Environmental Engineering Reference
In-Depth Information
Grande Composite Curve (G.C.C)
Should Be Drawn To Scale
T* (K)
Total hot utility required is equal to 2620 kW
600
Hu3
Hu2
500
Hu1
400
300
Enthalpy ( kW)
200
700
1400
2100
2800
Figure 15. Grand Composite Curve
Multiple Utility Targeting/Selection using Grand Composite Curve (GCC)
Upon maximizing heat recovery in the heat exchanger network, those heating duties and
cooling duties not serviced by heat recovery must be provided by external utilities.
The most common utility is steam. It is usually available at several levels. High
temperature heating duties require furnace flue gas or a hot oil circuit. Cold utilities might be
refrigeration, cooling water, air cooling, furnace air preheating, boiler feed water preheating,
or even steam generation at higher temperatures.
Although the composite curves can be used to set energy targets, they are not a suitable
tool for the selection of utilities. The grand composite curve drawn above is a more
appropriate tool for understanding the interface between the process and the utility system.
The GCC is obtained via drawing the problem table cascade as we shown earlier.
The graph shown above is a typical GCC. It shows the heat flow through the process
against temperature. It should be noted that the temperature plotted here is the shifted
temperature T* and not the actual temperature. Hot streams are represented by ∆Tmin/2
colder and the cold streams ∆Tmin/2 hotter than they are in the streams problem definition.
This method means that an allowance of ∆Tmin is already built into the graph between the
hot and the cold for both process and utility streams. In other words, the actual hot utility
