Page 20 - Design and Operation of Heat Exchangers and their Networks

Heat exchangers and their networks: A state-of-the-art survey 7

the heat transfer coefficient significantly. A performance comparison of
commercial enhanced tubes shows the enhancement ratios from 1.40 to
3.75 (Webb, 1994).
There are many other enhancement techniques, either passive or active.
Sharafeldeen et al. (2017) summarized the available experimental investiga-
tions of heat transfer performance and pressure drop in coiled wire inserted
tubes and conducted their own experiments in turbulent flow regime with
an extended range of Reynolds number of 14,400 Re 42,900. More
research work has been carried out on different kinds of twisted-tape inserts.
The heat transfer and pressure drop characteristics in a circular tube fitted
with V-cut twisted-tape inserts were investigated by Murugesan et al.
(2011) experimentally. Bhuiya et al. (2013) presented their experimental
results and correlations of heat transfer and friction factor characteristics in
turbulent flow through a tube fitted with perforated twisted-tape inserts.
The experimental and numerical studies on various types of twisted-tape
inserts were reviewed and discussed by Hasanpour et al. (2014) and Varun
et al. (2016). The application of twisted tapes inserted in the shell side outside
of the tubes was reported by Ayub et al. (2018). Recently, Ponnada et al.
(2019) published their experimental results and correlations of perforated
twisted tapes with alternate axis, perforated twisted tapes, and regular twisted
tapes inserted in a circular tube for heat transfer and pressure drop charac-
teristics under constant heat flux condition.
Some other examples are coated surfaces for dropwise condensation,
porous surfaces for boiling, and recently the use of nanofluids.

1.4 Optimal design of heat exchanger networks

In the last four decades, extensive efforts have been made for heat integration
problem and energy recovery technology because of the steadily increasing
energy cost and CO 2 discharge. A heat recovery system consisting of a set of
heat exchangers can be treated as a heat exchanger network (HEN). By the
use of HENs, a large amount of utility costs such as the costs of heating
medium, cooling water, heaters, and coolers can be saved. The optimization
problem of a HEN is named as heat exchanger network synthesis (HENS)
and has raised considerable research interest. A wide variety of HENS
procedures were developed.
The available HENS procedures can be classified into two categories:
sequential and simultaneous synthesis techniques. Sequential targeting
methods progressively cut down the problem feasible region by successively