In a double-pipe heat exchanger, the cold fluid is water with an inlet temperature 20°C and mass flow rate 20 kg/s and the hot fluid water inlet temperature 80°C and mass flow rate 10 kg/s. Assume that for water Cp = 4.2 kJ/kg°C, independent of temperature. What is the maximum temperature to which the cold fluid can be heated in a parallel flow and in a counter flow heat exchanger?

• A. 80°C in both parallel flow and counter flow
• B. 50°C in both parallel flow and counter flow
• C. 40°C in parallel flow and 50°C in counter flow
• D. 40°C in parallel flow and 80°C in counter flow