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The droplets
size is the most important parameter for characterizing the physical behavior
of the Aerosol. Most Aerosol generators do not produce Aerosols with droplets
of a single size but a range of droplet distributions
of different sizes. Droplets are measured in Microns. One Micron is 106
of a meter.
Scientists studying the field of Aerosols came up with 10 different
definitions of droplet diameter size and 9 different techniques for
measuring statistical droplet diameter size distributions.
For practical reasons, it is more convenient to work with calculated mean
diameter sizes of droplets than with many statistical size distributions.
Thus, the concept of "Mean Diameter Size" of droplets is usually used.
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The variety of natural
and artificial Aerosols can be classified
according to:
- The physical state of matter
- The method of Aerosol generation
- Types of liquid
- Shape of droplets/particles
- Naturally or artificially produced.
Example of some Aerosols are: dust, fumes, smoke, mist,
fog, and
bioaerosols. Researchers maintain that Aerosol vary in droplet/particle
size from 0.01 to 50 microns.
Green Clouds definition of its Aerosols is: Droplets from 0.1 to 5
microns(mean diameter size) suspended in air or gas resulting from the
disintegration of a liquid by means of Ultrasonic energy.
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Smaller droplets evaporate faster then larger ones
under the same temperature and relative
humidity conditions. This is because smaller droplets have larger Facial
Surface Area relative to
their Volume.
Larger droplets will evaporate substantially slower than smaller droplets
in low temperature and high relative humidity.
Evaporation of liquid from a droplet decreases its mass. It also increases
the drift distance
of the droplet.
The factors that influence droplets drift are:
- Droplets mean diameter size
- Wind Velocity
- Relative Humidity
- Ambient Temperature
- Droplet Discharge height & Velocity
- Wind Turbulence (Velocity, Direction)
- Evaporation Rate of Droplets
A number of experiments were conducted to test the
effectiveness of Aerosols with different
droplet sizes in disinfecting a contaminated room. Aerosol-A had droplets
of one micron.
Aerosol-B had droplets of 50 microns. The findings showed that Aerosol-A
disinfected the room
using about 1/10 of the quantity of the disinfectant liquid used by Aerosol-B.
Both Aerosols
disinfected the room.
These results show that small droplets are more cost effective and are
also friendlier
to the environment than larger droplets because the amount of liquid chemicals
required is
significantly reduced.
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