VacTran 3 Manual (779748), страница 21
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See also: Permeation calculationsEnter permeation data using a permeation library© 2011 Professional Engineering ComputationsSystem models201After selecting the permeation library, the above dialog appears. Click once on the material in the scrolling list, andenter the section area and thickness in the entry fields. Click OK when done, and the new gas load will be addedto your model.See also:PermeationPermeation dialog descriptionPermeation calculationsExponential out gas entryLeak entryO-ring entryOut gas entryRaw data gas loads© 2011 Professional Engineering Computations202VacTran 38.14.6 Raw data gas loadsRaw data files are gas loads, which are characterized by an array of values representing gas load vs. time.
Thesefiles are created separately and described in Raw data gas load models. The raw data model represents a look uptable of gas load values that are interpolated during pump down calculations. It is designed to give maximumflexibility to the user when the modeling functions built into VacTran are not sufficient, or when measured data isavailable.See also:Exponential out gas entryLeak entryO-ring entryOut gas entryPermeation entry© 2011 Professional Engineering ComputationsConductance models9203Conductance modelsA conductance model is consists of a conductance elements displayed as a list (conductance list) that representsthe physical piping path between the pump and the vacuum vessel. VacTran has series conductance models andparallel conductance models.Conductance models can be saved as separate files.
They are handy for inserting into System Models, Pumpstation models, or another Series conductance model. They are added into the conductance list in series withother conductance elements.After saving a Conductance model (series or parallel), inserting them into another model's conductance list isaccomplished by using the conductance palette as shown below.See also:Adding conductances to a modelConductance element definitionConductance list definitionConductance StudiesCreating conductance modelsEditing conductance models© 2011 Professional Engineering Computations2049.1VacTran 3Creating conductance modelsTo create a new Conductance Model: Under the File menu, select the New... command, or click on the icon asshown:In the New Document dialog that appears, select Series conductance orParallel conductance and click on OK.Alternatively, use one of the conductance model speed buttons as shown below:To create a newseries conductancemodel...To create a newparallel conductancemodel...The following shows the series model before any data is added.See also:About conductance modelsAdding conductances to a modelConductance element definitionConductance list definitionConductance StudiesEditing conductance models© 2011 Professional Engineering ComputationsConductance models10205Conductance StudiesConductance Studies provide a fast, convenient means to explore the effects of varying pipe diameter onconductance and delivered speed.An essential factor in the design of vacuum systems is an efficient conductance path between the pump station andthe vacuum vessel.
A critical parameter is the choice of pipe diameter. A larger diameter will decrease theconductance loss, but increase the pumped volume, outgassing contamination, space required in the facility, andcost of the piping system. Therefore, an optimization study should be performed which first examines theperformance of different conductance sizes.Conductance studies are interactive dialogs that evaluate several conductance geometries on one graph for either agroup of series or parallel conductance elements. For example, one can vary the diameter of a several pipes andelbows in series, and compare all cases on one graph.Conductance equations are given Calculation Formulas.The graph shows a delivered speed curve for a pump connected to a pipe of fixed length.
The diameter is varied foreach curve of delivered speed vs. pressure.© 2011 Professional Engineering Computations206VacTran 3Two types of studies are provided in the main menu:The Series conductance study calculates conductance vs. pressure curves for series connected conductanceelements. The Parallel conductance study calculates conductance vs. pressure curves for a group of parallelconductance elements. In either type study, pumps can be added to simulate a pump station as in a SystemModel.
Delivered speed vs. pressure and Delivered throughput vs. pressure for the pump/conductance combinationcan be graphed for numerous conductance diameters. You can then select the optimum conductance path.See also:Activating a conductance studyCalculations in conductance studiesSummary of conductance study functionsHow to calculate backing pump speedExample backing pump problem© 2011 Professional Engineering ComputationsConductance Studies10.1207Activating a conductance studyTo activate a Series Conductance Study, click on theconductances from the Tools menu as shown.speed button below the main menu, or select SeriesTo activate a Parallel Conductance Study, click onspeed button below the main menu. All operations for Series and Parallel Conductance Studies are identicalexcept for the calculation of conductance.Speed buttons:© 2011 Professional Engineering Computations208VacTran 3The following dialog will be activated:See also:Calculations in conductance studiesSummary of conductance study functionsHow to calculate backing pump speedExample backing pump problem© 2011 Professional Engineering ComputationsConductance Studies10.2209Calculations in conductance studiesIn a Series Conductance Study, all conductance elements on the list are considered to connected in series, andcalculations of combined conductance will follow the following formula:Given a series of conductance elements, Ca, Cb, … Cn,1/Cs = (1/Ca) + (1/Cb) + … + (1/Cn)whereCs = combined conductanceCa = conductance of element aCb = conductance of element bCn = conductance of element nNote:The result of this equation is that the combined conductance of elements in series can never be greater than thesmallest conductance in the series.
The smallest conductance is the limiting conductance in the series.In a Parallel Conductance Study, all conductance elements on the list are considered to connected in parallel, andcalculations of combined conductance will follow the following formula:Given a set of parallel conductance paths, Ca, Cb, ... Cn,Cp = Ca + Cb + ... +whereCsCaCbCnCn====combined conductanceconductance of element aconductance of element bconductance of element nNote:The result of this equation is that the combined conductance of elements in parallel can never be smaller than thelargest conductance element in the group.See also:Activating a conductance studySummary of conductance study functionsHow to calculate backing pump speedExample backing pump problem© 2011 Professional Engineering Computations21010.3VacTran 3Summary of conductance study functionsWhat to do first:Click on the conductance list (initially empty) once.Add a conductance to the list in any of the following ways:Type Ctrl+A to activate the Add Conductance dialogClick on the Conductance Palette, and click on one of the conductance buttons:Click on thespeed button to activate the Add Conductance dialog.Click on a conductance icon in the conductance palette to add it to the list.
Note that only conductance elementsthat have a diameter dimension are activated, because the conductance study only works by varying the diameter ofthe cross section.© 2011 Professional Engineering ComputationsConductance Studies211What to do next:Click on the pump list (initially empty) once.Add a pump to the list in any of the following ways:Type Ctrl+A to activate the Open dialogClick on thespeed button to activate the Open dialog. Pick a pump from a folder on your hard disk.All the buttons are now active. Click on the pump buttons to graph each pump on the list separately to comparethem, or to show the total pumping speed of the combined list of pumps.Change the From and to settings to vary the diameter of the conductances between these two values.
Try theDelivered Speed button to see a series of curves for each diameter.Change the # curves setting to change the number of diameter variations to graph between the From and tosettings.Use the Generate New Pump button to create a new pump file. This pump will have a pressure vs. speed curve thatis the same as the delivered speed curve for the conductance study using the “From” diameter for each of theconductances on the list.Use the Generate Raw Conductance button to create a new raw data conductance file.
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