Page 139 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

136 Chapter 5 Heat exchanger network analysis

5.4.6 Network simplification: heat load loops and heat load paths
In practice, the resulting network is usually complex and can be further simplified and capital-energy
optimisation can be carried out. Heat load loops and heat load paths leave scope for the designer to
simplify the network or to reduce the overall network cost.
Heat load loops are closed pathways in a system of connections in a HEN and usually arise when
two heat exchangers (one below and one above the pinch) are placed across the same pair of streams.
In this case, one of the exchangers can be eliminated by shifting duties around the loop without
affecting the heat duties on other units that do not belong to the loop. However, such a change will
affect the temperature driving forces in the network. The temperature driving force may become less
than DT min in some cases, resulting in temperature infeasibility. This implies that some energy flow
across the pinch may be necessary to obtain a practical network design.
A heat load path eliminates DT min violation that may arise during network simplification by
providing a continuous pathway between a utility heater and a utility cooler. This leads to transfer of
heat loads between heat exchanger units and utilities. Heat loads can be shifted along a path by
alternately adding and subtracting a duty from each successive unit. This procedure does not affect
stream duties but changes intermediate stream temperatures due to change in exchanger duties.
Some commercial software e.g. SuperTarget from M/s KBC can do the optimisation using heat
load loops as well as heat load paths.

5.5 Targeting for multiple utilities
The composite curves in Figs. 5.4 and 5.9A indicate the requirement for the extreme utility levels e
hot utility at temperature higher than the supply temperature of the hot
stream(s) and cold utility cooler than the cold stream(s). However, in
practice the plant utilities are available at several different temperature
Grand Composite Curve
levels e.g. furnace flue gas, steam levels, hot oil circuit, cooling water,
refrigeration levels, etc. The general objective is to maximise the use of
the cheaper utilities and conserve the expensive utility. For example, it
is preferred to use LP steam instead of HP steam and cooling water instead of refrigeration. These can
be incorporated in the composite curve as shown in Fig. 5.9B which shows the construction of the hot
composite curve if we use LP steam to replace part of the HP steam requirement. The maximum LP
consumption that can replace the HP steam consumption is obtained for a temperature difference of
ðDT min Þ between the composite curves. It can be seen that the shape of the composite curves change
with addition of every new utility level and the overall construction becomes complex for several
utility levels. Such a curve is commonly termed as grand composite curve.
The Grand Composite Curve is also a convenient tool for setting multiple utility targets. It is a plot

of adjusted heat flow ðDH Þ computed from Eq. (5.16) versus shifted temperatures T and T of the

h c
hot and cold streams. The points to be plotted are obtained from the Problem Table. Fig. 5.9C illus-
trates the construction of the grand composite curve from the enthalpy (horizontal) differences be-
tween the shifted composite curves (shown by distance DH in the figure) at different temperature

levels. The two end points of the curve give the minimum hot and cold utility requirement where HP
steam is used for heating and refrigeration is used for cooling the process.