To cool a house, air is routed though a plastic pipe (k=0.15 W/mk Di=0.15m, Do= 0.17m) in a pond whose water temperature is 17C a convection coefficient of h= 1500 W/mK is maintained on the outside surface of the pipe. If air from the house enters the pipe at 29C and a volumetric flow rate of 0.025 m/s, what length pipe is required for the discharge temperature of the air to be 21 C? [ans: 13.7m]