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.