68 RADIATION BIOLOGY 



source on its axis of a length greater than the diameter of the zone has 

 been shown to be that on a concentric cylindrical surface of one-half the 

 radius. It follows then that the ultraviolet intensity at any given dis- 

 tance from a germicidal lamp, as indicated in Fig. 2-9, will be the average 

 intensity throughout a cylindrical zone of a radius twice that distance. 

 Thus the GSGTO lamp of Fig. 2-9 provides an intensity of 2800 mw/sq ft 

 at a distance of 2'^^ in. and so an average of that intensity throughout 

 a cylindrical space of about the length of the ultraviolet source and a 

 radius of 53^ 2 ii'^-i or a cross-section of 0.95 sq ft. Although two such tubes 

 would provide an effective initial average intensity in the cross section 

 of the duct, three would be specified to increase the minimum intensity 

 at remote parts of the duct and for an effective intensity at the end of tube 

 life. Luckiesh and HoUaday (1942a) have developed the theory of 

 ultraviolet duct-air disinfection in minute detail, and Buttolph (1945, 

 1951) has given it practical application. 



Turbulent Flow for Average Exposure. In small ducts that require only 

 one or two germicidal tubes which are of necessity placed parallel with the 

 direction of air flow, there may be a 10-to-l variation in the ultraviolet 

 intensity at distances 1-10 in. from the tube. To ensure that all the air 

 receives an average intensity exposure in its travel through the irradiated 

 zone, either the streamlined flow of the air must be broken into turbulent 

 flow by baffles or more germicidal tubes must be used. In the latter case, 

 as in all cases where many tubes are used, they may be spaced to provide 

 a sufficiently uniform ultraviolet intensity to take care of the streamlined 

 air flow. 



Increase of Average Intensity and Uniformity by Reflective Duct Walls. 

 Duct walls of pure aluminum of 65-75 per cent reflectance for 2537 A 

 will nearly double the effectiveness of the germicidal tubes by at once 

 nearly doubling the average intensity and by greatly increasing the uni- 

 formity of distribution by multiple reflection. 



Disinfected Duct Air as Alternative to Make-up Air in Sanitary Ventila- 

 tion. The most that ultraviolet disinfection can do is to make all the air 

 handled by a duct bacteriologically equivalent to make-up air. When- 

 ever the use of enough ultraviolet to provide a theoretical 99 per cent 

 disinfection of the duct air may be impractical, it should be noted that 

 one-half as much ultraviolet will still prox'ide 90 per cent disinfection, and 

 one-fourth as much will provide about 70 per cent disinfection. In such 

 cases, disinfection provides the equivalent of 90 and 70 per cent make-up 

 air in contrast with the 10-20 per cent usually believed to be economically 

 practical in the winter. 



Outdoor air is usually considered satisfactory for the sanitary ventila- 

 tion of living and assembly (juarters, and its usefulness is limited only by 

 the considerable cost of heating and circulating it in adeciuate quantities. 

 In food and pharmaceutical plants it may, however, carry enough mold 



