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Cyclones 125

700.0
cost = US$ 3, 744
b
376.3
cost = US $, 964 .66
6
b
4. CYCLONES FOR AIRBORNE PARTICULATE SAMPLING
Because of the complex flow pattern of the gas within the cyclone, its removal effi-
ciency tends to increase substantially for small body dimensions. Cyclones with some
centimeters, or even millimeters, in body diameter can reach high collection efficiencies
for fine particles (diameters below 5 µm), unlike their “grown-up” relatives.
For these reasons, cyclones have been widely utilized as sampler devices for particulate
matter (PM) monitoring in environmental and occupational applications (59). Their size
and geometry, allied to suitable collection efficiencies, make possible the design of
portable monitors, very flexible in their use. They can, for example, be carried around,
clipped on a person’s body, continuously sampling the ambient air one is breathing.
The design criteria, in this case, are centered on the cyclone collection efficiency
performance and its comparison with the standards for monitoring devices, defined by
the governmental agencies and/or legislation. These standards constantly change, and a
brief overview of their present status is given below.
4.1. Particulate Matter in the Atmosphere
It is well known that the inhalation of particles is harmful to people’s health. Studies
carried out mainly in the second half of the last century also verified that the degree of
penetration of particles in the respiratory system is a function of the particle size. These
findings led to the establishment of criteria for aerosol monitoring, which are normally
presented in the form of precollectors acting as parts of the respiratory system: the par-
ticles that penetrate through these precollectors are equivalent to those that penetrate
through the corresponding part of the human body. The increasingly rigorous standards
for air quality, which include definitions of particle size fractions in relation to their
penetration through standard precollectors, fall into four categories, according to the
American Conference of Governmental Industrial Hygienists (ACGIH) (35):
• The inhalable fraction (IPM) is the mass fraction of total airborne particles inhaled
through the nose and mouth and is given by
IPM = 0.5 1 + [ exp 0.06D ae )] (58)
− (

where D is the particle aerodynamic diameter (in µm), defined as the diameter of an
ae
equivalent spherical particle of unit density and the same terminal velocity as the particle
in question. The International Standards Organization (ISO) expects to adopt a similar
definition that includes the ambient wind speed, U (in m/s), and can be written as (36)
IPM = [ exp 0.06D ae )] + 10 U 2.75 exp 0.05D ae ) (59)
(
− (
0.5 1 +
−
5
• The thoracic fraction (TPM) is the mass fraction of inhaled particles penetrating the res-
piratory system beyond the larynx and is given by
[
TPM = IPM * 1 − ( )] (60)
Fx