In a concentric counter flow heat exchanger, water flows through the inner tube at 250C. The engine oil enters at 1000C and flows in the annular flow passage. The exit temperature of the engine oil is 500C. Mass flow rate of water and the engine oil are 1.5kg/s and 1 kg/s, respectively. The specific heat of water nd oil are 4178 J/kg.K and 2130 J/kg.K, respectively. The effectiveness of this heat exchanger is ________