A very thin electric heater is wrapped around the outer surface of a long cylindrical tube whose inner surface is maintained at 50C. The tube wall has inner and outer radii of 25 mm and 75 mm respectively and a thermal conductivity of 10 W/(m-K). The thermal contact resistance between the heater and the outer surface of the tube per unit length is 0.01 (m-K) / W. The outer surface of the heater is exposed to a fluid with a temperature of -100C and a convection coefficient of 100 W/ (m2K). Determine the heater power per unit length of the tube required to maintain a heater temperature at 250C.