Pinch temperature, significance

A low temperature of approach for the network reduces utihties but raises heat-transfer area requirements. Research has shown that for most of the pubhshed problems, utility costs are normally more important than annualized capital costs. For this reason, AI is chosen eady in the network design as part of the first tier of the solution. The temperature of approach, AI, for the network is not necessarily the same as the minimum temperature of approach, AT that should be used for individual exchangers. This difference is significant for industrial problems in which multiple shells may be necessary to exchange the heat requited for a given match (5). The economic choice for AT depends on whether the process environment is heater- or refrigeration-dependent and on the shape of the composite curves, ie, whether approximately parallel or severely pinched. In cmde-oil units, the range of AI is usually 10—20°C. By definition, AT A AT. The best relative value of these temperature differences depends on the particular problem under study. 

To understand the true significance of the pinch, first divide the composite curves at the pinch, as illustrated in Fig. 6. Above the pinch (in temperature terms), the process is overall a heat sink. Heat recovery is possible from the composite hot stream to the composite cold stream above the pinch, but there is a residual amount of heat (Gh, ) that is required to be imported from hot utility to satisfy the enthalpy imbalance. Below the pinch (in temperature terms), the process acts overall as a heat source. Heat recovery is possible from the composite hot curve to the cold composite curve below the pinch, but a residual amount of cooling (2c ,i ) is required to satisfy the enthalpy imbalance. Overall, the process above the pinch therefore acts as a heat sink, and that below the pinch acts overall as a heat source. 

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