Derive an expression for the effectiveness of a parallel flow heat exchanger in terms of the number of transfer units, NTU, and the capacity ratio Cmin/Cmax . In a parallel-flow double-pipe heat exchanger water flows through the inner pipe and is heated from 20° C to 70° C. Oil flowing through the annulus is cooled from 200° C to 100° C. It is desired to cool the oil to a lower exit temperature by increasing the length of the heat exchanger. Determine the minimum temperature to which the oil may be cooled.