M Q = initial application amount, grams 

 U = air velocity at surface of fuel bed, cm. /sec. 



p v = vapor pressure at surface of retardant, mm. Hg 



o 



p v ^ = partial pressure of moisture in free stream air, mm. Hg 



t = time of drying, minutes 



w Q = initial application concentration, grams/cm. 2 . 



The lines through the data points (fig. 8) are computed from equation (5). The lines fit the data 

 until the point on the curve where the retardant is nearly dry. As explained, this is believed to 

 result from the depletion of water on the surface of the needles and slower evaporation of 

 moisture from within the needles. 



On fine or closely spread fuels such as brush, grass, ground Litter, or logging slash, 

 equation (5) provides a good estimate of how much moisture will remain on the treated fuel 

 after drying in a known environment. Logs or other large fuels present a different problem. 

 Equation (5) may also be used to predict effective holding time of short-term retardants by 

 incorporating into the equation an expression for the amount of total moisture required for 

 effective retardation. 



Effective Holding Time- -Short-Term Retardants 



Short-term retardants derive their fire -inhibiting powers from the water they contain. 

 This water, when applied onto a fuel, must evaporate or be driven off by the heat of the fire 

 before the fuel's temperature can be raised to ignition point. Twenty to 22 percent is the gen- 

 erally accepted Limit of fuel moisture that will permit combustion in dead fuels without forced 

 convection. Taking the Limit of fuel moisture to be at least 22 percent, an expression can be 

 developed for the minimum amount of short-term retardant necessary to be effective against 

 a fire. 



Let G = minimum amount of retardant per unit area necessary for effective retardation, 

 wf = amount of dead fuel per unit area , and 

 Mf = fuel moisture content, percent, 

 then: G = w f (,22-M f ) (°) 



G and Wf must be in the same units such as lb. /sq.ft. , or a conversion constant must be in- 

 cluded in the equation. 



The equation for retardant drying rate (5) may now be combined with the amount of mois- 

 ture necessary, equation 6, to estimate the effective time of short-term retardants. The 

 expression for G in equation (6) is substituted for M in equation (5). The initial amount of 

 retardant M Q is changed from total amount to the amount per unit area, and the units are 

 converted to those in general fire control use, resulting in equation (7). 



20 



