Page 26 - Design and Operation of Heat Exchangers and their Networks

CHAPTER 2

Basic thermal design theory for

heat exchangers

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

The basic theories of heat exchangers are based on the relationships between
the amount of heat exchanged and the heat transfer surface area, mass flow
rates, entrance and exit temperatures of fluids, etc. These relations are based
on the energy balance, mass balance, momentum balance, and the exchange
mechanism. Thus, the fundamental equations for all types of heat exchangers
include the mass, momentum and energy conservation equations, and the
equation describing the heat transfer rate. These equations are the starting
point for the design of heat exchangers and also for the transient analysis
of heat exchangers. Depending upon different specific cases, the general
forms of these equations may be simplified.

2.1 Heat transfer fundamentals

In most cases, recuperative heat exchangers are operated in a steady state
under preset operation conditions. Although some small disturbances in
environment, inlet fluid temperatures, and flow rates might happen, the
mean values of the operation parameters can be well maintained by auto-
matic control systems; therefore, the steady-state design methods can still
be applied, and they are the basic background of the design and operation
of heat exchangers and their networks not only for steady-state operation
but also for transient operation such as startup, shutdown, and operation
switching.

Design and Operation of Heat Exchangers and their Networks © 2020 Elsevier Inc.
https://doi.org/10.1016/B978-0-12-817894-2.00002-9 All rights reserved. 13