- #1

dzj633

## Homework Statement

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: A

_{o}=π*d

_{o}

Inside area of pipe: A

_{i}=π*d

_{i}

q

_{12}=h

_{r}A

_{1}(T

_{1}-T

_{2})

h

_{r}=σε

_{1}(T

_{1}+T

_{2})(T

_{1}

^{2}+T

_{2}

^{2})

## 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?