Introduction to Process Control 过程控制简介论文翻译

406荣誉出品 1 Unit 1 Introduction to Process Control

In recent years the performance requirements for process plants have become increasingly difficult

to satisfy. Stronger competition, tougher environmental and safety regulations and rapidly

changing economic conditions have been key factors in the tightening of plant product quality

specifications. A further complication is that modern processes have become more difficult to

operate because of the trend toward larger, more highly integrated plants with smaller surge

capacities between the various processing units. Such plants give the operators little opportunity to

prevent upsets from propagating from one unit to other interconnected units. In view of the

increased emphasis placed on safe. efficient plant operation, it is only natural that the subject of

process control has become increasingly important in recent years. In fact, without process control

it would not be possible to operate most modern processes safely and profitably, while satisfying

plant quality standards.

近年来，对于过程系统的执行的必要条件越来越难满足了，在紧缩的工厂产品质量规范中，强大的竞争，严峻的环境和安全规范，快速变化的经济状况，这些重要因素都是我们必须考虑的。由于工厂朝着规模更大，更集成化，在各种生产过程中，我们将面临又一个困难是越来越难操作和运行。对于操作员来说，阻止误差从一个单元到另一个互联的单元的可能性很小。鉴于把重点放在安全和系统运行，自然而然，过程控制变得越来越重要了。事实上，要是没有过程控制，我们不可能安全和效率的处理大多数现代过程，也不能满足工厂质量标准。

1.1.1 Illustrative Example

As an introduction to process control. Consider the continuous stirred-tank heater shown in Fig. 1.

1. 1 The inlet liquid stream has a mass flow rate w and a temperature T . The tank contents are

well agitated and heated by an electrical heater that provides Q watts. It is assumed that the inlet

and outlet flow rates are identical and that the liquid density p remains constant ,that is the

temperature variations are small enough that the temperature dependence of p can be neglected.

Under these conditions the volume V of liquid in the tank remains constant.

在图1.1.1中连续的搅拌槽加热器的入口液体有质量流速w和温度Ti，这个被加热的和搅拌的系统提供Q，假定入口和出口的流速一样和液体的密度保持常量，也就是，这个温度是足够的小以至于温度对密度的相互关系可以忽略，在这种情况下液体的体积保持常量。

The control objective for the stirred-tank heater is to keep the exit temperature T at a constant

reference value TR. The reference value is referred to as a set point in control terminology. Next

we consider two questions.

搅拌槽加热器的控制目标是为了保持出口温度T总是维持在一个恒定的参考值TR。在控制术语中，这个参考值被称为设定值。下一步，我们考虑两个问题。

Question 1. How much heat must be supplied to the stirred-tank-heater to heat the liquid From an

inlet temperature T, to an exit temperature TR?

问题1。需要给搅拌槽加热器提供多少热量来加热液体，以使液体温度从由入口温度T达到出口温度TR？

To determine the required heat input for the design operating conditions, we need to write a

steady-state energy balance for the liquid in the tank. In writing this balance, it is assumed that the

tank is perfectly mixed and that hear losses are negligible. Under these conditions there are no

temperature gradients within the rank contents and consequently, the exit temperature is equal to

the temperature of the liquid in the tank. A steady-state energy balance for the tank indicates that

the heat added is equal to the change in enthalpy between the inlet and exit streams:

Q= wC(T-Ti) (1)

406荣誉出品 2 为了设计工作条件，以确定所需的热量输入，我们需要写一个水箱中的液体稳态能量平衡。在写这个平衡时，假设水箱中的液体是完全混合的并且忽略热损失。在这些条件下，水箱里没有温度梯度，因此，出口温度等于水箱中的液体温度。一个水箱的稳态能量平衡表明，增加的热量等于进口流和出口流之间热含量的改变：Q = WC（T -TI）（1）

where Ti T w and Q denote the nominal steady-state design values of Ti T w and Q, respectively.

and C is the specific heat of the liquid. We assume that C is constant. At the design conditions,

T=TR(the set point). Making this substitution in Eq. (1) gives an expression {or the nominal heat

input Q: Q=wC(TR-T) (2)

其中，TI ，T ，Tw和Q表示TI， T，w和Q的稳态设计值，C是液体的比热。我们假设C为常数。在设计条件下，T = TR（设定值）。用Eq来替代。（1.1.1）给出了一个标称热输入的表达式：Q = WC（TR-T），（2）

Equation (2) is the design equation for the hearer. If our assumptions are correct and if the inlet

flow rate and inlet temperature are equal to their nominal values, then the heat input given by Eq.

(2) will keep the exit temperature at the desired value, TR. But what if conditions change? This

brings us to the second question:

方程（2）为加热器的设计公式。如果我们的假设是正确的且如果进水流量和进水温度等于其标称值，然后由方程2给定的输入热量式将出口温度保持在所需的值，TR。但如果条件发生变化呢？这给我们带来了第二个问题：

Quesrion2. Suppose that inlet temperature Ti changes with time. How can we ensure that T

remains at or near the set point TR?

问题2. 假设入口温度Ti随时间变化。我们如何确保T处于或接近设置点TR. ？

As a specific example. assume that Ti increases to a new value greater than Ti If Q is held constant

al the nominal value of Q, we know that the exit temperature will increase so that T>TR. (cf.Eq.

(1)). 作为一个特别的例子，假设Ti增加到一个比Ti（划横线的）更大的值。如果Q是常数，我们知道出口温度将会增加以至于T>TR 。

To deal with this situation, there are a number of possible strategies for controlling exit