Exergoeconomic optimisation of a heat exchanger with a two-phase refrigerant

In this paper, the exergoeconomic theory is applied to a heat exchanger for optimization purposes. The investigation was referred to a tube-in-tube condenser with the single-phase fluid to be heated flowing in the inner annulus and the two-phase refrigerant flowing in the external annulus. First, the irreversibility due to heat transfer across the stream-to-stream temperature difference and to frictional pressure drops is calculated as a function of two design variables: the inner tube diameter and the saturation temperature of the refrigerant, on which the heat-exchange area directly depends. Then, a cost function is introduced, defined as the sum of two contributions: the amortization cost of the condenser under study and the operating cost of the conventional electric-driven heat pump in which this component will have to work. The latter contribution is directly related to the overall exergy destruction rate in the plant, whereas the amortization cost mainly depends on the heat-exchange area. So, design optimization of the device can be performed by minimising this cost function with respect to the selected design variables. The so-called structural approach (Coefficients of Structural Bond) is used in the optimization, in order to relate the local irreversibility in the condenser to the overall exergy destruction rate in the heat-pump plant. A numerical example is discussed, in which, for a commercial heat exchanger, the design improvements needed to obtain a cost-optimal configuration are investigated. The results show that significant improvements can be obtained with respect to devices based on conventional values of the design parameters.