Page 183 - Instrumentation Reference Book 3E

Analysis methods that measure terminal velocity 167
particles. The system, however, is not perfect. are very sensitive in the range 2-12p1-1, while
Proper Stokesian conditions do not exist because others are better at larger sizes. Some are capable
practical considerations make the tubes too short of a continuous and large throughput which
compared with their diameters. Also, the velocity makes them especially suitable for producing
at the cylinder walls is considerably less than at the quantities of closely-sized particles. The devices
center, causing a circulation of some particles up are then being used for their original purpose
the center and down the walls. The cut point there- which is classification rather than size grading.
fore is not sharp. A multiple elutriator consists of a A cyclone is a centrifugal device normally used
number of elutriators connected in series but with for extracting dust from carrier gases. It consists of
increasing diameters, Each section therefore collects a conically shaped vessel. The dusty gas is drawn
progressively smaller particles. tangentially into the base of the cone, takes a
Both water and air elutriators are commercially helical route towards the apex where the gas turns
available. In use. the sample is introduced into sharply back along the axis and is withdrawn axi-
the smallest section and the fluid velocity set to a ally through the base. The device is a classifier in
preset value. When the fluid has cleared in all the which only dust with terminal velocity less than a
sections, the system is switched off and the sedi- given value can pass through the vortex formed
ment in each weighed. A filter usually terminates and out with the gas. The particle cut-off diameter
the last section. In some designs. the separating is calculable for given conditions. Systems have
section is short and followed by a conical redu- been designed using a set of cyclones in series with
cing zone to hasten removal of the undersize increasing gas velocities for size analysis.
particles to the next section. The Cyclosizer Analyzer uses liquid instead of
The elutriators described so far are counter- gas and has five equal-sized cyclones with differ-
flow elutriators. Acceleration (Le., gravity) and ent inlet and outlet diameters to obtain the vel-
drag are in opposite directions. Elutriators have ocity variation. The cyclones are arranged, apex
been designed for transverse flow. If the sample uppermost. Thus the coarse particles retained in a
of articles is introduced slowly into the upper given section fall back many times for reclassifi-
level of a horizontal laminar stream of fluid, cation, thereby obtaining good separation. In this
two-layer sedimentation takes place but the sedi- case the range is 8-50pm.
ment is separated horizontally, different sized
particles foliowing different trajectories. The par-
ticles are collected on judiciously placed pre- 11.8.3 Impaction
weighed sticky plates. When a fluid containing a suspension of particles
is made to turn a corner, particles with terminal
velocity in excess of a value determined by the
11.8.2. I Centrifirgal elutrintion
fluid velocity and the geometry of the bend are
Elutriation is also used in the centrifugal field, deposited or impacted. A cascade impactor con-
usually with air as the fluid. In principle, air with sists of a series of orifices each accurately pos-
entrained particles travels inwards against the itioned above a collector plate. The orifices can
centrifugal force. Particles with terminal vel- be round holes or slots. The holes in successive
ocities sufficiently low also pass inwards; the stages are reduced in size to increase the impac-
rest pass outwards. tion velocity. The particles pass through the holes
The natural flow of air in a spinning system is and are either deposited on the adjacent plate or
radially outwards. In order therefore to obtain a pass on to the next stage. There are typically
counterflow. the air must be driven. In some between six and ten stages covering an aero-
systems a separate pump is used. In others, the dynamic size range from about 0.4 pm to 15 ,urn.
air is introduced at the center, passes radially The Andersen cascade impactor is designed to
outwards under centrifugal force, turns 180” to work “on-line,” incorporating a nozzle for isokin-
travel radially inwards for a short distance, of the etic sampling from a gas stream. A precyclone
order of half a radius, and finally through removes particles >15pm and a filter catches
another 180” to pass to the circumference. Elu- those <Q.4 p,m. The Sierra, designed for room or
triation takes place in the inwards-flowing sec- atmospheric air measurement, covers a range
tion. In this case, no pump is necessary because 0.05pm to 1Qpm. The collected particles are
the net airflow is outwards. removed and weighed. California Measurements
Adjustment of either rotation speed or air vel- Inc. markets an instrument with piezoelectric
ocity will affect the cut size. Air velocity is usually crystal mass monitors at each of ten stages giving
set by a throttling mechanism. immediate automatic readout.
A variety of centrifuge systems is available, Impaction surfaces frequently require the aid
their design and size making them particularly of an adhesive to prevent re-entrainment from
suitable for different size ranges. Some. for example, one stage to the next.