Page 256 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

228 Applied Process Design for Chemical and Petrochemical Plants

Figure 4-2. Particle-size ranges for aerosols,
dusts, and fumes. Courtesy, H. P. Munger, Bat- ,
telle Memorial Institute. PARTICLE SIZE - MlcRchlS

93% efficient. Selection of a good wet collector will show The fundamentals of separation for a particle moving
an efficiency of 98%. The effluent leaving this collector with respect to a fluid are given by the drag coefficient of
will have a concentration of 2.25(1.00 - 0.98) = .045 Figure 46.
grains/cu ft. Using the line initially drawn, at the point The motion of particle and fluid are considered rela-
where it intersects the line of 0.045 grains/cu ft will indi- tive, and the handling of the relations are affected only by
cate a mean particle size in the effluent of 1.6 microns. conditions of turbulence, eddy currents, etc.

Guide to Dust Separator Applications Tmminal Velocity

Table 45 [ 101 summarizes dry dust particle separators When a particle falls under the influence of gravity it
as to general application in industry, and Table 46 and will accelerate until the frictional drag in the fluid bal-
Figures 44 and 45 [42] compare basic collector charac- ances the gravitational forces. At this point it will contin-
teristics. Figure 45 presents a typical summary of dust col- ue to fall at constant velocity. This is the terminal velocity
lection equipment efficiencies which have not changed or free-settling velocity. The general formulae for any
significantly for many years except for specialized equip- shape particle are [13]:
ment to specialized applications.

Guide to Liquid-Solid Particle Separators
(4-1)
Table 47 summarizes liquid particle separators as to
the general process-type application.

Gravity Settlers

The use of these settlers is not usually practical for (4- 2)
most situations. The diameters or cross-section areas
become too large for the handling of anything but the
very smallest of flowing vapor streams. In general, gravity (a) Spherical particles between 1500 and 100,000
settlers of open box or tank design are not economical for microns; Newton’s Law:
particles smaller than 325 mesh or 431.1 [23].
They are much more practical for solids or dusts,
although even for these situations the flow quantities iJLDPbpS
must be small if the sizes are not to become excessive. ut = 1.74 - p, (4- 3)
With unusually heavy and/or large particles the gravity
separator can be used to advantage. C = 0.445 average drag coefficient