VacTran 3 Manual (779748), страница 17
Текст из файла (страница 17)
To changethese values, go to the system Environment Dialog.© 2011 Professional Engineering Computations1468.13VacTran 3Adding conductances to a modelConductance elements are added using the Conductance palette shown below.If a conductance list is active, conductance elements such as pipes can be added. To activate the list, click on it.A conductance palette is used to add conductance elements to other models, such as System models,Conductance models, Pump station models, and Conductance studies.To add a conductance element using the Conductance palette, press thebutton, use the Ctrl+A short cut,or select the Add command from the Model menu.
You may select the Use pipe library option to select thediameter of the conductance from a pipe library file on disk. The following the selection of a conductance elementfrom the Conductance palette, a geometry-specific dialog (such as an orifice) will let you enter the geometryinformation.Click on any of the buttons for m ore detailThe Conductance Palette will only be active if there is something on the screen that has a conductance list focusedand waiting for input. For example, if you want to add a conductance to the conductance list in a System Model,bring the System Model window to the front by clicking on it, and then click on the conductance list to make it thefocus.
The Conductance palette will always activate when a conductance list has the focus of the program.If the Conductance Palette is not visible, you can activate it by selecting it under the Window menu. If it is visiblebut not active, it will look like this:You may select the Use pipe library option to select the diameter of the conductance from a pipe library file ondisk.Use Pipe Library optionIf the check box is checked, the library option will ask you to select a pipe library file. Pipe libraries enable you tostore and retrieve frequently used pipe diameters for future conductance element entries. A number of standardpipe sizes is provided with VacTran in the /Pipes directory.© 2011 Professional Engineering ComputationsSystem modelsSee also:Annular pipe entryCombination conductancesCone entryConstant entryElbow entryElliptical pipe entryMiter entryOrifice entryPipe bend entryPipe entryPump station modelsRaw data conductance modelsRectangular pipe entrySlit entryTriangular pipe entry© 2011 Professional Engineering Computations147148VacTran 38.13.1 Annulus entry dialogDefinitionAn annulus (annular pipe) is a conductance element having a circular cross section, non-zero length, and consisting of twoconcentric cylinders with the gas flow in the annular space.Entrance and exit lossesDepending on upstream and downstream geometry relative to the pipe, it can have an entrance loss, and exit loss, or both.The user must determine whether to select the entrance loss or exit loss options.
Since limited data has been found forentrance and exit losses associated with annular pipe, circular pipe formulas are used for end effects. This will introducesome level of error that has not been quantified. Use the entrance and exit functions for estimating purposes.Where is it used?Annulus conductance elements are used in System Models and Conductance Studies.This dialog is a multi-tabbed interface that has several functions.The Data Entry tab contains the basic information needed to create an annular pipe.
You need not go any further than this© 2011 Professional Engineering ComputationsSystem models149tab if you know the dimensions, and end effects. Quantities greater than one will be calculated as conductances in series.Note, however, that the entrance and exit loss value will also be multiplied. In most cases, a separate annular pipe entrywill be appropriate for each length of pipe in the model, each with its own assessment of applicable end effects.The Use Recent Entry tab allows selection of a previous conductance element that was entered, with the most recent entryshown at the top of the list. This function is intended to be a time saver for frequently used geometries.© 2011 Professional Engineering Computations150VacTran 3The Entrance Detail tab contains more detail on the type of entrance present, and applies to the pipe calculations only ifthe entrance loss option is selected.
The three images below show the three different types of entrances applicable topipe. Note that K factor and r/d ratios will only apply to viscous flow calculations, and not molecular flow calculations.A projecting pipe is assumed to be square-edged, with no entrance radius.A radius-edge entrance tends to minimize turbulance. For an edge radius r and pipe diameter d, the r/d ratio is used todetermine the loss factor. At r/d >0.15, there is no significant improvement in flow loss.A sharp edge entrance has a radius of zero, and maximizes the turbulent entrance loss effect in viscous flow.
