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 (eta(II)) was presented as a function of number of transfer units (Ntu), ratio of heat capacity rates C-t*, ratio of channel-flow inlet temperature to dead-state temperature (theta*), number of rectangular tubes (N) and pressure-drop ratio on the channel-flow side (Delta P-c/P-o). The result shows that the eta(II) value increases with the Ntu value. As the Delta P-c/P-o value increases, the eta(II) value would decrease. For a set of Ntu, theta*, N and Delta P-c/P-o values, a maximum eta(II) value (eta(II,max)) was found. At theta* = 1.205, the C-t*, value corresponding to the eta(II,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 eta(II) value. (C) 2009 Elsevier Ltd. All rights reserved.