TABLE OF CONTENTS So far, we have assumed that all streams are equally free to exchange heat and that there is no preference on pairings except on temperature grounds. Often, this is not the case. Two specific situations are:
Where two parts of a plant are physically separated and exchanging heat between the two involves long pipe runs with high capital cost, heat losses and pressure drop.
Where part of the plant forms a natural "subset" which can be operated on its own, for example if the rest of the plant is shutdown for maintenance.
In such situations, we can make use of zonal targeting. The different streams are allocated to subsets or zones, and targets for each of these are obtained separately. The targets for various combinations of zones can then be obtained by combining the relevant sets of stream data, and the difference in the targets between the separate and combined data is found. If the difference is small, the zones can be treated as independent entities. If the difference is large, and is a high proportion of the total energy savings available, this gives a clear incentive for heat exchange between the zones, which must then be traded off against the extra cost and operating inconvenience.
This technique has been used since the early days of pinch analysis, but was developed into a formal zoning concept by Ahmad and Hui (1991) and modified by Amidpour and Polley (1997). It is closely...
