AFT Blog

Very large and complex models may be constructed by building smaller models and merging them together. It is a good idea to break the large system into logical parts where there are known boundary conditions (flowrates or pressures).

Models may be merged together by selecting Merge... from the File menu. Before starting this process it is always advisable to save your current model. You will be asked for the file name to merge into the current Workspace. The model being merged in will be placed on the Workspace and each object will be selected. Move the merged model by positioning the mouse pointer on a selected junction and holding down the left mouse button and dragging it to the desired location on the Workspace. If you accidentally click somewhere off the selected items they will be come unselected and...

Flow direction arrows are for reference only, the actual direction being determined by the analysis and, if the direction is opposite the reference direction you've specified, values for flow, velocity and pressure drop will be negative. If you don't want negative values, simply select the pipe, press F3 to toggle the direction arrow and rerun your model. While this is fine for one or a few pipes, what if you have many pipes with negative flow values? Within the Workspace, open Select Special (on the Edit menu or by pressing F6). For 'Selection Type', select 'Pipe Output Properties', then 'Volumetric Flow...

Click on the junction to be moved once to select it. Then hold the Ctrl key down while you drag the junction icon and the pipes will remain in their current location. Note this results in the pipes being disconnected from the junction.

While we usually think of building a system model in the Workspace (and, of course, we can have multiple models within a file in different scenarios), you can have multiple, disconnected system models within the work space simultaneously. This has many uses, including facilitating parametric modeling. For example, you're modeling a system with a defined supply flow and want to evaluate the results for various supply flow rates. After building the model, select the whole model (Edit/Select All or ctl+a), then duplicate it (Edit/Duplicate or ctl+d) as many times as desired. For each instance specify a different supply flow and, after...

Using “Specified Heat Rate In Constants” for the thermal model of heat exchangers can often cause problems in system models.  The reason why is because this thermal model type causes the heat exchanger to act like an “assigned heat input” junction as does an assigned flow junction does for providing constant flow rates.  Another problem is that this thermal model can cause unrealistic temperature changes across a heat exchanger.  When the heat rate is specified and the mass flow rate and heat capacity are calculated based on the system solution, the temperature change will be whatever is required to maintain the...

Many components, such as isolation valves, elbows, and adjacent area changes, do not need to be modeled graphically when a simple k factor will suffice. In these cases, the AFT Fathom user can simplify and avoid unnecessary workspace clutter by integrating this data into a single pipe specification, using the Fittings & Losses tab. Fathom has a built library of component loss data provided by Miller, Crane, and Idelchik. The Fathom user can quickly and easily pick from a database of components, enter the quantity, and click OK. It is important to note, however, that Fathom uses the size of the...