3.7 Determine the heat transfer surface area required for a heat exchanger constructed from a 25.4 mm OD tube to heat helium from -7°C to 0°C using hot water at 85°C.

The water and helium flow rates are 0.6 and 2.4 kg/s. Helium flows within the tubes. The overall heat transfer coefficient is 120 W/m² K. The specific heats for water and helium are 4.18 and 5.20 kJ/kg - K, respectively. Consider (a) a counterflow exchanger, and (b) a 1-2 TEMA E exchanger. Solve the problem by both the e-NTU and MTD methods.