Introductions to temperature control
　　　　　　　　　　　　　　　　and PID controllers
　　　　　　　　　　　　　　　　
　　Process control system.
　　Automatic process control is concerned with maintaining process variables temperatures pressures flows compositions, and the like at some desired operation value. Processes are dynamic in nature. Changes are always occurring, and if actions are not
taken, the important process variables-those related to safety, product quality, and production rates-will not achieve design conditions.
In order to fix ideas, let us consider a heat exchanger in which a process stream is heated by condensing steam. The process is sketched in Fig.1