-
Lesson 1.1 Introduction to Process Control
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In
recent
years
the
performance
requirements
for
process
plant
have
become
increasingly difficult to satisfy.
近年来，对过程系统的性能改善需求变得越来越困难
.
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Stronger
competition,
tougher
(
更加严苛的
)
environmental
and
safety
regulations
(
法
规
),
and rapidly changing economic conditions have been
key factors in the tightening of
plant
product quality specifications (
产品质量规范<
/p>
).
更为激烈的竞争，
更加严格的环境<
/p>
和安全规范，以及快速变化的经济条件都是制约工厂产品质量规范的关键因素。
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A
further complication (
复杂
) is
that modern processes have become more difficult
to
operate because of the trend toward
larger, more highly integrated plants with smaller
surge
capacities
(
谐振能力
)
between
the
various
processing
units.
更为复杂的情况
是，由
于现代制造业朝着规模更大
,
集成度更高的方向发展
,
而使不同的加工环节之
间的协调能力更低
,
所以加工过程更难控制
.
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Such
plants
give
the
operators
little
opportunity
to
prevent
upsets
(
扰乱
)
from
propagating from one
unit to other interconnected units.
在这种
工厂中，要想让一个
生产环节出现的问题不对其相连的另一个生产环节产生影响
,
几乎是不可能的
.
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In
view
of
(
考虑到，由于
)
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.
近
年来，考虑到工业制造逐渐加强的安全、高效需求，过
程控制这个课题变得越来越受重视
.
In
fact,
without
process
control
it
would
not
be
possible
to
operate
most
modern
processes safely and
profitably (
有利的
), while
satisfying plant quality standards.
实际上，
对于大多数现代工业，要满足安全、高效，产品质量的要求，没有控制系统是不可
能的
As an
introduction to process control, consider the
continuous (
连续
) stirred-tank
heater
(
加热搅拌器
)
The inlet liquid stream has a mass flow
rate
w
and a temperature
T
i
. The tank
contents
are well agitated
(
搅动
) and heated by an
electrical heater that provides Q watts
(
瓦特
).
输入液态流体的质量流量
率为
w
，温度为
Ti.
槽内成分搅拌均匀，并且用电热器加
热，功率为
Q<
/p>
瓦特。
It
is
assumed
that
the
inlet
and
outlet
flow
rates
are
identical
(
相同的
)
and
that
the
liquid density
ρ
(rho) remains constant,
假设输入和输出流量率是相等的，
并且液体密
度保持恒定
that is, the temperature variations are
small enough that the temperature dependence
of
ρ
can be
neglected.
也就是说温度变化足够小，密度对温度的影响可以忽略不计
Under these conditions the volume V of
liquid in the tank remains constant.
在这
些条
件下，槽内液体的体积保持恒定
The control objective for the stirred-
tank heater is to keep the exit temperature
T
at a
constant
reference value
T
R
.
加热
搅拌器的控制目标是保持输出温度
T
在一个恒定参
考值
T
R
上
The reference value
(
参考值，给定值
) is referred to as
a set point (
设定值，给定值
) in
control terminology
(
术语
). Next we consider two q
uestions.
在控制术语中，
参考值指的
是给定值
.
下面我们考虑两个问题
Question 1: How much heat must be
supplied to the stirred-tank-heater to heat the
liquid
from an inlet temperature
T
i
to an exit temperature T<
/p>
R
?
把加热搅拌器内的液体从输入
?
?
?
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温度
T<
/p>
i
加热到输出温度
T
R
，需要多少热量？
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To
determine
the required heat
input
for
the
design
operating
conditions,
we
need
to
write a
steady-state (
稳态
) energy
balance (
能量平衡
) for the
liquid in the tank.
要确定
达到设计运
行条件下的热量需求，我们需要写下槽内液体的稳态能量平衡式
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In writing this
balance, it is assumed that the tank is perfectly
mixed and that heat losses
are
negligible.
在写平衡式之前，假设槽内是完美搅拌的，同时忽略热损耗
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Under
these
conditions
there
are
no
temperature
gradients
(
梯度
)
within
the
tank
contents
and
consequently,
the
exit
temperature
is
equal
to
the
temperature
of
the
liquid
in
the
tank.
在这些条件下，槽内成分的温度保持一致，因此，输出温度等于
槽内液体
温度
A steady-state energy
balance for the tank indicates that the heat added
is equal to the
change in enthalpy
(
焓
,
表示物质内能，可以理解为衡量物质温度的物理量
)
between
the inlet and exit stream.
根据稳态能量守恒，加入的热量等
于输入和输出流体之
间的焓变化量
....Thus,
if
T
is
too
high
we
would
decrease
w
to
reduce
the
energy
input
rate
in
the
stirred tank relative to the mass flow
rate and thereby reduce the exit temperature.
因此，
如果温度太高，我们将降低流量率
w
，使得搅拌槽内的与质量流量率相关的能量输
入速率减少，因此使输出
温度得以降低
…
Place a
heat exchanger on the inlet stream.
The
heat
exchanger
is
intended
to
reduce
the
disturbances
in
Ti
and
consequently
reduce the
variation in T. This approach is sometimes
ca
lled
“hog
-
tieing” (
把脚或
腿束缚
住，这里意思与
restraint
相近，解释为约束、束缚
) an
input.
方法
7.
在输入流安置
一个热交换器
.
热交换器意图减少
Ti
的干扰，因此可以减少
温度
T
的扰动
.
这个方
法有时又叫做输入束缚法
.
Method 8. Use a larger tank.
If a larger tank is used,
fluctuations (
波动
) in Ti will
tend to be damped out
(
阻尼，衰减
)
due to
the larger thermal capacitance of the tank
contents. However, increased volume
of
tankage would be an expensive solution for an
industrial plant due to the increased
capital costs of the larger
tank.
方法
8.
使用一个更大的槽
.
如果使用更大的
槽，因为
更大的热容，
Ti
的波动会趋
向于衰减
.
然而，体积增加使得开支增加，会使工厂系
统的解决方案变得更加昂贵
.
Note that this approach is analogous to
the use of water baths in chemistry laboratories
where the large thermal
capacitance of the bath serves as a heat sink
(
散热装置
) and
thus
provides an isothermal (
恒温的
)
environment for a small-scale research apparatus
(
仪器
).
要指出的是这个方法类似于化学实验室中水缸的使用，
水缸大的热容量可以
看作散热装置，因此可以为小型实验仪器提供一个恒温环境
.
Next, we will classify the eight
control strategies of the previous section and
discuss their
relative advantages and
disadvantages.
接下来，我们将给这
8
种控制方法进行分类，
同时讨论他们各自的优缺点
Methods
1
and
3
are examples
of
feedback
control
(
反馈控制
)
strategies.
In feedback
control, the process variable to be
controlled is measured and the measurement is used
to adjust another process variable
which can be manipulated
(
操作，操纵
).
方法
1
和
3
是反馈控制的例子
.
在反馈控制中，
测量被控过程变量，该测量值用于调整另一个
可以操做的过程变量
.
（即测量变量，操作变量，测量变量用于调整操作变量
.<
/p>
）
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