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.

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/m
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?