ABSTRACT
This chapter presents the beginning of a fascinating field of study and work, namely, process control theory and practice. Since all control equipment and processes exhibit inherent qualities of gain, or the difference between the magnitude of an input resulting in the output, it is important that we understand what is really happening. The gain of a process stage or control equipment is the relationship between the input and the resultant output. The effect that a disturbance will have on a plant depends on its point of entry into the control system. Positive feedback, when applied to the control unit, has the effect of increasing the overall gain and will preclude any possibility of stability. In a real plant, the most important cyclic changes that occur are those brought about under automatic control during the period of recovery after a process upset.
