An expert system capable of assisting the process engineer in thermal processes design selection can improve both the quality and the variety of the solutions proposed. Conceptual design is part of the engineering design process which converts design requirements into an acceptable design solution. Nowadays, engineering design problems have become more and more complex and the increasing complexity has made synthesis a very difficult task: it is during this stage of the design process that the creativity and experience of an engineer are most needed. Inappropriate component requirements and/or coupling can significantly reduce the capability of a process to meet design demands. The prototype presented in this paper (COLOMBO) is an attempt to show that an expert system can be of valid help in finding answers to conceptual process selection. An exhaustive search across the data base allows the user to evaluate plant configuration constituted by virtually every technically feasible combination of components. Generally, the problems under exam result in a goal-driven search process and, therefore, a backward chaining approach is used here. The data base has been subdivided into two main classes: equipment and resources. Each ＇＇equipment＇＇ object has in-flows and out-flows (energy, matter,...) which define its operational capabilities. Interconnection of these objects is possible when there is compatibility between the out-flows of one and the corresponding in-flows of the next object. Of course, connectivity must satisfy all the constraints imposed to the problem domain. Presently only a few components have been included in the data base and, therefore, only few conventional configurations are produced by the expert system. Nevertheless COLOMBO produced four configurations for a power generation plant and three process configurations for a cogeneration plant which resolve to be equivalent to the human expert choices.