Figure 3-25 Temperature Control Of An Oil Heated Chemical Reaction Chamber - Honeywell VPR100 User Manual

Cascade Control
An example of a cascade control application is featured in Figure 3-25. Cascade control is typically used
when two process values must be simultaneously controlled, with one process value directly influencing the
behavior of the other. In this control strategy, each process value is supported by its own dedicated control
loop. The term "cascade" is used because it describes how this control approach literally attaches both
control loops together. This act of linking control loops allows for the regulation of both process values
using one and only one % output control signal.
INSTRUMENT
PV
SP
OUT 83.5%

Figure 3-25 Temperature Control Of An Oil Heated Chemical Reaction Chamber

In Figure 3-25, the temperature in a chemical reaction chamber is determined by the temperature of the
heated oil surrounding it. Heating the oil is done by an electric heating element driven by a 4 to 20 mA
controlled SCR and external power source. In this application the instrument controls the temperature of
the chemical reaction chamber through control of the heat emitted by the jacket tank oil. The instrument
must provide a single 4 to 20 mA control output to govern the voltage switched by the SCR and, hence, the
heat applied to the entire system. Temperature is monitored with thermocouples.
The function block diagram of the required instrument configuration is featured in Figure 3-26.
Note that this diagram illustrates the classic cascade arrangement of two control loops that defines the
cascade control strategy. The first control loop, LP1, is designated as the primary cascade loop by the
notation "CAS_P." The notation "CAS_S" indicates LP2's designation as the secondary cascade loop.
Note how both control loops are joined together. In addition to the back-calculated feedback path set up
between the two (LP2 BC), LP1's output is connected to an input on LP2 that at this time must be
introduced. Denoted as SP2, this input is LP2's remote set point input.
12/00
THERMOCOUPLES
200
500
4 TO 20 mA
(CAT)
SCR
+ ~ -
AC POWER
SOURCE
VPR & VRX – User Manual
Programming and Operating Concepts
CHEMICAL
REACTION
VESSEL
OIL
ELECTRIC
HEATING
ELEMENT
OIL JACKET
67