Encyclopedia of Environmental Science and Engineering, Volume I and II

(Ben Green) #1

26 AEROSOLS


a particle and air at the outlet of a converging nozzle is
detected (Wilson and Liu, 1980).

Sedimentation Method

By observing the terminal settling velocities of particles it
is possible to infer their size. This method is useful if a TV
camera and He–Ne gas laser for illumination are used for the
observation of particle movement. A method of this type has
been developed where a very shallow cell and a TV system
are used (Yoshida et al., 1975).

Centrifuging Method

Particle size can be determined by collecting particles in a
centrifugal flow field. Several different types of centrifugal

IMPACTION SURFACE

SMALL
PARTICLE S

T

LARGE
PARTICLE

STREAMLINE
OF GAS

MEAN GAS FLOW

U 0

W

Re =
25000
3000
500
10

S/ W = 0.5, T/ W = 1
Re = 3000, T/ W = 2

0.3 0.4 0.5 0.6 0.7 0.8 0.9

St k

0

20

40

60

80

100

S/W=
0.25
0.5
5.0

COLLECTION EFFICIENCY (%)

FIGURE 8 Principle of operation of an impactor. Collection effi-
ciency of one stage of an impactor as a function of Stokes number,
Stk, Reynolds number, Re, and geometric ratios.

BEAM
SPLITTER

He–Ne
LASER

CLEAN
AIR

AEROSOL

NOZZLE

PHOTOMULTIPLIER
SIGNAL PROCESSING

CHAMBER PRESSURE
GAUGE

PUMP

FIGURE 9 Measurement of aerosol particle size by laser-
doppler velocimetry.

chambers, of conical, spiral and cylindrical shapes, have been
developed for aerosol size measurement. One such system is
illustrated in Figure 10 (Stöber, 1976). Particle shape and
chemical composition as a function of size can be analyzed
in such devices.

Electrical Mobility Analyzers

The velocity of a charged spherical particle in an electric field,
v e , is given by Eq. (20). The velocity of a particle having unit
charge ( n p  1) in an electric field of 1 V/cm is illustrated
in Figure 3. The principle of electrical mobility analyzers is
based upon the relation expressed by Eq. (20). Particles of
different sizes are separated due to their different electrical
mobilities.

FIGURE 10 Spiral centrifuge for particle size measurements.

AEROSOL

DISTRIBUTOR

CLEAN AIR

AEROSOL
CLEAN AIR

ROTATION

PLASTIC FILM

EXHAUST

DISTRIBUTOR

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