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288 Cha p te r T w e l v e

where three streams are extracted. The correct data extraction for this
case involves two streams, as shown in Figure 12.3(b).
General guidelines for data extraction may be summarized as
follows:

1. Heat losses. In most cases, heat losses can be neglected. However,
they should not be neglected when streams (mass and heat
flow in pipes) are long or subject to varied temperatures. In
such cases, the solution is to introduce hypothetical coolers (or
heaters) that represent the heat loss.
2. Extracting utilities. The utilities should never be extracted from
the existing plant or flowsheet, for then the solution would
likely arrive at the same utility values and perhaps neglect
some options that would be more efficient. This rule of thumb
applies especially to cases where utilities can be generated
on-site and thereby (at least partially) replace costly existing
utilities. In this connection it should be remembered that
steam is not always a utility—sometimes it is also a process
stream (e.g., stripping steam in separation columns). Process
streams should remain in place and not be removed.
3. Generating utilities. The Heat Integration analysis may indicate
some valuable options for using otherwise wasted heat or
cold to generate utilities. The Grand Composite Curves can
be used to locate such options. However, when extracting
data it must be recognized that steam requires the boiler
feedwater (BFW) to be heated, water to be evaporated, and
the steam to be superheated; see Figure 12.4. Many mistakes
have been caused by designers who simply matched the

T

Specified Steam
Conditions

BFW Conditions

ΔH

FIGURE 12.4 Extraction of a cold stream including segments for BFW
preheating, evaporation, and superheating.