温度控制简介和pid控制器------- 外文翻译.doc

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


　　过程控制系统 自动过程控制系统是指将被控量为温度、压力、流量、成份等类型的过程变量保持在理想的运行值的系统。过程实际上是动态的。变化总是会出现，此时如果不采取相应的措施，那些与安全、产品质量和生产率有关的重要变量就不能满足设计要求。
　　为了说明问题，让我们来看一下热交换器。流体在这个过程中被过热蒸汽加热，如图1所示。
　　这一装置的主要目的是将流体由入口温度乃(f)加热到某一期望的出口温度T(f)。如前所述，加热介质是过热蒸汽。
　　只要周围没有热损耗，过程流体获得的热量就等于蒸汽释放的热量，即热交换器和管道间的隔热性很好。