For manydesigns where a pipe is welded into a vessel at a machined port location, sharp edged entrances are common.© 2011 Professional Engineering ComputationsSystem models151The Summary tab contains calculation information for this conductance element. The text in this section can behighlighted, copied, and pasted into another application. The information is intended to provide significantly more detailthan can otherwise be gained from looking at conductance curves, and serves to allow the user additional scrutiny into thebasis for flow calculations.See also:Annulus calculationsCombination conductancesCone entryConstant entryElbow entryElliptical pipe entryMiter entryOrifice entryPipe bend entryPipe entryPump station modelsRaw data conductance modelsRectangular pipe entrySlit entryTriangular pipe entry© 2011 Professional Engineering Computations152VacTran 38.13.2 Combination conductancesCombination models can be series conductance models or parallel conductance models.
Each of these is createdas a separate file which contains multiple conductance elements. A series model can contain a parallel model andvisa versa. This can lead to significantly complex combinations. For example, the parallel model may have 5parallel pipes. The series model may have a pipe in series with an elbow. Each of these models can be added tothe conductance list to create a pipe and an elbow in series with the 5 parallel pipes.We can create a series or parallel model by clicking on the following speed buttons:The series conductance example is shown below.
Conductance elements are added in the same manner as on thesystem mode. Click on the conductance list, then select the conductance element from the conductance palette.After saving the series model, it can be inserted into a system model as described above. The parallelconductance model works exactly the same way.© 2011 Professional Engineering ComputationsSystem models1538.13.3 Cone entry dialogDefinitionA cone is a conductance element having a circular cross section, finite length, and different inlet and outletdiameters.Entrance and exit lossesWhen a cone is relatively short in length, it can be a randomizing element that can disrupt fully developed flow indownstream piping.
If the entrance diameter is much larger than the exit diameter, the cone will approach thebehavior of a pipe entrance. If the exit diameter is much larger than the entrance diameter, the cone will approachthe behavior of a pipe exit. Click here for more detail.Where is it used?Cone conductance elements are used in System Models and Conductance Studies. They are often used astransition components to join two pipes of different diameters. In fact the equations used are valid only when thecone is connected to pipes that have matching entrance and exit diameters.© 2011 Professional Engineering Computations154VacTran 3See also:Conical pipe calculationsConical pipe examples[****]Annular pipe entryCombination conductancesConstant entryElbow entryElliptical pipe entryMiter entryOrifice entryPipe bend entryPipe entryPump station modelsRaw data conductance modelsRectangular pipe entrySlit entryTriangular pipe entry© 2011 Professional Engineering ComputationsSystem models1558.13.4 Constant entry dialogThe constant conductance is used for entries about which a known conductance value has been measured.
Themost common application for this function is the use of traps in the molecular flow regime or orifices in the chokedflow regime.On all other conductance elements, volume is calculated based on selected geometry. In this case, no geometricinformation is known about the conductance element. Use the Volume field if you want the volume of thisconductance to be factored into system calculations. Remember to click on the Add cond volume check box in thesystem model dialog to make this happen.A constant conductance is simply a flat line conductance vs.
pressure model, which may be valid in molecular orchoked flow conditions.Use this type of element only if accurate data is available and you are not concerned with viscous flow, for whichconductance changes with pressure. Otherwise, use other elements such as pipes and elbows, and let VacTrancalculate the conductance at each pressure increment.© 2011 Professional Engineering Computations156VacTran 3See also:Annular pipe entryCombination conductancesCone entryElbow entryElliptical pipe entryMiter entryOrifice entryPipe bend entryPipe entryPump station modelsRaw data conductance modelsRectangular pipe entrySlit entryTriangular pipe entry© 2011 Professional Engineering ComputationsSystem models1578.13.5 Elbow entry dialogDefinitionAn elbow is a conductance element having a circular cross section a single abrupt bend, and a short length of pipeat the entrance and exit.Entrance and exit lossesAn elbow is considered a randomizing element that disrupts fully developed flow coming from upstream piping.Therefore, no option for an entrance loss is provided because the randomizing affect of the entrance is alreadyimplicitly included in the elbow formula.
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