Counter flow:

Counter flow, as described in below figure, exists while the two fluids flow within opposite directions. Every of the fluids enter the heat exchanger at opposite ends. Since the cooler fluid exits the counter flow heat exchanger at the end where the hot fluid enters the heat exchanger then the cooler fluid will approach the inlet temperature of the hot fluid.

Counter flow heat exchangers are the most efficient of the three categories. In compare to the parallel flow heat exchanger, the counter flow heat exchanger could have the hottest cold fluid temperature greater than the coldest hot-fluid temperatue.

Figure: Counter Flow Heat Exchange