Thermodynamic design of condensers and evaporators: Formulation and applications

This paper assesses the thermo-hydraulic design approach introduced in a previous publication (Hermes, 2012) for condensers and evaporators aimed at minimum entropy generation. An algebraic model which expresses the dimensionless rate of entropy generation as a function of the number of transfer units, the fluid properties, the thermal-hydraulic characteristics, and the operating conditions is derived. Case studies are carried out with different heat exchanger configurations for small-capacity refrigeration applications. The theoretical analysis led to the conclusion that a high effectiveness heat exchanger does not necessarily provide the best thermal-hydraulic design for condenser and evaporator coils, when the rates of entropy generation due to heat transfer and fluid friction are of the same order of magnitude. The analysis also indicated that a high aspect ratio heat exchanger produces a lower amount of entropy than a low aspect ratio one.