The targeting involves setting appropriate loads for the various utility levels by maximising cheaper utility loads and minimising the loads on expensive utilities.

1.6 Exergy Analysis

The use of pinch analysis techniques has already resulted in a significant impact on savings in utility consumption. However, traditional techniques tend to be mainly effective in situations where utility costs are dominated by heat loads. In contrast, in the case of utility systems which involve power as well as heat they cannot directly account for the power component. Such problems can now be addressed by the combined pinch and exergy approach, which was originally developed for refrigeration shaftwork targeting. Most importantly, the method can also be extended to above ambient applications involving heat and power. Exergy analysis techniques are therefore applicable to the design and optimisation of a wide range of overall site utility systems.

Exergy Analysis provides the information needed to consider shaftwork targets at the same time as considering the heating or cooling requirements of a chemical plant or site. The information is used as follows:

1.For a sub-ambient plant, the engineer can determine the minimum refrigeration shaftwork that will be required in the target network. This can be used for costing and for site-wide combined heat and power studies.

2.For an above-ambient plant, the engineer can determine the maximum potential for the plant to produce shaftwork. This is most relevant when there is a nearby sub-ambient plant to which this work can be exported. In some cases, however, the work may also be exported to an electrical grid. If none of the potential is recovered then the target represents the lost work in the process.

1.6.1 Carnot factor calculations

For the calculation of exergy targets the point targeting curves are redrawn using Carnot Factor, instead of temperature, as the Y axis co-ordinate. The Carnot factor is defined as follows:

nc = (1 - TTo )

where:

To = ambient temperature

T = stream temperature

The exergy value of a constant temperature stream can then be calculated from: Exstream = H*(1 - TTo )

where:

Pinch Technology Introduction