Solve Problem 15.7 if all specified conditions remain the same but radiant energy exchange from glass to the surroundings at the air temperature is also considered.

Data From 15.7

Plate glass, k = 1.35 W/m · K, initially at 850 K, is cooled by blowing air past both surfaces with an effective surface coefficient of 5 W/m² · K. It is necessary, in order that the glass not crack, to limit the maximum temperature gradient in the glass to 15 K/cm during the cooling process. At the start of the cooling process, what is the lowest temperature of the cooling air that can be used?