The maximum amount of heat or mass transfer that can be obtained is higher with countercurrent than co-current (parallel) exchange because countercurrent maintains a slowly declining difference or gradient (usually temperature or concentration difference).

Why is counter-current flow better?

One of the great advantages of counter-current flow is the possibility of extracting a higher proportion of the heat content of the heating fluid. It is important to note that the LMTD value for counter-current flow is much larger than for cocurrent flow at the same terminal temperature (see Figure 1.9).

Which one is better countercurrent flow or concurrent flow Why?

In Chemical engineering most of the operations are counter-current in nature because, the mass transfer or heat transfer will be maximum in the counter-current flow. Co-current flow is preferred in rare cases of heat transfer where a quick heat transfer is required.