VacTran 3 Manual, страница 10

Oncesuch a configuration is defined, pump down time can be calculated and graphed. If the results are undesirable,parameters are interactively changed, and calculations are repeated within seconds.© 2011 Professional Engineering ComputationsPump models663Pump modelsA pump model is a file containing pump speed vs pressure data for a specific pump and a specific gas.This section explains how to create and edit pump models, which are fundamental building blocks for vacuumsystems in VacTran.A vacuum pump is the essential active vacuum system element tasked with removing or capturing gas from avacuum vessel.

Over the past 50 years, many types have been developed which provide a range of performancecharacteristics under different operating conditions. Most can be categorized as positive displacement pumps,momentum transfer pumps, or capture pumps. For example, a rotary vane pump uses positive displacement, aturbo pump uses momentum transfer, and a cryogenic pump uses capture mechanisms as the mean of removinggas from a vacuum vessel.Most pumps have a limited range of operating pressures, and are often staged with other specialized pumps tocomplete the pump down of a system to the desired pressure. For example, some rotary vane pumps provideuseful pumping from atmospheric pressure down to about 10-1 torr. Below this pressure, a different type of pump isrequired, such as a roots blower, which provides efficient, economical pumping down to about 10-4 torr.

However,roots blowers cannot be operated above differential pressures of about 10 torr, and require mechanical pumpsbacking them for initial pumping. If the target pressure for the vacuum vessel is less than 10-4 torr, yet another typeof pump is required for "high vacuum" pumping. Therefore, it may take three or more types of pumps, valved on andoff in separate stages for their effective pumping ranges, to achieve the desired pressure.VacTran can be used to size a backing pump and fore line for a pump which requires it. Diffusion pumps and rootsblowers typically require a minimum backing pump speed for efficient operation.

Conductance Studies explainshow to calculate the effective backing pump speed using delivered speed calculations.See also:Modeling a pump in VacTranCreating pump modelsPump dialog descriptionPump dialog commandsMenu commands for pump modelsRight-click optionsOpening existing pump modelsCreating pump models with the pump digitizerAdditional data in the tabbed pagesExample 1 Mechanical pumpExample 2 Roots pumpExample 3 Combining two or more pumpsImporting DOS pump filesImporting VacTran 2 pump filesExporting pumps to Microsoft ExcelTips on modeling pumps© 2011 Professional Engineering Computations646.1VacTran 3Modeling a pump in VacTranModeling a pump in VacTran involves entering pump speed vs inlet pressure data that represents the actualperformance as published by the manufacturer or as measured by the user.Operation and application of the many types of pumps available are covered in several excellent texts on the subjectgiven in the References section.

Despite the multitude of vacuum pump types available today, all have one thing incommon:A given pump or pump combination can be characterized by its performance expressed as volumetric pumpingspeed vs. inlet pressure, as shown for the diffusion pump below.Data for commercially available pumps is often provided in vendor catalogs in graph form. These graphs show theuseful range of operating pressures, and the pumping speeds for different gasses.

From a modeling standpoint, thiscurve is the single useful set of data needed for pumping calculations. However, from a system engineeringstandpoint, additional familiarity with pump technology (not covered by this manual) is needed to determine whethera given pump is being used appropriately.© 2011 Professional Engineering ComputationsPump models6.265Creating pump modelsTo create a new Pump Model: Under the File menu, select the New... command, or click on the icon as shown:In the New Document dialog that appears, select Pump Model and click on OK.Alternatively, use the pump model speed button as shown below:The following shows the pump model before any data is added.© 2011 Professional Engineering Computations666.3VacTran 3Pump dialog descriptionThe data list in the center of the dialog contains a scrolling window of pump data.

In a new pump model, thislist will be initially empty. If more data is in the list than can fit in the window, use the scroll bar to move upand down the list. Clicking on one row of the list will update the input fields at the top of the dialog. Tochange the value of these numbers, click in each input field and edit the number. The number you edit in theinput field will not change the list values below until you either add it to the list using the Add command, orreplace the selected list value with the Replace command.At the bottom of the dialog, two pull-down menus are provided to change the number format of the displayeddata.

The format menus can be changed at any time, and all data in the list will be immediately updated.Buttons on the right side of the dialog are provided for the basic editing functions, such as Add, Replace,and Delete. These commands are also available under the Model menu.© 2011 Professional Engineering ComputationsPump models6.467Pump dialog commandsAdd (Ctrl+A): Adds the data in the input fields to the list. If the global graph update option is selected, and at leasttwo data points are in the list, the graph window is updated.

