Page 242 - Design and Operation of Heat Exchangers and their Networks

232 Design and operation of heat exchangers and their networks

optimization algorithm (Silva et al., 2010), and knowledge-based system
(Expert System) (Chen et al., 1989; Souto et al., 1992). Till now, researches
are still progressing along these three lines with the most attention to the
latter two methods. To solve real-life industrial problems, the engineer
should take advantage of all these disciplines.
There are three areas of heat exchanger network synthesis: targeting,
synthesis, and optimization. Targets include energy consumption (utilities),
heat transfer area, number of heat exchange units, and finally total annual
cost. The targets can be served as a motivation or to give the designer con-
fidence that a network is close to “optimal.” Synthesis methods include the
matching of hot and cold streams and the sequencing of the resulting heat
exchangers. Optimization involves both topological and parameter
improvements that reduce the total annual cost.
According to whether the three elements consisting the total cost,
namely, utilities, area and unit number, are considered simultaneous or
separately, the available synthesis methods can be reclassified into two cat-
egories: targeting sequential methods and simultaneous synthesis methods.
The methods in the first group progressively cut down the problem feasible
region by successively imposing a series of design targets arranged by their
decreasing impact on the total annual cost of the network. Usually, the top
goal is the least utility usage to be achieved through a minimum number of
units (the second-level target) at the lowest capital investment (the bottom-
level target). Though one cannot guarantee that a sequential method ends
up with the network featuring the lowest total annual cost, it often pro-
vides a very good network design. Pinch design techniques are the typical
representative in this group of sequential methods. The methods in the sec-
ond group are aimed at finding the optimal heat exchanger network in a
single step. These methods are no longer based on the assumption that the
total annual cost is dominated by the utility requirements, and all of
methods in the second group belong to the mathematical programming
area and use a mixed-integer nonlinear programming (MINLP) problem
formulation to seek the heat exchanger network featuring the least total
cost at once.
The design objective includes a quantitative part (cost of heat exchange
equipment and external utilities) and a qualitative part (safety, operability,
flexibility, and controllability). The quantitative part is the main topic of this
chapter, and the qualitative part will be discussed in Chapter 9.