Double tube heat exchangers can be classified by flow direction. Parallel and counterflow can be deployed in these exchangers, it all depends on the position of the inlet and outlet. The choice of co-current and counter-flow affects the heat transfer and pressure drop in the system, by the way, in some applications you should choose one of the other flows.
Countercurrent double tube heat exchanger
The best design for a double tube heat exchanger is counter flow. In this mode, the heat exchanger has the best heat transfer coefficient and can cool or heat the outlet as required.
The location of the entrance and exit is explained. As indicated by this type, the fluids flow in opposite directions to each other, and at the ends of the two heads, the temperature difference between the fluids is greatest. Check the counterflow diagram and consider that fluid 1 is hot and fluid 2 is cold. The outlet cold junction temperature (T2out) can obtain a temperature close to T1in, and we know that this temperature is greater than T1out. In this type, the cold fluid temperature can reach beyond that of the outlet on the hot side, while in the parallel type, this is not possible.