I • ENGINEERING CALCULATIONS OF RADIANT HEAT EXCHANGE 



multiplied (based on the data of Howard, et al. {2Jf\ and on unpublished 

 data of HoLeong and Wu [25,26]). The absorption, by the gas, of radi- 

 ation from the surface is aTla^i, where a^\ is the absorptivity of the gas 

 at Tg for black or gray radiation from the surface at Ti. Absorptivity is 

 evaluated approximately by reading the gas emissivity at Tx and at 

 pJLTi/Te (the value giving the same molal concentration as at the gas 

 temperature), then multiplying the result by {TJT])^-^^. The same cor- 

 rection factor Cc applies to ojgi as to €g if the total pressure is not 1 atm. 



0.004 



0.003 



500 1000 1500 2000 2500 3000 3500 4000 4500 5000 



Temperature, °R 



Fig. 1, 3a. Emissivity of carbon dioxide. 



For water vapor, the gas emissivity depends on Tg and -p^L and on 

 total pressure Pt as before, and in addition on the partial pressure of 

 water vapor pw Emissivity due to water vapor is given in Fig. 1, 3c (based 

 on experimental data [22,28,27,28,29]) as a function of Tg and p^L, for 

 Pt = 1 atm and for the "ideal" system with pw = 0. Approximate allow- 

 ance for departure from these special conditions is made by multiplying 

 eg from Fig. 1, 3c by a factor Cw read from Fig. 1, 3d as a function of 

 (pw + Pt) and pwl/. The absorptivity of water vapor for black body radi- 



< 514 ) 



