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In a counter-flow double pipe heat exchanger, oil ($\dot m = 2 \text{ kg/s}, C_p= 2.1 \text{ kJ/kg}.\:^{\circ} C$) is cooled from $90^{\circ}C$ to $40^{\circ} C$ by water ($m = 1 \text{ kg/s}, C_p = 4.2 \text{ kJ/kg.}\;^{\circ}C$.) which enters the inner tube at $10^{\circ}C$. The radius of the inner tube is $3 \text{ cm}$ and its length is $5\text{ m}$. Neglecting the wall resistance, the overall heat transfer coefficient based on the inner radius, in $\text{kW/m$^{2}$.K}$, is

- $0.743$
- $7.43$
- $74.3$
- $2475$