A thin-walled copper tube of radius ri= 5mm is used to transport a low-temperature refrigerant and is at a temperature Tithat is less than that of the ambient air at Taround the tube.

I. Find the critical thickness of insulation
2. Calculate the total thermal resistance per unit length of the tube when it has an insulation layer of thickness 0, 2, 5, 10, 20, 40mm (the insulation is composed of cellular glass (k 0.055Wm-K),and the outer surface convection coefficient is 5 W/m2-K.)