An open feedwater heater operates at 7 bars. Compressed liquid water at 35 C enters at one section, while superheated vapor enters at another section. The fluids mix and leave the heater as a saturated liquid. To = 20 C and Po = 1 bar. Find: a) The change in flow exergy (kJ/min) if the flow rate of compressed liquid is 4370 kg/min and the mass flow rate of superheated vapor is 1000 kg/min and b) exergetic efficiency.