A cross-flow air-to-water heat exchanger with an effectiveness of 0.65 is used to heat water (Cp=4180 J/kg · °C) with hot air (Cp=1010 J/kg · °C). Water enters the heat exchanger at 20°C at a rate of 4 kg/s, while air enters at 100°C at a rate of 9 kg/s.
If the overall heat transfer coefficient based on the water side is 260 W/m²-K, determine the heat transfer surface area of the heat exchanger on the water side. Assume both fluids are unmixed.