Page 465 - Design and Operation of Heat Exchangers and their Networks

448 Design and operation of heat exchangers and their networks

y
Feedback
controller

Z Z ref

c C ˙
h h
t² h,set
t²
t¢ h t¢ E,h t² E,h h
C ˙
h
(1–c )C ˙ h
h
C ˙ c
t² c t¢ c

Heat
exchanger
Fig. 9.5 Feedback control system.

manipulation of input parameters is carried out only after the deviation of the
controlled output variables has been detected; therefore lag error and some-
time undesired fluctuations and/or instability cannot be avoided.
A feedforward control method was developed to overcome these short-
comings. In a feedforward control system, the controller senses the deviation
of inlet parameters and give the corresponding adjustments based on the
predication of dynamic response of the system, as is shown in Fig. 9.6.
For the traditional feedforward control, dynamic behavior of the system
is determined by analytical methods or experimental methods, which may
be expressed in differential equation form or transfer function form, and they
can be converted each other by mathematical methods. In order to improve
the control quality of feedforward control in its application in a HEN, to
predict the transient responses of the HEN more accurately becomes nec-
essary for the proper manipulation. Guan et al. (2004) used a mathematical
model based on a distributed parameter approach to predict the dynamic
behaviors of a plate-fin HE and the change of output parameters caused
by certain disturbances can be calculated. The dynamic analysis model is
embedded in the controller. When a disturbance of input parameter occurs,
or the system should be switched from one state to a new operation state, the
predictive model in the controller will predict the response of output param-
eters based on the sense of input parameter change, and then the controller
instructs the manipulation of bypass valves; thus, the change of output