Content
 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
 Heat
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.
 The 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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