Why the effectiveness of counter flow heat exchanger is more than that of parallel flow heat exchangers??

   For counter flow HEX: the temperature difference between the hot & cold fluids is relatively constant across the HEX ( from inlet to outlet ) . which means that the rate of heat transfer is constant across the HEX pass.

   For Parallel flow HEX: the temperature difference is high at the HEX inlet while it decrease gradually across the HEX pass. which mean the rate of heat transfer decrease through the HEX.