A double pipe heat exchanger installed in a flow system to cool the oil which will be transferred to a large turbine. The coolant used here in the system is water and is end to the heat exchanger in a counter direction to the oil. The inlet flow rate and temperatures of water and oil are $200\frac{g}{sec,30C}$ and $100\frac{g}{sec,100C}$ respectively. The machine oil will be sent in the annulus whereas water will be sending in an inner pipe.

Use the below data and calculate the tube length for outlet temperature of the oil to be $60C.$

Diamter of inner pipe $=25mm$

Diamter of outer pipe $=45mm$