Energy Targeting Using Graphical Method

Energy Targeting Using Graphical Method

As previously explained, any heat exchanger can be represented as a hot stream that is cooled by a cold stream and/or cold utility and a cold stream that is heated by a hot stream and/or hot utility with a specified minimum temperature approach between the hot and the cold called ΔT_min.

The process exhibited below in the graph shows the situation when the two streams do not have a chance of overlap that produce heat integration between the hot and the cold.

Energy Targeting Using Graphical Method

Two Non-Overlapping Streams

Heat Integration between Hot and Cold

Moving the cold stream to the left on the enthalpy axis without changing its supply and
target temperatures until we have a small vertical distance between the hot stream and the cold stream we obtain some overlap between the two streams that result in heat integration between the hot and the cold and less hot and cold utilities. As seen depicted in the graph below with shrinkage in the red and blue lines span.

Now we want to represent all the hot streams in the process by one long hot stream and we will call this line the hot composite curve. We will also do the same thing with all the cold streams in the process.

The next step will be drawing the two composite curves/lines on the same page in a
Temperature (T)-Enthalpy diagram with two conditions:
1. The cold composite curve should be completely below the hot composite curve,
and
2. The vertical distance between the two lines/curves in terms of temperature should
be greater than or equal to a selected minimum approach temperature called global
ΔT_min
The resulting graph is depicted below and known as thermal pinch diagram.

Composite Curves

Constructing the Composite Curves

1. Draw the hot composite curve and the cold composite curve via developing the
following tables.
2. These tables list all the hot and cold streams temperatures in an ascending order
with the cumulative enthalpy corresponding to the lowest hot temperature and
lowest cold temperature respectively equal to zero.

3. In every temperature interval the cumulative hot load is calculated using the
following formula:
H= FCp * (Tsupply – Ttarget)

4. In every temperature interval the cumulative cold load is calculated using the following formula:

As we mentioned before the cold composite curve shall lie completely below the hot composite curve and this can be done via dragging the cold composite curve to the right on the enthalpy axis (H). This process shall stop at a vertical distance between the cold and the hot composite curve for a temperature equal to the minimum temperature approach selected earlier.

Temperature (T) Enthalpy (H) Diagram 1

Temperature (T) Enthalpy (H) Diagram 2

 


 

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Document Category Engineering
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