Optional Input for Junctions
A user defined initial guess can be entered for pressure (or head) and temperature (if heat transfer is modeled) for most junctions. This is only an initial guess for the Solver - the solution will vary from this value, and the value has no effect on the end solution.
A good initial guess can be helpful to decrease the number of Solver (the part of AFT Fathom that applies the governing incompressible flow equations to obtain a solution to the pipe system) iterations required, and potentially help with convergence issues.
The Special Condition can be changed for some junctions. For example, a valve can be set to closed, preventing all flow. A symbol (default of "X") will be placed before the junction label indicating that a special condition is applied.
A design factor can be used to add safety margins into the hydraulic analysis. For junctions, three types of design factors exist:
-
Pump Junctions - head rise at the pump is multiplied by the design factor assigned to the pump.
-
Heat Exchanger Junctions - total rate of heat transfer is multiplied by the Heat Transfer design factor assigned to the heat exchanger. The total loss factor is multiplied by the Friction Loss design factor assigned to the heat exchanger.
-
All other junctions - total loss factor is multiplied by the design factor assigned to the junction.
Note: The design factor for pumps has the opposite effect on a system that the design factor for other junction types has. A design factor of 1.1 for junctions with a pressure loss will give an extra 10% pressure drop at that junction, adding resistance to the system. A design factor of 1.1 for the pump junction will give an extra 10% pressure rise, effectively modeling the pump as having extra capacity than expected. To add a 10% safety margin to the pump junction, a design factor of 0.9 should be used.
The junction icon can be changed to better represent a specific junction type. This is accomplished by clicking the Change Icon button. Custom images can be added by clicking the Add Image button. The size of the junction can be changed via Workspace Layers.
Restricted Area
This is an optional parameter in several junction Properties windows which describes the effective flow area through the junction. This is done for the purpose of assessing cavitation. In most cases, information on the size of the restricted area must be obtained from handbook or test data.
