Water at 15°C is to be heated to 65°C by passing it over a bundle of 4-m-long, 1-cm-diameter resistance heater rods maintained at 90°C. Water approaches the heater rod bundle in normal direction at a mean velocity of 0.8 m/s. The rods are arranged in-line with longitudinal and transverse pitches of S_L = 4 cm and S_T = 3 cm. Determine the number of tube rows N_L in the flow direction needed to achieve the indicated temperature rise.

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Water 15°C 0.8 m/s SL = 4 cm - D=1 cm T, = 90°C ST = 3 cm