Effective design of a chemical process requires a deep understanding of all areas of chemical engineering as well as many other aspects. For example, government agencies require that all industries comply with certain rules regarding environmental impact. The chemical engineer tasked with a process design must take these rules into account while attempting to design the best possible process that will maximize profits. Exergy can be used as a measure of how valuable a system is, in terms of efficiency of the process. A process is highly thermodynamically efficient and as a consequence profitable and eco-efficient if the exergy values of the products of a process are close to those of the raw materials. In this paper we present a new tool, the Relative Exergy Array (REA) to measure the relative exergetic efficiency and also the controllability of a process when a proposed process and control structure is postulated. The Relative Exergy Array is a means to compare thermodynamic efficiency for control structures of multi-input multi-output processes. Although only steady-state information is employed it is a valuable tool for quick comparison between several candidate process control structures. General guidelines for interpreting the results obtained from the REA are presented and explained in the paper.