Collector diameter

Vertical surfaces collect spray primarily by impaction. Plants and other entities with a vertical component will collect some material by impaction, and typically also by deposition, as discussed in the previous section. While horizontal collectors will tend to collect all or most material that falls out on to them, vertical collectors will have collection efficiencies that are more closely related to their physical characteristics, especially collector diameter. The SDTF used a-cellulose cards oriented perpendicular to the ground and strings made of cotton or Teflon in its field studies to assess spray volumes at locations above the ground. Many other researchers have used strings for assessing airborne spray volumes in drift studies. ... 

Researchers should be aware of, and account for, factors that can affect the performance of field studies with respect to precision, bias and possible error influences. The major factors affecting collectors, tracers and analytical approaches have been discussed elsewhere in this article. In summary, these are collection efficiency, stability and detection levels, respectively. Collection efficiency (or impaction parameter) for field samplers is related fo parficle/collector diameter and wind speed relationships, as summarized by the following equation developed by May and Clifford ... 

Often it is thought that the effectiveness of a riffler can be demonstrated by the uniformity of weight accumulated in each collector. This reasoning is incorrect if one considers that each collector will necessarily acquire a slightly different amount of sample if the collector diameters vary slightly. The only correct test for the effective performance of a riffler is to compare the contents of each collector in terms of particle size distribution or specific surface area. 

Interception The inertial impaction model assumed particles had mass, and hence inertia, but no size. An interception mechanism is considered where the particle has size, but no mass, and so they can follow the streamlines of the air around the collector. If a streamline which they are following passes close enough to the surface of the fiber, the particles will contact the fiber and be removed (Figure 8.4). The interception efficiency depends on the ratio of the particle diameter to the cylindrical collector diameter (k= dp/Dc) ... 

When the particles are much. smaller than the collector, and in sufliciently low concentration, the How Helds for the particle and collector can be uncoupled. For the gas flow Held around the collector, (he velocity distribution is detenntned by the Reynolds number based on collector diameter, independent of the presence of the particles. The particle is assumed to be located in a How with a velocity at intinity equal to the local velocity for the undisturbed gas How uroimd the collector the drag on the particle is determined by the local relative velocity between particle and gas. 

Figure 4.8 Comparison of experiment and theory for impaction of monodisperse droplets on single solid spheres (after Hahner el al., 1994). Points correspond to different collector diameters. The numerical simulations were for a Reynolds number of 500 based on sphere diameter. The two cases considered, potential flow and boundary layer (low, gave similar results. Both agree well with experiment for Slk > 0.2. However, the experimental results indicate that measurable deposition occurs at values of Stk < Stkcrii = 1/12.

C cell number density, number of cells/m c specific heat of medium, J/kg K collector diameter, m dp particle diameter, m rfj pipe diameter, m D axial dispersion coefficient, m /s Dsr diffusivity due to the Brownian motion, m /s Erf activation energy for thermal cell destruction in Arrhenius equation, J/kmol... 

With As being a parameter [35], primarily dependent on the porosity s and PCj being the Peclet number. The Peclet number is proportional to the fluid velocity and the ratio between collector diameter dc and difliision coefficient for Brownian motion Dbm-... 

Another method, namely laser ablation, was first used to synthesize CNTs in 1996 hy Thess et al. [31]. In this method, a graphite target mixed with a Co and Ni catalyst is evaporated hy laser irradiation under flowing inert atmosphere at lower temperature, that is, 1200°C. The carbon products are swept by the flowing gas and deposition occurs on a water-cooled collector. Diameter and size distribution of CNTs can be controlled by adjusting the reaction temperature during the growth procedure and the catalyst composition. 

Fig. 22. Performance cut diameter predictions for typical dry packed bed particle collectors as a function of bed height or depth, packing diameter and packing porosity (void area) S. Bed irrigation increases collection efficiency or decreases cut diameter (271). SoHd lines, = 25 mm dashed lines,...

In the case of speed changes, the pump efficiency is not affected except for a minor change owing to Reynolds number change, but the diameter cut may reduce the efficiency appreciably on account of increased gap and losses between the impeller OD and a collector (casing or diffusor). 

FIG. 14-107 Overall integrated penetration as a function of particle-size distribution and collector cut diameter when B = 2 in Eq. (14-224). (Calveti, Gold-shmid, Leith, and Mehta, NTIS Puhl PB-213016, 213017, 1972. )... 

Dt Diameter or other representative dimension of collector body or device m ft ... 

From the standpoint of collector design and performance, the most important size-related property of a dust particfe is its dynamic behavior. Particles larger than 100 [Lm are readily collectible by simple inertial or gravitational methods. For particles under 100 Im, the range of principal difficulty in dust collection, the resistance to motion in a gas is viscous (see Sec. 6, Thud and Particle Mechanics ), and for such particles, the most useful size specification is commonly the Stokes settling diameter, which is the diameter of the spherical particle of the same density that has the same terminal velocity in viscous flow as the particle in question. It is yet more convenient in many circumstances to use the aerodynamic diameter, which is the diameter of the particle of unit density (1 g/cm ) that has the same terminal settling velocity. Use of the aerodynamic diameter permits direct comparisons of the dynamic behavior of particles that are actually of different sizes, shapes, and densities [Raabe, J. Air Pollut. Control As.soc., 26, 856 (1976)]. 

On approaching a collecting body (fiber or liquid droplet), 0 porticle corried along by the gas stream tends to follow the stream but may strike the obstruction because of its inertia. Solid lines represent the fluid streamlines oround a body of diameter Dt, and the dotted lines represent the paths of particles that initially followed the fluid streamlines. X is the distance between the limiting streamlines A and B The fraction of particles initially present in a volume swept by the body that is removed by inertiol interception is represented by the quantity X/Dt, for a cylindrical collector and (X/Dt,) for a sphericol collector... 

By using an anionic collector and external reflux in a combined (enriching and stripping) column of 3.8-cm (1.5-in) diameter with a feed rate of 1.63 ni/n [40 gal/(h ft )] based on column cross section, D/F was reduced to 0.00027 with C JCp for Sr below 0.001 [Shou-feld and Kibbey, Nucl. AppL, 3, 353 (1967)]. Reports of the adsubble separation of 29 heavy metals, radioactive and otheiwise, have been tabulated [Lemlich, The Adsorptive Bubble Separation Techniques, in Sabadell (ed.), Froc. Conf. Traces Heavy Met. Water, 211-223, Princeton University, 1973, EPA 902/9-74-001, U.S. EPA, Reg. 11, 1974). Some separation of N from by foam fractionation has been reported [Hitchcock, Ph.D. dissertation. University of Missouri, RoUa, 1982]. 

Figure 11 shows a t pical liquid collector plate for a column that uses one side downcomer to withdraw the liquid. The maximum diameter for such a design is about 12 ft, which is limited by the hydraulic gradient necessary for such a liquid flow-path length, For larger diameter columns, two opposite side downcomers or a center downcomer normally is used unless the total amount of liquid collected is relatively small. 

Older existing cartridge collector types have a range of actual operating efficiencies of 99 to 99.9% for PM,q and PMj j. Typical new equipment design efficiencies are between 99.99 and 99.999 -t- %. In addition, commercially available designs are able to control submicron PM (0.8 m in diameter or greater) with a removal efficiency of 99.999 + %. 

---