The issue of temperature pinch points

The so-called pinch point is the minimum allowable heat transfer temperature difference in a heat exchanger network; it limits the maximum amount of heat that can be recovered from such a network. It was first introduced by B. Linnlloff. Pinch analysis is based on chemical engineering thermodynamics; it uses an objective economic function as a criterion to select appropriate pinch temperature differences in order to minimize utility consumption and achieve the most economical heat allocation for energy savings. Its main design steps are as follows: 1. Specify an initial pinch temperature difference, or determine the appropriate pinch temperature difference based on energy recovery requirements, in order to determine the location of the pinch point. 2. Divide the network into sub-networks: at the pinch point, the heat exchange network is split into an upper and a lower heat exchange sub-network; the area above the pinch point corresponds to the endothermic section, while the area below it corresponds to the exothermic section. 3. Starting from the pinch point, apply the pinch point design rules (flow rate rules, branching rules, and heat capacity CP rules) to design the network above the pinch point as well as the network below it. 4. Combine the two sub-networks. To form the overall initial network, it is necessary to achieve the predetermined energy recovery target. 5. The energy relaxation method is employed to break circuits and reduce the number of units, thereby simplifying the initial network. 6. The network is further optimized through the analysis of remaining problems and drive force diagrams. 7. The temperature at the pinch points is changed, and the above steps are repeated until a heat exchange network with the lowest cost is found; the corresponding temperature difference at the pinch points is referred to as the optimal pinch point temperature difference, and the design is complete.