Pasquill-Gifford

PasquiU Atmo.spheric Diffusion, Van Nostrand, 1962) recast Eq, (26-60) in terms of the dispersion coefficients and developed a number of useful solutions based on either continuous (plume) or instantaneous (puff) releases, Gifford Nuclear Safety, vol, 2, no, 4, 1961, p, 47) developed a set of correlations for the dispersion coefficients based on available data (see Table 26-29 and Figs, 26-54 to 26-57), The resulting model has become known as the Pasquill-Gifford model. 

TABLE 26-28 Atmospheric Stability Classes for Use with the Pasquill-Gifford Dispersion Model... 

TABLE 26-29 Equations and Data for Pasquill-Gifford Dispersion Coefficients... 

FIG. 26-54 Horizontal dispersion coefficient for Pasquill-Gifford plume model, Reprinted ffomD. A. Ct owl and J. F. Louvar, Chemical Process Safety, Fundamentals with Applications, Z.9.90, p. 138. Used hy permission of Ft entice Hall)... 

Fig. 19-6. Pasquill-Gifford (left) and (right). Source From Gifford (12).

As an example of the use of the Gaussian plume equations using the Pasquill-Gifford dispersion parameters, assume that a source releases 0.37 g s of a pollutant at an effective height of 40 m into the atmosphere with the wind blowing at 2 m s . What is the approximate distance of the maximum concentration, and what is the concentration at this point if the atmosphere is appropriately represented by Pasquill stability class B ... 

The Pasquill-Gifford dispersion parameters and Brigg s plume rise equations. 

The Pasquill-Gifford dispersion parameters are functions of downwind distance and meteorological conditions. The parameters, and Oy may be obtained from Figure 6(a) and 6(b) respectively. The user must know the atmospheric stabihty as well as downwind distance from the somce to select the appropriate dispersion parameters... 

Figure 6- The Pasquill-Gifford dispersioo coefficients versus downwind distance for various dispersion classes, (a) The lateral dispersion coefficient and b) the vertical dispersion coefficient are plotted against x.

In the calculations that were made to predict ground level concentrations from a VCM reactor blow off, the Pasquill-Gifford-Holland dispersion model was used as a basis for these estimations. Calculations were made for six different stability classes and ground level concentrations, and at various distances from the point source of emission. 

The Pasquill-Gifford-Holland dispersion method was used for the above example. Note that vinyl chloride monomer (VCM) constitutes the primary active ingredient in the reactor. It was therefore assumed that ... 

From Figures 2 and 3, the Pasquill-Gifford dispersion coefficients are obtained for a downwind distance of 2000 meters and for atmospheric stability Class B. 

The Pasquill-Gifford-Holland method predicts GLC s based upon 10 minute averaging times. However, what is also of interest is the concentration of HjS... 

Figure 5-11 Dispersion coefficients for Pasquill-Gifford plume model for urban releases.

Table 5-2 Recommended Equations for Pasquill-Gifford Dispersion Coefficients for Plume Dispersion12 (the downwind distance xhas units of meters)...

Distance downwind, km for Pasquill-Gifford puff model. 

The equations for cases 1 through 10 were rederived by Pasquill8 using expressions of the form of Equation 5-37. These equations along with the correlations for the dispersion coefficients are known as the Pasquill-Gifford model. 

Pasquill-Gifford or Gaussian dispersion applies only to neutrally buoyant dispersion of gases in which the turbulent mixing is the dominant feature of the dispersion. It is typically valid only for a distance of 0.1-10 km from the release point. 

Pasquill-Gifford plume model At a given downwind distance x, the maximum (average) concentration for a (continuous) passive plume from a point source is... 

When denser-than-air effects are important, use the Britter-McQuaid (plume or puff) models. Otherwise, assume the release is passive and use the Pasquill-Gifford (plume or puff) models. Adjust values for the virtual source correction s) as appropriate. 

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