From experiments will be that the convection of counter flow to give the overall heat transfer coefficient as well as the efficiency higher than the parallel flow . By the volume flow rate of cold and hot water increase it will affect to the overall heat transfer coefficient increase along .
But the efficiency of the heat convection is well during the low hot water volume flow rate and the high cold volume flow rate because the low hot water volume flow rate is make the heat convection transfer to cold water better up cause high efficiency.
Which, if compared with the hot water volume flow rate that much although it make the overall heat transfer coefficient high up.
But. When consider the efficiency , it will not equal the volume flow rate of hot water at a lower