Water at 0.025 kg/s and 20°C enters an annular region formed by an inner tube of diameter 20 mm and an outer tube of diameter 80 mm. Saturated steam flows through the inner tube, maintaining its surface at a uniform temperature of 100°C, while the outer surface of the outer tube is well insulated. If fully developed conditions may be assumed throughout the annulus, how long must the system be to provide an outlet water temperature of 75°C? What is the heat flux from the inner tube at the outlet?