An oil cooler is to be designed in a process industry.

A parallel flow heat exchanger is designed to cool oil $(C_p=2\mathrm{.}0K\frac{J}{k}gK)$ from $120C$ to $85C$ by flow of water. The water flows at the rate of $75k\frac{g}{m}in$ and gets heated from $40C$ to $75C.$ What heat exchanger area is required for an overall heat transfer coefficient of $0\mathrm{.}35K\frac{W}{m^2}K?$

A change in operating conditions occurs and the water flow rate drops to $50k\frac{g}{m}in$ for the same oil flow rate and the same inlet temperatures of both the fluids. Work out the exit temperature of the oil and water under the changed conditions.

$($Marks $40)$