Possible processes are shown in Fig. 1.36.

Fig 1.36 Possible plant to heat and cool four fluid streams for a minimum 7°C temperature difference.

Example 2: Pinch point temperature of 23.5°C

Similar to example1 the cold stream (composite) is moved from left to right on the diagram (Fig 1.37) horizontally, keeping the hot composite curve fixed, until the temperature difference at pinch point is 23.5°C. It is then seen that the external heating load of 130 kW and external cooling load of 202kW are required for the system, all other energy changes can be achieved by the heat exchangers between the various streams.

Fig 1.37 Temperature verses rate of change enthalpy change for composite hot and cold streams for Pinch point temperature of 23.5°C

Mathematically to obtain the pinch point at 23.5°C the values of cold steams are increased by 200KW, keeping the values of hot streams constant. Required estimation of hot and cold streams for 23.5°C pinch point is given in Table. 1.15.

Table 1.15 Estimation of hot and cold streams for 23.5°C pinch point