Page 22 - Design and Operation of Heat Exchangers and their Networks

Heat exchangers and their networks: A state-of-the-art survey 9

balance relationship among fluid streams involved in heat exchangers and
the time-independent temperature profiles of these streams, but it pro-
vides no information about the dynamic behaviors of heat exchangers.
Besides being in steady-state operation, heat exchangers often experience
transient processes that might be induced for different reasons.
During start-up, shutdown, and deliberate variations in flow rates and
temperatures of incoming fluid streams, heat exchangers are in a transient
operation phase. In addition, fouling on heat transfer surfaces also induces
time-dependent operation of heat exchangers. Strictly speaking, heat
exchangers are always in dynamic operation, although they are
usually assumed to be in the steady-state operation in most cases. After
the transient operation, a heat exchanger may either approach a new
steady state or return to its original steady state. It is also possible that a
heat exchanger is in a transient operation all the time, for example, in
the case of periodic variations of flow rates and temperatures of incoming
streams.
Optimal operation, treatment of accidents, dynamic design, and real-
time control demand more accurate description of the time-domain
behavior of heat exchangers. Although the steady-state design and rating
procedure is still necessary, dynamic analysis of heat exchangers provides
information about transient responses subject to various disturbances. There
are many cases in which the system dynamic behavior is of prime design con-
sideration. For example, the dynamic analysis is very important in nuclear
plants where severe transients may cause accidents and one may have to esti-
mate the possibility of accidents and may be able to prevent them by means
of accurate knowledge of dynamic behavior. Since the 1950s, more and
more attention has been paid to the transient behavior of a variety of heat
exchangers with or without phase change, and there are plenty of published
papers on the dynamics of heat exchangers. Compared with the steady-state
procedure, dynamic simulation gives more information about the opera-
tional situation and effects of exchanger structures. Some of the advantages
of dynamic analysis of a heat exchanger were assessed by Gupta (1986).
In all, dynamic simulation can serve as an aid to heat exchanger operation
(i.e., control and regulation) and a design tool. Increasing safety and envi-
ronmental concerns and stricter regulation have provided strong incentives
for the study of the dynamics of heat exchangers.
The dynamic analysis of heat exchangers can be performed either
experimentally or theoretically. With respect to complexity of dynamic
simulation, both the lumped parameter and distributed parameter
approaches can be used for equation-oriented dynamic simulation.