Two very large parallel plates are maintained at uniform temperatures t1 = 800 k and t2 = 500 k and have emissivity ε1 = 0.2 and ε2 = 0.7, respectively. determine the net rate of radiation heat transfer between the two surfaces per unit surface area of the plates.

Awall of 0.5m thickness is to be constructed from a material which has average thermal conductivity of 1.4 w/mk. the wall is to be insulated with a material having an average thermal conductivity of 0.35 w/mk so that heat loss per square meter shall not exceed 1450 w. assume inner wall surface temperature of 1200°c and outer surface temperature of the insulation to be 15°c. calculate the thickness of insulation required.