A thin-walled pipe of D = 20 mm internal diameter is subjected to a constant heat flow on the surface of strength q'=400 W / m. Water flows through the pipe at a rate m. = 0.1 kg/s. The entry bulk temperature is Tm,i = 20 degree C. Find the required length of the pipe if the bulk outlet temperature Tm,o = 40 degree C. Find the pipe wall temperature at the exit.