82 RADIATION lUOLOGY 



Effective Exposures. Even with absorption minimized by the thinnest 

 fihn.s provided in centrifugal devices, exposures at least as great as for 

 \vater-l>orne organisms would be re(iuired: 200 300 ultraviolet ^iw-min/ 

 cm-. The minimum exposure for the thicker films pro\'ided by gravity 

 devices might i)e increased se\-eralfold to 700-1000 ultraviolet /iw-min 

 cm- such as is retiuired in the disinfection of absorptive water through 

 several inches of depth. Since it is difficult to provide ultraviolet inten- 

 sities greater than 10,000 ultraviolet ^w/cm- (10 ultraviolet watts/scj ft), 

 the exposure times in such devices should not be less than 1-2 sec for the 

 thiimest films to 4-6 sec for the thickest. 



Although the speed of 500-1000 rpm of the centrifugal devices provides 

 linear film speeds of 200-400 ft/min, this is in a helical, nearly circular 

 path. Only the relatively slow, forward component contributes to the 

 exposure time. The centrifugal force provides this forward component 

 directly only in rotors essentially conical in shape (Fig. 2-1 8C). In a 

 cylindrical rotor the centrifugal force spreads the film in both directions 

 perpendicular to the helical path, l)ut there is effective movement of the 

 film as a whole only in so far as the gravity-pressure equilibrium is dis- 

 turbed by the delivery of liquid from the film and only in so far as the 

 rotor is so inclined as to permit gravity flow. Note also in this connec- 

 tion that the rate of liquid flow through these devices defines the exposure 

 time only through the amount actually and momentarily being processed 

 in the film, an amount usually very difficult to measure accurately. 



Operating Controls. Where there may be variations in ultraviolet 

 absorption or penetration into an irradiated film such as are shown in Fig. 

 2-16, there must be provision in commercial devices by which the film 

 thickness and the ultraviolet exposure are completely controlled bj' the 

 licjuid absorption. The ideal control would provide an automatic adjust- 

 ment of the exposure to the absorption with the film thickness held con- 

 stant mechanically. The exposure should, in turn, be based directly on 

 the ultraviolet intensity at the film surface, rather than on the electrical 

 characteristics of the sources, to compensate for their output depreciation. 

 Fast-acting relays and valves should stop the delivery of material 

 instantly upon the shortest power failure and should provide for rejection 

 or reprocessing of material remaining in the device. 



DISINFECTION OF SURFACES OF GRANULAR MATERIALS 



There is practically no penetration or reflection of ultraviolet energy 

 in the irradiation of granulated or powdered materials. Only the upper 

 fourth or fifth of the surface of individual particles can be irradiated at 

 any given instant of time. However actively stirred or agitated, the sur- 

 face of such particles is therefore efTectively irradiated only one-fourth 

 to one-fifth of the time. The particles shade each other as soon as there 

 is a layer more than one particle thick, so that there is a "coefficient of 



