Deposition by impaction

When the wind blows past an obstacle, the streamlines of air flow diverge to pass round it. Particles carried in the wind tend to carry straight on and may impact on the obstacle. The efficiency of impaction C, is defined as the ratio of the number of impacts to the number of particles which would have passed through the space occupied by the obstacle if it had not been there. If vg is the velocity of deposition relative to the profile area of the obstacle, then C = vglux where ux is the free stream air velocity. C, is thus analogous to Cd, the drag coefficient of the obstacle. 

Aerodynamic theory shows that C,- is a function of the Stokes number defined 

Sp is the stopping distance of the particle, which is the distance it would travel in still air if given a velocity uv L is the characteristic dimension of the obstacle (the diameter of a cylinder or sphere). For particles in the 1-50 pm range, and less than 5 m s-1, the approximate stopping distance is given by 

Thus Sto is proportional to the wind speed, the density of the particle and the square of its diameter, and inversely proportional to the size of the obstacle. Both vv and Sto are functions of dA, and aerodynamic diameter is used to classify particles deposited by impaction or sedimentation. 

Experiments on the impaction of monodisperse water droplets on cylinders (May Clifford, 1967) have shown C,- correlating with Sto as predicted theoretically. Droplets are almost always captured on impact, but this is not true of solid particles, for which the efficiency of capture Cp may be less than C(. The same factors which tend to increase Q, namely large dp, large ux and small L, also tend to increase the possibility of the particle bouncing off the surface, and this may result in a decline in Cp with increase in Sto. Bounce-off of particles from fibres is a well known factor limiting the efficiency of filters. 

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The previous section described active samplers where the air is swept of particles using mechanical mechanisms. This section describes passive samplers that do not move, but collect material that deposits by impaction or sedimentation deposition. These types of collector are the most common type for field studies aimed at assessing exposure of aquatic and terrestrial organisms to pesticides. 

The probability of an inhaled particle being deposited by impaction is a function of the dimensionless Stokes number Stk, which relates particle properties (mass mP, diameter dP, and density, pP) to parameters of the airflow (air velocity vA, viscosity i)A, and airways radius rA) ... 

Particles in the micrometre size range can be deposited by impaction in the TB region, particularly at the carina where the two main bronchi diverge. Using hollow casts of the trachea and main bronchi, Schlesinger et al. (1977) and Chan Lippmann (1980) found that the efficiency of deposition correlated with the impaction parameter Ppdpq. 

Smaller particles in the size range 0.5-5 pm may escape impaction in the upper airways and will deposit by impaction and sedimentation in the lower TB and A regions. If the aerosol particle size is between about 3 and 5 pm then deposition will predominantly occur in the TB region. If the particles are less than about 3 pm then appreciable deposition in the A region is likely to occur. 

FIGURE 4-30 Particle deposition by impaction. In this example, airflow is from front to back past the car. A particle following streamline A, which curves gently, may be deflected sufficiently to avoid contact with the obstruction. Particles with sufficient mass and density that travel along streamline B, which has very sharp turns, may be carried to the surface of the object by their own momentum. Although the process is illustrated for a large object (the car) and a centimetersized particle (the moth), the same phenomenon also occurs for much smaller particles and objects. 

Example In certain types of heat exchangers, a gas flows normal to a bank of tubes carrying fluid at a different temperature, and heat transfer occurs at the interface. Fouling of the outside surface of the tubes by particles depositing from the flow reduces the heat transfer rate. If the tubes are I in. (outside diameter) and the gas velocity is 10 ft/sec, estimate the diameter of the largest particle [pp — 2 g/cm ) that can be permitted in the gas stream without deposition by impaction on the... 

When we sub.stitute and solve for dp, the result is dp 10 /im. Particles smaller than this will not deposit by impaction on the tubes according to calculations based on potential flow theory. In reality, there may be a small contribution by impaction due to boundary layer effects and direct interception. Diffusion may also contribute to deposition. 

Particles that penetrate beyond the oropharynx and enter the lower airways may deposit in two broad regions of the lungs the tracheobronchial zone and the pulmonary zone, as shown in Fig. 3 (13). Anatomically, it is assumed that the tracheobronchial zone is composed of the trachea and the larger conducting airways, whereas the pulmonary zone contains the smaller airways and alveoli. The extent to which particles deposit in either region is dependent on particle size, inspiratory flow rate, and lung volume at the time of inhalation (14). Whether a particle will impact or sediment also depends on the value of these parameters. For example, a small particle can deposit by impaction mechanisms when inhaled fast or by sedimentation when inhaled slowly. 

For particles that deposit by impaction mechanisms (i.e., particles > 3 pm), high inspiratory flow rates (>30 L/min) lead to enhanced losses of drug in the oropharynx and increased deposition in the tfachea and larger centtal airways. Slower inspirations should enhance deposition of these particles in the lung periphery. 

Influence of pneumoconstriction on dust deposition. Bronchoconstriction in man can be caused by exposure to cigarette smoke as shown by Loomis (1956), Nadel and Comroe (1961), and Guyatt et al. (1970), or by exposure to inert dusts as shown by Dautrebande et al. (1948), and Dubois and Dautrebande (1958). The mechanisms involved have been discussed by Nadel et al. (1965), Nadel (1968) and Dubois (1969). Bronchoconstriction, by reducing the cross section for flow in the conductive airways, results in increased air velocities and turbulence. Increased velocity can result in greatly increased deposition by impaction at the airway bifurcations, while increased turbulence can account for an increase in deposition by eddy diffusion (Lehmann 1938, Worth and Schiller 1951). 

Figure 1 Deposition by impaction A schematic drawing of the respiratory tract, which can be seen as three filters in line, to protect the fragile alveoli from particles. The first two filters, mouth and throat and tracheobronchial airways, work by impaction (i.e., particles tend to continue forward and deposit when the gas flow changes direction). Impaction is the most important deposition mechanism for medical aerosols in the upper airways and in larger bronchus, and correlates well with the impaction parameter AD F. In the last filter, the bronchioles, impaction is insignificant owing to the large total cross-sectional area, leading to low velocities.

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