Impaction efficiency

Fig. 4. Electron-impact efficiency curves at m/e = 32 and rri/e = 16 for a supersonic beam of O2. Arrows indicate the literature values72 of the ionization energy of O2 and of the appearance energy of 0+.

Fig. 5. Electron-impact efficiency curves at (a) m/e = 41 and m/e = 13, and (b) m/e = 14, recorded at a lab angle of 30° during the CMB study of the reaction 0(3P) + C2H2 at Ec = 12.6kcalmol 1. The thick arrow marks the electron energy (17eV) used for product angular and TOF distribution measurements. In (b) the thin arrow marks the literature value50 of the IE of triplet methylene (see text).

The impaction efficiency (17) for particles depends directly on the particle diameter (D), the flow velocity of the air (V), and the particle density (p) it varies inversely with the gas viscosity (p,) and with a parameter (Db) that is representative of the impactor s physical dimensions (e.g., the inlet nozzle diameter) and that is related to the curvature of the airstream. 

Thus, the impaction efficiency should be greatest for larger, denser particles and higher flow velocities. The factors involved in particle impaction on surfaces are discussed in detail by John (1995). 

Quantifying Environmental Impact Efficiency, E-factors, and Atom Economy... 

Figure 5 shows representative spectra for test 4, an unstable case. The maximum of the deposited spectrum occurs at 10 y. Since 10 y droplets have quite poor impaction efficiency, this finding suggests that the proportion of very small drops emitted by the aircraft must be very large indeed. Earlier studies (10) of the TBM emitted spectrum have severely underestimated these numbers due to problems associated with measuring in-flight drops smaller than about 30 y. Recent studies using a laser spectrometer are expected to clarify this point (11). 

Droplets were collected on 1-inch ethylcellulose strips, placed laterally across the wind tunnel floor at various distances downwind, and on 11-mm. glass rods located 40 inches downwind from the spray nozzle. The effective fall distance for droplets impinging on the wind tunnel floor was 2 feet. Droplet sizes that reach the glass rods will not all impact efficiently, and the efficiency varies with droplet diameter (2, 4). Nonetheless, the size droplet reaching a given glass rod was approximately... 

One may plot the data as shown in Figure 7 for water sprays and for a particulate spray (14). [A particulate spray is formed from a liquid imbibed in sized particles of a lightly crosshnked, swellable polymer.] Air (glass rod) and floor samples are treated separately because of the uncertain impaction efficiency on the rods, as previously discussed. The large apparent error for the particulate spray data of Figure 7b results from the proximity to the detection limits of the analytical method used. 

During recent years, the tendency to apply smaller and smaller volumes of spray per hectare has necessitated the use of small drops to maintain adequate distribution. However, the size of the drops cannot be reduced indefinitely without affecting their ability to impact on the target, and in any case, as the drop size is reduced, the tendency of the spray to drift from the target is increased. Thus, there is an optimum drop size for most spraying operations which represents the smallest drop size consistent with satisfactory impact efficiency. To determine this optimum size for any particular application, one must take into account the mode of action of the toxicant, the nature of the crop, the type of spray equipment, and the volume of spray to be applied. 

Particle size control may also be desirable to facilitate solids handling during formulation. Solids produced by uncontrolled crystallization or precipitation processes can have a broad size distribution that can result in poor flow properties or tendency to segregate. The particle size of APIs can also impact efficiency of blending with excipients, compressibility, flow/ suspension behavior, and compaction performance in downstream equipment. Small particle sizes may be important for controlling dose uniformity in the final formulation, especially for low doses. ... 

Figure 138. Aerodynamic diameter vs. Flowrate through Anderson Sampler for an impaction efficiency of 95%.

For Stokesian particles, rvi o impaction regimes are similar when the Stokes, interception, and Reynolds numbers are the same. The impaction efficiency. as in the case of diffusion, is defined as the ratio of the volume of gas cleared of particles by the collecting element to the total volume swept out by the collector. (Refer to Fig. 4.5 for the case of the cylinder.) If all panicles coming within one radius of the collector adhere, then we obtain... 

Flow around single cylinders is the elementary model for (he fibrous filter and is the geometry of interest for deposition on pipes, wires, and other such objects in an air flow (Chapter 3). The flow patterns at low and high Reynolds numbers differ significantly, and thi.s affects impaction efficiencies. For Re > 100. the velocity distribution outside the velocity boundary layer can be approximated by inviscid flow theory. This approximates the velocity distribution best over the front end of the cylinder which controls the impaction efficiency. The components of the velocity in the direction of the mainstream flow, x, and normal to the main flow, y, are... 

For Stk < 1 /46. the roots are real and both are negative. The u versus x diagram has a nodal point that corresponds to zero particle velocity at the forward stagnation point and zero impaction efficiency (Fig, 4,6b). For Stk = I/4h, the roots are equal and the system... 

Figure4.6 (a) Spiral point corresponding to StJoStk ni. The particle velocity at the surface (.v = 0)is positive, and the impaction efficiency is nonvanishing. Only the solid portion of the ctirve is physically meaningful, (b) Nodal point corresponding to Stk > Stkcrii< Particle velocity vanishes at the surface and the impaction efficiency is zero.

This analysis provides a lower anchor point for curves of impaction efficiency as a function of Stokes number. It applies also to non-Stokesian particles, discussed in the next section, because the point of vanishing efficiency corresponds to zero relative velocity between particle and gas. Hence Stokes law can be used to approximate the particle motion near the stagnation point. This is one of the few impaction problems for which an analytical solution is possible. 

The analysis neglects boundary layer effects and is probably best applied when the particle diameter is larger than, or of the order of, the boundary layer thickness. The change in the drag law as the particle approaches the surface is also not taken into account. Hence the criterion provides only a rough estimate of the range in which the impaction efficiency becomes small. 

Figure 6.9 Schematic diagram of jet impactor efficiency showing Stk corresponding to 50 impaction efficiency. For round jets, the lower tail of the cfticicncy curve may not exist (Miirpk and Liu, 1974),...

Impaction is a function of several variables. According to Ranz and Wong (1952) the impaction efficiency (defined in Subsection 4.1.2) depends on the parameter ijj ... 

The raindrop will wash out a fraction eMt of this mass, where e is the impaction efficiency as defined in Subsection 4.1.2 (for its numerical value see Mason, 1957). If the drop radius lies between 50-2000 pm the impaction efficiency is 1 for 10 pm g r < R and decreases with decreasing particle size. It ranges from 0.5 to 0.1 if the particle radius varies between 5 pm and 2 pm. This indicates that only giant particles in the coarse mode (Subsection 4.3.2) are significantly washed out below the clouds. Let us multiply the right-hand side of equation [5.16] by e and divide the result by the volume of the raindrop. This yields the trace concentration in the drop (C2a) due to the capture of aerosol particles ... 

---