The NTU (number of transfer units) is given by
where U is the overall heat transfer coefficient,
A is the heat transfer area,
Cmin is the smaller heat capacity of the two fluids .
Since the flow rates of the fluids remain the same in both cases, the overall heat transfer coefficient U is the same too. The area of the heat exchanger and the smaller heat capacity of the fluids remain the same. Thus, the NTU the same for both cases too.
The figure above shows the dependencies of the effectiveness from NTU and Cmin/Cmax ratio for the paralle-flow and counter-flow double pipe heat exchangers. In our case the maximum to minimum thermal capacity ratio is 0.5. Thus, from the plot (b) we can define NTU = 0.8 for the counter-flow. Now, from the plot (a) we can define the effectiveness of the parallel-flow heat exchanger as @$\varepsilon=48\%@$.