Performance of a serpentine heat exchanger: Part II – Second-law efficiency

A second-law analysis of a cross-flow serpentine heat exchanger for waste heat recovery was performed. The heat exchanger is composed of rectangular tubes. The flow outside the tubes (channel flow) is unmixed and the flow inside the tubes (tube flow) is mixed. In the analysis, the channel flow and the tube flow were considered as an ideal gas flow and an incompressible flow, respectively. In addition, the inlet temperature of the tube flow was the dead-state temperature and the pressure drop on the tube-flow side was neglected. The second-law efficiency (ηII) was presented as a function of number of transfer units (Ntu), ratio of heat capacity rates , ratio of channel-flow inlet temperature to dead-state temperature (θ∗), number of rectangular tubes (N) and pressure-drop ratio on the channel-flow side (ΔPc/Po). The result shows that the ηII value increases with the Ntu value. As the ΔPc/Po value increases, the ηII value would decrease. For a set of Ntu, θ∗, N and ΔPc/Po values, a maximum ηII value (ηII,max) was found. At θ∗ = 1.205, the value corresponding to the ηII,max value is very close to 1.0. The result also shows that increasing the number of passes of the heat exchanger can largely increase the ηII value.