I • ENGINEERING CALCULATIONS OF RADIANT HEAT EXCHANGE 



Temperature/ °K 



300 600 1000 3000 6000 

 l.OrtT-^ 



Key 



1. Slate composition roofing 



2. Linoleum, red brown 



3. Asbestos slate 



4. Soft rubber, gray 



5. Concrete 



6. Porcelain 



7. Vitreous enamel, white 



8. Red brick 



9. Cork 



10. White Dutch tile 

 1 1. White chamotte 



1 2. MgO, evaporated 



13. Anodized aluminum 



14. Aluminum paint 



15. Polished aluminum 



16. Graphite 



The two dotted lines bound the 

 limits of data on gray paving 

 brick, asbestos paper, wood, 

 various cloths, plaster of Paris, 

 lithopone, and paper 



500 1000 2000 4000 10,000 



Temperature, °R 



Fig. I, lb. Effect of source temperature on the absorptivity 

 of surfaces for black radiation. 



represented by aoT^T^, with n considerably greater than m for non- 

 metals, and both negative; and n about the same as m for bright metallic 

 surfaces, and both positive. Then, since ei = ai.i = aoT'l'+"', the net flux 

 at Si is 



gi^2 = cxSiao{Ti+"'+"- TfTI+^) (1-3) 



It may readily be shown that, over a moderate temperature range, this 

 reduces to 



gi-2 = crSi 



(-j) 



(^1 - Tl) 



(1-4) 



with €av evaluated at the arithmetic mean temperature. Thus, although 

 € approaches a as T2 and Ti approach each other, the emissivity factor 



< 506 ) 



