CONDUCTION HEAT TRANSFER Heat Transfer Q Q A Q 2  p  r_o  L 5.92  x  10⁵ Btu hr 2  (3.14)  (0.54  ft)  (35  ft) 4985 Btu hr   ft² Example: A 10 ft length of pipe with an inner radius of 1 in and an outer radius of 1.25 in has an outer  surface  temperature  of  250°F.   The  heat  transfer  rate  is  30,000  Btu/hr.   Find  the interior surface temperature.   Assume k = 25 Btu/hr-ft-°F. Solution: Q 2  p  k  L(T_h T_c) ln(r_o/r_i) Solving for T_h: T_h Q  ln(r_o/r_i) 2  p  k  L T_c 30,000^Btu hr ln 1.25  in 1  in 2  (3.14) 25 Btu hr   ft   °F (10  ft) 250°F 254°F The evaluation of heat transfer through a cylindrical wall can be extended to include a composite body composed of several concentric, cylindrical layers, as shown in Figure 4. HT-02 Page 14 Rev. 0