Partial control is a decentralized control strategy whereby a number of economic operating objectives are controlled either at their setpoints or within a specified range by feedback control of a few dominant variables. In many processes that have been studied the economic variables are related to the flow and production rates in the process. These rates also determine the magnitude of the energy exchange between the plant＇s different energy carriers. Thus, a thermodynamic process description can be used to identify the dominant variables affecting the rate of energy exchange within the plant. This approach to dominant variable identification is very efficient in that it focuses on a single expression containing all the dominant variables for each process unit and it ignores all irrelevant process states. In this first part the method is demonstrated on simple units as well as on the more complicated fluidized catalytic cracker. In part 2, the method is applied to the Tennessee Eastman challenge process.