Determining the optimal configuration of a heat exchanger (with a two-phase refrigerant) using exergoeconomics

In this paper, the exergoeconomic theory is applied to a heat exchanger for optimisationpurposes. The investigation was referred to a tube-in-tube condenser with the single-phasefluid to be heated flowing in the inner annulus and the two-phase refrigerant flowing in theexternal annulus. First, the irreversibility due to heat transfer across the stream-to-streamtemperature-difference and to frictional pressure-drops is calculated as a function of twodesign variables: the inner-tube’s diameter and the saturation temperature of the refrigerant,on which the heat-exchange area directly depends. Then, a cost function is introduced, definedas the sum of two contributions: the amortisation cost of the condenser under study and theoperating cost of the conventional electric-driven heat-pump in which this component willhave to work. The latter contribution is directly related to the overall exergy destruction ratein the plant, whereas the amortisation cost mainly depends on the heat-exchange area. So,design optimisation of the device can be performed by minimising this cost function withrespect to the selected design variables. The so-called structural approach (Coefficient ofStructural Bond) is used in the optimisation, in order to relate the local irreversibility in thecondenser to the overall exergy destruction rate in the heat-pump plant. A numerical exampleis discussed, in which, for a commercial heat-exchanger, the design improvements needed toobtain a cost-optimal configuration are investigated. The results show that significantimprovements can be obtained with respect to devices based on conventional values of thedesign parameters.