The heat capacity flow rate is given by CP (kW/K) = FCp(T-To). The heat
exchanger network featured four cold streams with combined enthalpy of
141,195.65 kW, and ten hot streams having total enthalpy of 138,583.12
kW. It is clear from the capacity of hot and cold streams presented in
Table 4 that the load available in all hot streams is more than those
available in the cold streams.
In order to generate targets for minimum energy targets, the
ΔTmin value was set for the problem, with an initial
value of 10 oC. The ΔTmin or minimum
temperature approach is the smallest temperature difference that was
allowed between hot and cold streams in the heat exchanger where
counter-current flow was assumed. This parameter reflects the trade-off
between capital investment (which increases as the ΔTminvalue gets smaller) and energy cost (which goes down as the
ΔTmin value gets smaller). For the purpose of this
study, typical ranges of ΔTmin values that have been
found to represent the trade-off for each class of process have been
used.
The retrofitted HEN grid diagram for the base case design is shown in
Figure 5. It can be observed from the CDU network that there is little
restriction on heat exchange between the cold process and the large hot
process streams. This results in wastage of useful heat energy which is
also unsafe for the environment and the plant operators. Hence, in the
retrofitted network, cold utility (cooling water - CW) was introduced in
order to make up for the deficient heat sink (cold process streams) to
exchange heat with the heat source (hot process streams). The
temperature versus enthalpy plot or composite curve is shown in Figure
6.