Figure 7 Composite curves with utilities
As shown in Figures 6 and 7, the hot stream (red line) enters the heat
exchanger from the right side 620.42 K and leaves at the left side at
313.14 K. As a counter current heat transfer, the cold stream (blue
line) enters the exchanger from the left side at 303.14 K and exits at
the right side at 624.67 K. The horizontal distance between the red and
blue lines corresponds to the heat-transfer rate from the hot stream,
QH, to the cold stream, QC. The slope of
the hot or cold stream line is inversely proportional to the ability of
the stream to give off or accept heat. The total heat transferred from
the hot stream ∆HH is given by Eq. (5) or to the cold
stream ∆HC is given by Eq. (6):
\({H}_{H}\left(\text{kW}\right)=\left(MC_{p}\right)_{H}\left(\frac{\text{kW}}{K}\right)\left({T_{H}}^{\text{supply}}-{T_{H}}^{\text{target}}\right)(K)\)(5)
\({H}_{C}\left(\text{kW}\right)=\left(MC_{p}\right)_{C}\left(\frac{\text{kW}}{K}\right)\left({T_{C}}^{\text{target}}-{T_{C}}^{\text{supply}}\right)(K)\)(6)
Where the capacity flow rate, (MCp)irefers to the product of the mass flow rate, Mi, and the
heat capacity, Cp,i, of each stream i.
As shown in Figure 6, the composite curves are graphical representation
of the heating and cooling demands of the entire system. It is used in
identifying the minimum utility requirements. The construction of
composite curves is an essential step in process integration by pinch
analysis. Individual hot and cold streams are represented on a single
diagram in order to determine the minimum utility duties for the entire
system. The vertical overlap represents possible heat integration in the
system of heat exchanger network. The composite curves also showed where
to apply heating and cooling utilities to the cold and hot streams
respectively.
The composite curves give the energy targets before the design. Energy
targets from the composite curve are heating 138.58 MW and cooling load
141.196 MW. The Grand composite curve (GCC), which is a plot of shifted
temperatures against the cascaded heat between each temperature
interval, is shown in Figure 8. This was obtained at
DTmin of 10 oC.