A composite pipe made of steel pipe ( ks = 16 W/m.K) of inner and outer radius of 0.5 and 1.0 cm and a plastic pipe (kp = 0.092 W/m.K) of inner and outer radius of 1 and 1.25 cm, respectively. If the inner surface of the steel is maintained at 900 °C and outer surface of plastic is maintained at 300 °C. The rate of heat transfer per unit length is close to?
2. Relevant equation
Outside area of pipe: Ao=π*do
Inside area of pipe: Ai=π*di
The Attempt at a Solution
Sigma is boltzmann's constant (6.6667E-8 W/m^2.K^4) I do not know if these equations are the right ones to use because i cannot find an equation for the emissivity. What equation am i missing to solve?