Gifford

ELU0RINEC0MP0UNDS,0RGANIC - FLUORINATED ACETIC ACIDS] (Volll) Gifford-McMahon cryocooler... 

The Gifford-McMahon cryocooler consists of displacer, regenerator, compressor and intake/exhaust valves that can be staged to reach cryogenic temperatures. 

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. 26 57 Vertical dispersion coefficient for Pasqiiill-Gifford puff model. These data are based only on the data points shown and should not he considered rehahle elsewhere. (Reprinted from D. A. Cr owl and J. F. Louvar Chemical Process Safety, Fiiudanieutals with Applications, 1990, p. 140. Used hy permission of Prentice Hall. )... 

For estimating concentrations from more than one source, it is convenient to use map coordinates for locaHons. Gifford (4) has pointed out that the resulting calculated concentration is the same whether the preceding axis system is used or whether an origin is placed at the ground beneath the receptor, with the x-axis oriented upwind, the z-axis remaining vertical, and the y-axis crosswind. 

Pasquill s dispersion parameters were restated in terms of a-y and Cj by Gifford (14, 15) to allow their use in the Gaussian plume equations. The... 

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 ... 

For sources having a large component of emissions from low-level sources, the simple Gifford-Hanna model given previously as Eq. (20-19), X = Cqju, works well, especially for long-term concentrations, such as annual ones. Using the derived coefficients of 225 for particulate matter and 50 for SO2, an analysis of residuals (measured minus estimated) of the dependent data sets (those used to determine the values of the coefficient C) of 29 cities for particulate matter and 20 cities for SOj and an independent data set of 15 cities for particulate matter is summarized in Table 20-1. For the dependent data sets, overestimates result. The standard deviations of the residuals and the mean absolute errors are about equal for particulates and sulfur dioxide. For the independent data set the mean residual shows... 

A version of the Gifford-Hanna model was evaluated (50) using 1969 data for 113 monitoring stahons for particulate matter and 75 stations for SO2 in the New York metropolitan area. This version differed from Eq. (20-19) in considering major point source contributions and the stack height of emission release. This model produced results (Table 20-2) comparable to those of the much more complicated COM model (51). 

The Gaussian diffusion equation is known as the Pasquill and Gifford model, and is used to develop methods for estimating the required diffusion coefficients. The basic equation, already presented in a slightly different form, is restated below ... 

The PasquiU and Gifford approach described later, removes the need to concentrate on determining and Oy (refer to Figure 1) directly from weather data. In order to do this, Pasquill introduced the concept of the atmospheric stabihty class. 

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.

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