Counterflow and Parallel Flow in a Double Pipe Heat Exchanger
A primary advantage of a hairpin or double pipe heat exchanger is that it can be operated in a true counterflow pattern, which is the most efficient flow pattern. That is, it will give the highest overall heat transfer coefficient for the double pipe heat exchanger design.
Also, hairpin and double pipe heat exchangers can handle high pressures and temperatures well. When they are operating in true counterflow, they can operate with a temperature cross, that is, where the cold side outlet temperature is higher than the hot side outlet temperature.
For example, in the diagrams in this section, consider Fluid 1 to be the hot fluid and Fluid 2 to be the cold fluid. Then, in the counterflow diagram at the left, you can see that the cold side outlet temperature, T2out, can approach the hot side entering temperature, T1in, which is higher than the hot side outlet temperature, T2out. For the parallel flow shown at the right, T2out can only approach T1out; it could not be greater.