In this tutorial we examine the use of Flowsheet Designers to achieve a required result.
Flowsheet Designers are similar to Unit Designers which were examined in Tutorial 4. The difference is in the manner in which the calculation is performed.
The calculation of the Unit Designers is done at the same time as calculating the values for the Units. This makes Unit Designers fast and efficient.
However, they can only change variables that only affect that one unit (in order to achieve the required result). For example, they can manipulate a feed stream if it is fed directly into the unit in question.
They cannot manipulate a feed stream that is first fed to a mixing tank and then into the Unit in question.
Flowsheet Designers on the other hand, are calculated only after a few iterations of the entire flowsheet. They cause the entire flowsheet to be re-calculated for each iteration of the Designer.
This results in much slower performance. However, in certain cases they are unavoidable.
In this tutorial we add a Flowsheet Designer to the example build in the previous tutorial. If you haven't yet done that tutorial then please do it now because the concepts discussed there are necessary here.
The first step is to add a flowsheet designer. Do this so that the flowsheet looks like that shown in Figure 1.
Figure 1. Adding a Flowsheet Designer.
This is done in a manner similar to that for the Unit Designer.
Follow these steps:
- Activate the DataAndDesignPoints for the streams that you are going to link the Flowsheet Designer to if they are not already activated. This can be done as follows: select the stream, and in the Properties Viewer, press the ellipsis button (...) in the DataAndDesignPoints property. This will result in a blue diamond and an orange circle appearing on the stream.
- Add the Flowsheet Designer by dragging it from the Process Library and onto the p
- Connect the measurement point and design point to the Flowsheet Designer using the DesignConnectorTool. Connect from the Measurement Point on the Process Stream to the Flowsheet Designer and from the Flowsheet Designer to the Design Point on the design stream.
Set the properties for the Flowsheet Designer.
The required properties are shown in Figure 2.
These properties are changed in the same way as those for the Unit Designer.
Figure 2. The properties for the Flowsheet Designer.
In this case the Measurement Variable is the Temperature and we require it to be 140°C.
Follow these steps for changing and checking the Designer properties:
- Check that the measurement connector is properly connected. Look to see if the MeasurePoint and MeasurementPoinID have values in their property boxes.
- Check that the design connector is properly connected. Look to see if the DesignPoint and DesignPointID have values in their property boxes.
- Set the Measurement Variable. To set the measurement properties, expand the MeasurementVariable option by clicking on the + sign in the left margin of the Properties Viewer. Under the "Variable" heading, select "Temperature" from the drop down menu. Under the "Value" heading, type in the number 140. (There is no need to select a component!)
- Set the Design Variable. Expand the DesignVariable option by clicking on the + sign in the left margin of the Properties Viewer. Under Variable, select "Flow".
Run the project, as shown in Figure 5.
Figure 3. The Process Toolbar, with the cursor hovering over the Run button.
When the project runs you will immediately notice the difference between the use of Unit Designers and Flowsheet Designers.
The project is re-run for different values of steam flow and is now slightly slower.
The results are shown in the Convergence Form, as shown in Figure 4.
Figure 4. Convergence form.
The first aspect to examine on the convergence form is the box labelled "Convergence of the Overall Mass Balance".
Here we see that the error is zero for all elements, so the flowsheet is in balance.
The second aspect to examine is the box labelled "Convergence of the Flowsheet Designers and Unit Designers". The errors here are small. This means that all the Designers have been successfully calculated.
The Designed column in this box indicates the value of the Design Variable that was required achieved the required measurement. For example, 0.176 t/hr of steam was required for the temperature to be 140°C.
We can accept these values since both mass balance and the designers converged.
At this point it should be noted that the three main differences between a unit designer and a flowsheet designer are as follows:
- A unit designer requires a ProcessUnitID to be specified in order to work, whereas a flowsheet designer does not.
- A flowsheet designer can be applied across any number of equipment on a page, whereas a unit designer can only be applied across a single piece of equipment on a page.
- Solving a unit designer is generally much faster than solving a flowsheet designer