Alph is no longer for sale.

I am afraid revenue generated from Alph has not proven to be sufficient to justify its continued sale. I greatly appreciate the support of the folks who have purchased Alph in the past and hope that it will continue to serve you well in the future.

This and related pages will be maintained for archival purposes only.

Craig

Alph Model Tool Example

From your device, you can download the completed Model One example

This example modifies the completed gas plant example of Tutorial 7 to use models for the gas/gas exchanger and the combined chiller/refrigeration unit. A compressor model with a specified polytropic efficiency is also added to the sales gas.

Using models in problems has two main benefits:

- Providing clarity by encapsulating calculations
- Providing reusability by allowing complete calculations to be copied and pasted to new problems

For this example the IFD for tutorial 7:

is modified by introducing three models:

**gasgas**- represents the gas-gas heat exchanger**refrig**- represents the chiller and the propane refrigeration unit**polycomp**- adds a sales gas compressor with a specified polytropic efficiency.

The resulting IFD now looks like:

The IFD for the **gasgas** model looks like the following:

The **side1in** and **side2in** objects are Model Input Tools, which are used to get information from the parent flow diagram. In this case the source for **side1in** is **feed.v** and the source for **side2in** is **lts.v**. Variables or tools in the parent can also be referenced by prefixing the name with the appropriate type symbol (**$** or **#**).

The **side2out** fluid is calculated using the **side2in** composition and flow, a temperature calculated from the **side1in** temperature plus a delta T specified in the **dt** variable. The pressure is simply the **side2in** pressure minus the specified **side2dp** pressure drop.

The side1out fluid flow, composition and pressure are calculated in a similar manner, but instead of temperature, the enthalpy is calculated by an energy balance of the other three inlets and outlets.

These calculations simply mirror those that were done in the main IFD of Tutorial 7 and of course could be modified for different circumstances.

Only a model's variables can be directly referenced by the parent, so **side1out** and **side2out** variables are added with a property type of **Reference**. This allows the parent to access the outlet fluids by means of these aliases. The **duty** and **lmtd** variables are added as convenience values.

Check the notes for the model itself as well as its various objects for additional information.

While the Alph Compressor/Expander tool calculates polytropic efficiency, it does not allow you to specify it directly. However it is easy to create a model which adds that capability, as illustrated by the **polycomp** tool:

This pairs a standard compressor tool with a single equation function solver that varies the compressor's adiabatic efficiency until the desired polytropic efficiency is obtained.

This model makes use of the **if** function of formulas in a couple of interesting ways. First the **OutletP** and **PolyEff** variables use it to check to see if the corresponding model input tools are actually connected to an object in the parent. If they are, then their values are used, but if not, the values of the **POut** and **PolyE** variables are used instead. This allows for optional inputs depending on whether an absolute value is used or one that is calculated by the parent in some fashion.

Also the formula for the **comp** efficiency uses an **if** function to first check if the **feed** fluid is in fact known. The purpose of this is to ensure that any solvers in the parent are triggered before the **polysolver** as the latter should probably be nested inside the formers solution if that is the case. This will not be an issue for the sales gas compressor, but can be for the Model Example Two case where a copy of this model is used inside a refrigeration model.

A number of variables have been added to allow this model to be referenced in formulas the same way a normal compressor/expander model can be.

The calculations are the same as discussed in Tutorial 6, with the addition of the appropriate model input tools and variables to communicate with the parent. Note the **X** model input tool is provided to allow the parent to designate the composition of the refrigerant; pure propane in this case.

From your device, you can download the completed Model Two example

This is identical to the Model One example, except the compressor in the **refrig** model has been replaced with a copy of the **polycomp** model to illustrate that models can be nested.

From your device, you can download the completed Model Three example

This is also a modification of the Model One example to specify the polytropic efficiency of the refrigeration compressor. However in this case solution of the polytropic efficiency is done by adding an equation to the main solver. While not terribly important in this example, the idea is to illustrate that it may sometimes be more efficient to consolidate solver functions and you should give thought to what calculations you are including when you add existing models to your problem.

The models used in Model One are included, along with other models, in the Model Library Case, in a form ready for copying and pasting into new problems. If you have models or ideas for models you would like to see included, please contact me.