TABLE OF CONTENTS In order to design, analyze, or commission a process control system, one must be familiar with the characteristics of the process itself. Although it is highly beneficial for the control engineer to have a good understanding of the physical and chemical phenomena that govern the process, his or her view of the process will usually differ from that of the process design engineer. The discussion in this chapter is meant to develop the thought processes of a control engineer. Though some of the following points may seem to overstate the case, they will enable us to highlight the differences in the ways the control engineer and process design engineer think:
The process design engineer is concerned with meeting production rate and quality specifications, which are often called the "design conditions." The control engineer is concerned with operating an existing process at other than design conditions, often with reduced throughput, variations in feedstock, or other abnormal conditions.
The process design engineer's objective is often to minimize the initial cost (or the life-cycle cost) of the processing equipment. The control engineer's objective is to make the most efficient use of the equipment that is already installed.
The process engineer considers those design parameters that can be specified as independent variables. Other parameter values that are derived from these are dependent variables. For example, the pressure of a saturated steam system might be an independent variable that can be specified during the design process; the temperature then becomes a dependent variable. The...
