H = Stream enthalpy

This is equivalent to calculating the area between the stream and the ambient temperature line, on a Carnot Factor vs. Enthalpy diagram. The exergy curves therefore provide a clear visual representation of where exergy is lost in the process and utility system.

For a stream which does not have a constant temperature the exergy value is calculated from:

where Ts and Tt are the stream source and target temperatures, respectively.

This is still equivalent to calculating the area between the stream and the ambient temperature line. However, the equation is now non-linear because a non-constant temperature stream actually appears slightly bent on the exergy curves. This is an obvious result of the non-linear relationship between Carnot Factor and stream temperature.

1.6.2 Constructing an exergy balance

The exergy equations can be used to construct an exergy balance around the process or site being analysed. In interpreting this balance the following table may prove useful:

1.7 Capital - Energy Trade-offs

The best design for an energy efficient heat exchange network will often result in a trade off between the equipment and operating costs. This is dependent on the choice of the DTmin for the process. The lower the DTmin chosen, the lower the energy costs, but conversely the higher the heat exchanger capital costs, as lower temperature driving forces in the network will result in the need for greater area. A large DTmin, on the other hand, will mean increased energy costs as there will be less overall heat recovery, but the required capital costs will be less. The trade-off is further complicated in a retrofit situation, where a capital investment has already been made. This section explains a rational approach to the complex task of capitalenergy trade-offs.