Composite curves minimum temperature approach

Minimum Temperature Approach AT For a feasible heat transfer between the hot and cold composite streams, a minimum temperature approach must be specified, which corresponds to the closest temperature difference between the two composite curves on the T H axis. This minimum temperature approach is termed as the network temperature approach and defined as AT an-Maximal Process Heat Recovery The overlap between the hot and cold composite curves represents the maximal amount of heat recovery for a given AT r - In other words, the heat available from the hot streams in the hot composite curve can be heat-exchanged with the cold streams in the cold composite curve in the overlap region. 

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. 

Each temperature is a fixed value on the vertical axis, and enthalpy change rates are relative quantities. We estimate the enthalpy changes rather than absolute enthalpies, and the horizontal location of a composite line on the diagram is arbitrarily fixed. The location of ATmm on the composite diagram is where the hot and cold curves most closely approach each other in temperature in a vertical direction. We move one of the two curves horizontally until the distance of the closest vertical approach matches the selected ATmin. The overshoot of the hot composite curve represents the minimum cold utility (qc mm) required, and the overshoot of the cold composite curve represents the minimum hot utility (gh mm) required for the process. 

The portion of the hot composite curve and the cold composite curve shows the minimum cooling and heating utilities, respectively, that must be supplied. Increasing the minimum approach temperature will shift both curves farther apart, reducing the total heat integration... 

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