Page 333 - Design and Operation of Heat Exchangers and their Networks

CHAPTER 7

Dynamic analysis of heat

exchangers and their networks

a
b
Wilfried Roetzel , Xing Luo , Dezhen Chen c
a
Institute of Thermodynamics, Helmut Schmidt University/University of the Federal Armed Forces Hamburg,
Hamburg, Germany
b
Institute of Thermodynamics, Gottfried Wilhelm Leibniz University Hannover, Hannover, Germany
c
Institute of Thermal Energy and Environmental Engineering, Tongji University, Shanghai, China

Transient thermal behavior of heat exchangers and their networks has gained
more and more interest because of its increasing importance in process
industries. In industry, heat exchangers and their networks frequently
undergo transients resulting from external load variations and regulations.
The environmental temperature variation, frosting, and fouling in a heat
exchanger might cause slow changes in operating conditions of heat
exchangers. Startup and shutdown of equipment in a system, switchover
of the streams in pipelines, and regulation of the process parameters would
bring the heat exchangers into transient operating processes with rapid and
large variations. In real processes, three kinds of transients often occur: (1)
The heat exchanger undergoes a transient from a steady state to a new steady
state; (2) the heat exchanger works with small disturbances around a fixed
operating condition; (3) the heat exchanger works in a steady periodic
variation.
Dynamic analysis of heat exchangers is very important for the design of
automatic control systems. For example, the time delays of the outlet fluid
temperature responses to a disturbance in an inlet fluid temperature or mass
flow rate might be relatively large. As the temperature sensors at the outlet of
an exchanger detect a deviation from the target value, the system might have
already been out of control. It is obvious that the feedback control system
might not be suitable in such a case. Dynamic analysis of heat exchangers
will offer the designers important design basis and parameters. Only if the
designer well knows the dynamic characteristics of the heat exchangers,
he can design a good and reasonable control system to realize the expected
control aims. With the computing ability of personal computer available

Design and Operation of Heat Exchangers and their Networks © 2020 Elsevier Inc.
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