1. A double-pipe counterflow heat exchanger uses hot water to heat cold water. Cold water enters at 20 C and leaves at 50C. Hot water enters at 80C and leaves at 40C. The mass flow rate of the cold water is 1.5kg/s. If the overall heat transfer coefficient (U) is 250W/m2K, a. What is the required surface area of the exchanger? b. What is the flow rate of the hot water (kg/s) ? For both hot and cold water, cp=4.18kJ/kgK