Process simulators, including a novel process simulator that utilizes simplified mathematical correlations to predict the performance of equipment or subsystems in a process unit. The process simulator comprises a process correlation that is a mathematical response model for a discrete process system in a process unit. The process correlation is developed by obtaining operating data over a range of operating conditions of the process unit and regressing the operating data to form the mathematical response model. The process simulator calculates a number of output parameters using a number of input parameters. The process simulator outputs the output parameters so they can be used by operating personnel to evaluate the current operations of the process plant and make operating decisions.