535-PROF User's Manual
Chapter 7
115
Applications
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
17
18
19
20
21
22
23
24
32
31
30
29
28
27
26
25
PV 1+
PV 1–
OUT 1–
OUT 1+
UNIT 1
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
17
18
19
20
21
22
23
24
32
31
30
29
28
27
26
25
PV 1+
PV 1–
RTD 3RD
OUT 1–
OUT 1+
raw
materials
MIXER
temperature sensor
pressure sensor
steam
HEAT
EXCHANGER
RSP+
RSP–
UNIT 2
In
Figure 7.20 we have a 535 set up to control a heat exchanger. In a PID-equipped
heat exchanger, pressure in the steam shell more quickly reflects fluctuations
in the steam supply than does the process liquid’s temperature.
Why? In this example, with PID control, the average temperature of the liquid in
the heat exchanger of 80°, but can vary by as much as five degrees because
the steam supply itself is not constant. Fluctuations in the pressure of the steam
supply cause fluctuations in the temperature of the steam within the heat
exchanger. So, the process liquid’s temperature begins to rise, but it takes
several minutes for the increased heat from the steam to travel through the process
liquid to reach the temperature sensor. By the time the sensor registers the higher
value and calls for a decrease in steam, the process liquid near the walls is already
at an even higher temperature. Although the steam supply is reduced, the process
liquid’s temperature continues to rise for a period of time. This delay in the transfer
of heat prevents the 535 controller from controlling the temperature more
precisely.
The solution to the problem is
Figure 7.21. Have the PID controller position the
steam valve, but add a sensor by means of another 535 controller that will monitor
the steam pressure. The pressure control system now creates a second feedback
control loop, which “cascades” from the first.
Figure 7.21
Cascade Control of Product
Temperature
