A modular production plant consists of predefined apparatuses with a fixed design, called equipment modules. Selecting the equipment module with the appropriate suitable capacity to compensate fluctuations in production rate is one of the key challenges in module-based plant design. The equipment module's operating window needs to fit and determines the point in time when a capacity expansion is required so that the operating window of the equipment module also affects a capacity expansion strategy. In order to avoid an unnecessary use of multiple apparatuses in parallel, equipment modules should be developed specifically for a large operating window in contrast to conventionally designed equipment. Therefore, an approach is required to design equipment modules for flexibility in production rate. In this work a method to design equipment modules for flexibility based on a global sensitivity analysis and the deduction of design rules of thumb is introduced. This method is applied to the design of a liquid/liquid heat exchanger without phase change. The resulting design for flexibility is compared to the conventional design with regard to the operating window and investment as well as operating costs. It is shown that for 14% higher annual costs a four-fold operating window can be obtained. This presents an important step towards increasing the competitiveness of modular production plants. (C) 2017 Elsevier Ltd. All rights reserved.