Example $4\mathrm{.}2$ : Calculate the rate of heat transfer by radiation from an unlagged steam pipe, $50mm,$ o$.$d$.$ at $393K(120C)$ to air at $293K(20C).$

Assume emissivity $\text{'}$e$\text{'}$ of $0\mathrm{.}9.$

Solution : Given :

$e=0\mathrm{.}90$

$T_1=393K,T_2=293K$