On the basis of constructal theory, structure design of a shell-and-tube (ST) evaporator with ammonia-water working fluid is conducted in this paper. A complex function composed of heat transfer rate (HTR) and total pumping power (TPP) is taken as optimization objective, and the heat transfer area of the tubes is taken as the constraint. The complex function of the ST evaporator is minimized, and the corresponding optimal external diameter (ED) of the heat transfer tube (HTT) and optimal mass flow rates (MFRS) of the ammonia-water and hot water are obtained. Compared with the initial design, the complex function, HTR and TPP are decreased by 17.24%, 22.88% and 64.14% after constructal optimization, respectively. It illustrates that the complex function is a tradeoff between the heat transfer and fluid flow performances of the evaporator. As the ammonia concentration increases, the overall performance of the evaporator is improved. The inlet temperature of hot water has certain influences on the optimal MFRS of the ammonia-water and hot water, but has no influence on the optimal ED of the HTT. The optimal results obtained in this paper can provide some guidelines for the optimal designs of the ST evaporators, and can be further extended to the optimal designs of the ST condensers and even the whole systems of Kalina cycles. (C) 2019 Elsevier Ltd. All rights reserved.