Retardants dry by a diffusion process that occurs at the interface between the retardant 

 and the atmosphere. The amount of moisture that leaves the surface and goes into the atmos- 

 phere is proportional to the number of water molecules on the surface of the retardant and the 

 environmental conditions that exist at the interface. Thus, M in equation (2) depends upon the 

 number of molecules on the surface of the retardant; and the constant r depends upon the envi- 

 ronmental conditions. The drying rate factor r remains constant only for a constant condition. 



All of the variables affecting retardant drying and all relations between these variables 

 must be known in order to solve equation (2) from first principles. The experiments conducted 

 in the wind tunnel, however, considered only three variables: 



1 . Area of retardant surface as controlled by amount of retardant applied to a uniform 

 fuel bed. 



2 . Difference in vapor pressure between the retardant surface and the atmosphere as 

 governed by relative humidity. 



3. Air velocity in close proximity to the fuel bed as governed by wind-tunnel velocity 

 blowing over uniform boundary conditions . 



Equation (1) could not be derived from equation (2) and made completely general primarily 

 because the actual value of the initial surface area of the retardant as it clung to the needles and 

 bridged between them could not be determined. However, a satisfactory equation was obtained 

 by considering the three variables tested that affect drying rate, and adjusting r for each 

 variable until a single value resulted. 



Surface Area 



The surface area of the retardant may be related to the amount of retardant applied uni- 

 formly and to the projected area 8 over which it is spread. Consider a unit area of fuel bed (a 

 cross section through the unit area is illustrated in figure 9); if a small amount of retardant is 

 spread uniformly over each needle, it will be thin and will have a relatively large surface area. 

 The maximum surface area possible is equal to that of the needles. As the amount of retardant 

 is increased on the same unit area of fuel bed, liquid bridges form between the needles, and the 

 surface area of the retardant decreases. The limiting value for maximum retardant and mini- 

 mum surface area equals the projected unit area. This occurs when retardant is applied to fill 

 all the crevices between the needles and only a flat surface of retardant is exposed. The drying 

 rate constant r will therefore be inversely proportional to the amount of retardant applied per 

 unit area. 



Vapor Pressure 



The correction to the drying rate factor r necessary to account for changes in humidity is 

 provided by considering the difference in vapor pressure. Drying rate is proportional to the 

 coefficient of diffusivity times the difference in vapor pressure between the surface of the 

 retardant and the free stream: 



Projected area refers to the flat plane surface area considering the fuel bed to be two- 

 dimensional. 



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