The Influence of Availability Costs on Optimal Heat Exchanger Design

Optimizing heat exchangers based on second-law rather than first-law considerations ensures that the most efficient use of available energy is being made. In this paper, second-law efficiency is used to develop a new technique for optimizing the design of heat exchangers. The method relates the operating costs of the exchanger to the destruction of availability caused by the exchanger operation. The destruction of availability is directly related to the second-law efficiency of the exchanger. This allows one to find the NTU at which the benefits of reduced availability losses are offset by the costs of added area; this is the optimal point. It can be difficult to determine the proper cost of irreversibility to be used in the optimization process. This issue can be handled by including the irreversibility cost in a dimensionless parameter that represents the ratio of annual ownership costs to annual operating costs that include irreversibility costs. In this way, each heat exchanger designer can estimate the costs of irreversibilities for his particular system, and then use the generalized method that is developed herein for determining the optimal heat exchanger size. The method is applicable to any heat exchanger for which the ε-NTU-R relationships are known.