Page 265 - Design and Operation of Heat Exchangers and their Networks
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254   Design and operation of heat exchangers and their networks


                  180
                                                          Q HU,min
                  160

                  140
                  120
                                                          Dt m
                  100
                t (°C)
                  80
                  60

                  40
                  20
                       Q CU,min
                   0
                    0          1000        2000        3000        4000
                                          H (kW)
          Fig. 6.3 Composite curves of Example 6.4 (H2C2_175R), Δt m ¼5K.

             Because of the polyline nature of the composite curves, they approach to
          each other most closely at the pinch point where Δt min occurs. The com-
          posite curves allow the designer to predict optimized hot and cold utility
          targets ahead of designing the network. Taking the composite curves shown
          in Fig. 6.3 as an example, if we move the cold composite curve to the right,
          Δt min will increase, which means a decrease in the heat exchanger area, that
          is, a decrease in the investment costs. However, both hot and cold utility
          costs will increase. If we move the cold composite curve to the left until
          Δt min approaches to zero, it is rather the opposite. An experienced engineer
          can make a good balance between the investment costs and utility costs to
          get a good design of the network.


          6.3.3 Pinch design method
          According to the composite curves, we can separate the network at the
          pinch into two sections. In the section above the pinch (hot end part),
          the composite hot stream gives all its heat to the composite cold stream with
          only residual heating required. The system is therefore a heat sink. Heat
          flows from the hot utility into the network, but no heat leaves it. Con-
          versely, below the pinch (cold end part), the system is a heat source. Heat
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