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


　　過(guò)程控制系統(tǒng) 自動(dòng)過(guò)程控制系統(tǒng)是指將被控量為溫度、壓力、流量、成份等類型的過(guò)程變量保持在理想的運(yùn)行值的系統(tǒng)。過(guò)程實(shí)際上是動(dòng)態(tài)的。變化總是會(huì)出現(xiàn)，此時(shí)如果不采取相應(yīng)的措施，那些與安全、產(chǎn)品質(zhì)量和生產(chǎn)率有關(guān)的重要變量就不能滿足設(shè)計(jì)要求。
　　為了說(shuō)明問(wèn)題，讓我們來(lái)看一下熱交換器。流體在這個(gè)過(guò)程中被過(guò)熱蒸汽加熱，如圖1所示。
　　這一裝置的主要目的是將流體由入口溫度乃(f)加熱到某一期望的出口溫度T(f)。如前所述，加熱介質(zhì)是過(guò)熱蒸汽。
　　只要周圍沒(méi)有熱損耗，過(guò)程流體獲得的熱量就等于蒸汽釋放的熱量，即熱交換器和管道間的隔熱性很好。