Since this is the default command, pressing theReturn key has the same effect as clicking on Add. After adding, VacTran will sort the data according to the sortoption currently selected. It is not necessary to enter an even spacing of data. Since VacTran uses linearinterpolation to determine pumping speed between data points, two points may represent a straight-line section of acurve. Other areas might be represented by large number of points to obtain a smoother curve. Pump model sizeis limited only by available memory and disk space, but usually no more than 10 to 20 points are needed.The Replace, Delete, and Multiply commands are available only if there is data in the list.Replace (Ctrl+R): Replace the currently highlighted list selection with the data in the input fields.

The list isresorted according to the current sort option, available under the Defaults menu. This option will be dimmed if themodel is empty.Delete (Ctrl-D): Delete the currently highlighted list selection. This option will be dimmed if the model is empty.Clear All:Deletes all the data from the model.Multiply:Multiply the entire list of pump speeds by a specified value.

The dialog shown is used to enter the multiplier. Thisis useful if the speed data needs to reduced to simulate a pump that is not operating efficiently. A multiplier of twoor more can be used if several of these pumps are to be added together in parallel. The multiplier only affects thepumping speed data, while the pressure data remains unchanged. The multiplier value must be greater than zero.© 2011 Professional Engineering Computations68VacTran 3This option will be dimmed if model is empty.Add from file: It is useful to combine pump files under certain circumstances.

This command allows anotherexternal file to be added to the current one. Where data overlaps in the pumps' pressure ranges, it is addedtogether. However, if there is a gap between the data of the two pumps, this will be ignored. Be careful about this.If the current pump has a lower pressure bound of 10-2 torr, and the external added pump has an upper pressurebound of 10-4 torr, the two pumps will be sewed together into a continuous model which behaves as if there is nogap in pressure range. The pump will be graphed continuously between 10-2 torr and 10-4 torr. When calculationsare made based on this new pump model, values for pump speed between 10-2 and 10-4 torr will simply beinterpolated.The resulting pump model will contain the number of entries equal to the larger of the two files, and ranging from thehighest to the lowest pressure the two combined pumps.

Resulting data is interpolated from the data from eachpump. This combination will be roughly equivalent to the two pumps running in parallel.6.5Menu commands for pump modelsIf there is more one entry in the pump model data list, the following options are available in the pull down menus:command:Add, Replace, and Deletemenu: ModelFunctions perform exactly as described in the dialog commands section.command:Pump speed vs. pressuremenu: GraphPump Speed vs. Pressure for the pump model is plotted on the currently selected axes in the Primary GraphWindow.

Pump speed is in current volume/time units. Pumps are usually plotted either in Y linear -X log or log-log.command:Pump throughput vs. pressuremenu: GraphPump Throughput vs. Pressure for the pump model is plotted on the currently selected axes. Throughput is incurrent pressure-volume/time units. Q vs. P is usually plotted log-log. Note that this does not change theappearance of the data in the list, which is always in pump speed vs. pressure.© 2011 Professional Engineering ComputationsPump models6.669Right-click optionsClicking the right mouse button over a pump model will show this menu:Sort pressure up and Sort pressure down determine the order that pump data is displayed.

The data is notaffected by this.Graph controls… activates the dialog for graphing optionsUpdate open system models searches for open system models and conductance studies that are using thispump, and forces an update. This function happens automatically when you save the pump model after changes.Save as version 2 pump model reverts the pump model to be compatible with an earlier version of VacTran. Notethat none of the additional data in the model, such as pump type and pump images will be saved in this olderformat.© 2011 Professional Engineering Computations706.7VacTran 3Opening existing pump modelsTo open an existing pump model: Under the File menu, select the Open...

command, or click on the icon asshown:In the Open Document dialog that appears, select Pump model and click on OK.(click to expand)You will then be presented with a file selection dialog from which you can choose the directory and name of thepump file.© 2011 Professional Engineering ComputationsPump models6.871Creating pump models with the pump digitizerThe pump digitizer is a powerful tool used to trace static images of pump curves into VacTran models. After goingthrough the initial steps of scaling the original artwork, you can zoom in and click along the curve. Each click willbe translated into a digitized point that becomes part of the pump performance curve.

When done, click on “Exportdata to pump model” to open a new pump model with the newly digitized data.To access the digitizer, use the speed button shown below or select Pump Digitizer from the Tools menu.Step 1: Start with a pump speed curveThe first step in digitizing a pump curve is finding a graphic of the curve. This can be scanned from a catalog orcopied directly from a vendor web site, but only if copyright notices allow this. Contact the vendor to be sure.Once a digital version of the pump curve graph is obtained, insert the image from a file orpaste it in from the Windows clipboard.