Momentum and heat transfer in complex systems always is the sum of single and simple transfer elements here called unit operations. They are usually characterised by head loss coefficients and Nusselt numbers as far as the flow and the heat transfer aspect is concerned. In our study we show that this is insufficient for a complete assessment and that additional parameters should be introduced. They take into account that energy, its use and conversion always is subject to the constraints set by the second law of thermodynamics. (C) 2010 Elsevier Ltd. All rights reserved. e