Hot oil at 110 ℃ heats water from 30 ℃ to 70 ℃ in a counter-current double-pipe heat exchanger. The flow rates of water and oil are 50 kg min⁻¹ and 100 kg min⁻¹, respectively, and their specific heat capacities are 4.2 kJ kg⁻¹ ℃⁻¹ and 2.0 kJ kg⁻¹ ℃⁻¹, respectively. Assume the heat exchanger is at steady state. If the overall heat transfer coefficient is 200 W m⁻² ℃⁻¹, the heat transfer area in m² is __________