerosol will break apart on the fan blades leaving an unwanted residue and reduced output.

Some equipment now offer compressed air to use in creating fog effects. This unique feature allows compressed air to be introduced right into the heat exchanger and, by displacing part of the fluid in the heat exchanger, the air can aerate the output of the machine. With a high volume of air (up to 35 psi) and a low output of fog fluid, the result is the increasingly popular "haze" effect. By alternating fog with short sharp bursts of air, a gunshot or cannon effect can be attained. These are just two examples of the many effects that compressed air can offer.

Chilling Fog
One of the most common effects required is a fog that stays close to the ground. The usual method of achieving this is by dropping solid carbon dioxide (dry ice) into hot water. However the dry ice only lasts a short time. Fifty pounds of dry ice can be gone in 10 minutes, making the effect quite costly.

When the output of a fog machine is chilled below ambient temperature, it will have the same look. There are several methods of doing this, but the most economical is to run the fog over ice or dry ice. When the fog is chilled, it lays low to the ground. If dry ice is used, it will last far longer than the conventional method. For example, 50 pounds of dry ice, when used in a device designed to achieve this effect, can last for an entire 3 hour performance. Rosco manufacturers and markets such a device, called a Chiller Module.

Most standard fog fluids, when chilled, will start to rise as they warm which can hurt the effect. A fast dissipating fluid, will dissipate as it rises, maintaining the integrity of the appearance of low lying fog.

Academy of Motion Pictu clare Arts and Sciences AwardThe citation from the Academy of Motion Picture Arts and Sciences reads:"...to Rosco Laboratories for the development of an improved non-toxic fluid for creating fog and smoke for motion picture production."

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