Thermodynamic optimal design of heat exchangers for an irreversible refrigerator

Abstract

Thermodynamic optimisation of energy systems is essential in reducing the environmental impact of energy utilisation. Yet, the refrigerators commonly used for this purpose have improvable efficiency levels. Their performance, as shown by the literature, is highly influenced by the size of the heat exchangers and by internal irreversibilities. In this paper the maximum coefficient of performance ( COP) is obtained for an irreversible inverse Rankine cycle refrigerator working with the environmentally friendly fluid R134a. This is a steady-state refrigerator working as an open system which consumes external work, subtracts heat from a cold fluid stream at an inlet fixed temperature and assigns it to a higher fixed inlet temperature stream. Heat transfer irreversibilities in the shell-and-tube heat exchangers and external friction losses in the water streams are considered, ignoring only the internal pressure drop of vapor. A simulation program was developed to search the maximum COP at given external fluid temperatures, as a function of mass flows, dimensions and temperature differences in the heat exchangers. Owing to the large number of control variables involved, a numerical optimisation method was used to determine the maximum COP. The proposed method is fast, producing the maximum with acceptable approximation. It provides the refrigerating fluid evaporating and condensing pressures, the heat exchanger dimensions, and the water flow rates for a given cooling power with predefined inlet temperatures of cold and hot water streams. The heat exchanger area closely conditions the COP, so each maximum represents the optimum thermodynamic working conditions for a given area of the heat exchangers.