IMPACTORS

BY:Rajashree Naik2014MSE

S012

Other ways of measuring size distribution or making size-classifications

cascade impactors cyclone separators

Operation Principle• Impactors utilize the mechanism of inertial

impaction to deposit particles onto impaction surfaces.• Technically, impactors are realized as nozzle

– impaction - plate configuration.• Flowing through the nozzle the aerosol flow

is accelerated.• The impaction plate causes a strong bending

of the gas stream lines. • .

• Small particles, because of their small inertia, can follow the gas stream lines and are not deposited.

• Large particles, due to their higher inertia, cannot follow the stream lines are deposited on the impaction plate.

• An impact or separates the aerosol particles into two size (mass) fractions

Inertia based instrumentsAn Impactor separates the particles into two size ranges,larger or smaller than a cutoff size

Cascade impactor: have multiple impaction stages in series (largest cutoff size is 1st stage, etc). Decrease the nozzle size each stage. Can get access to each impaction plate and then weigh the particles.

• Virtual impactor: replace the impaction plate with a collection

• probe. Particles with sufficient inertia go into the collection

• probe.

• Time – of – flight: have a nozzle emit particles, use two lasers

• at e.g. 100 m apart used to time the particles travel. Particle’s

• Aerodynamic diameter is based upon it’s travel time between the

• two beams.

Mathematical Description The important parameter for the description of impactors is the Stokes-Number:

P = particle relaxation timeu = gas velocityDj = nozzle diameter

The Stokes-number is defined as the ratio of particle stopping distance to the nozzle radius. In first approximation, the "Cut-Off„ diameter d50 of an impactor can be determined (500 < Re < 3000 and x / D j >1.5) by:

j

C2PP

j

Pp 9

2Stk

D

Cud

D

u

Determination of Particle Size Distributions

•For the determination of particle size distributions multiple impactors are operated in series.

•A device like this is called a cascade or multi-stage impactor.

• In a cascade impactor, the cut-off-diameter is decreased from stage to stage.

•For complete deposition of the small particles, down-stream of the last impactor stage, a backup filter is utilized.

•Each stage is equipped with an exchangeable impaction plate.

•The particle mass / number deposited in each stage is determined gravimetrically, chemically or optically.

•From these particle masses / numbers, the particle size distribution can be reconstructed if the transfer functions of the different stages are known.

To decrease the cut-off diameter from stage to stage is achieved by

a) decreasing the nozzle diameters (increase in particle/air velocity)

b) and/or decreasing the number of nozzles

c) and/or decreasing the pressure (increase in slip correction).