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Automated Continuous Process Control Part 5 pps
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Mô tả chi tiết
70 CASCADE CONTROL
Steam
Process
SP
Fluid
T
TT
22
TC
22
T(t)
Condensate
return
Ti(t)
T
Figure 4-3.1 Temperature control.
Steam
Process
SP
Fluid
T
TT
22
TC
22
T(t)
Condensate
return
Ti(t)
FT
21
FC
21
T
F
F
set
vp
(a)
Figure 4-3.2 Cascade control schemes in heat exchanger temperature control.
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resets the flow controller set point. Any flow changes are now compensated by the
flow loop. The cascade scheme shown in Fig. 4-3.2b accomplishes the same control,
but now the secondary variable is the steam pressure in the exchanger shell side.
Any change in steam flow quite rapidly affects the shell-side pressure. Any pressure
change is then compensated by the pressure loop. This pressure loop also compensates for disturbances in the heat content (superheat and latent heat) of the steam,
since the pressure in the shell side is related to the condensing temperature and thus
to the heat transfer rate in the exchanger. This last scheme is usually less expensive
in implementation since it does not require an orifice with its associated flanges,
which can be expensive. Both cascade schemes are common in the process industries. Can the reader say which of the two schemes gives a better initial response to
disturbances in inlet process temperature Ti(t)?
The cascade control systems shown in Fig. 4-3.2a and b are very common in industrial practice. A typical application is in distillation columns where temperature is
controlled to maintain the desired split. The temperature controller is often cascaded to the steam flow to the reboiler or the coolant flow to the condenser.
Finally, another very simple example of a cascade control system is that of a positioner on a control valve. The positioner acts as the inner controller of the cascade
scheme.
OTHER PROCESS EXAMPLES 71
Steam
Process
SP
Fluid
T
TT
22
TC
22
T(t)
Condensate
return
Ti(t)
PC
21
T
P
P
set
PT
21
(b)
Figure 4-3.2 Continued.
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