VacTran 3 Manual (779748), страница 25
Текст из файла (страница 25)
When building a system model, one often adds out gas data to see the effect on pumpdown time.The option of using stored library data for gas load entry, rather than entering it manually, can save time andeliminate errors, especially in unit conversion.© 2011 Professional Engineering Computations23612.2VacTran 3Units of measure for out gassingThe process of gas evolution from the surface of a material in a vacuum is commonly described in units ofFor a specified surface area, the units becomeDespite the common use of units as shown above, data entry can be in any combination of pressure-volume/timearea units available in VacTran. The data is always stored in a consistent set of units internally, and displayed inany units selected by the user.Out gas libraries store data as a rate per unit area.
As explained in System Models, the dimensional information isinput when a specific material is added to the gas load list. Each of these gas load files can contain as manymaterial entries as desired. For convenience, separate files should be created for each general type of material,just to make them easier to find. For example, an out gas library called "Low carbon steel" could be created whichhas out gas data for baked and unbaked steel, or steel that is rusty or polished.12.3Caveats - out gas librariesVacTran will calculate the out gas contribution to total gas load, by facilitating the modeling of out gasmaterials. Much published data exists for outgassing rates of various materials.
However, the usermust ultimately determine the validity of the data and results. The following represents a partial list offactors that may contribute to differences between real and calculated outgassing rates.Geometry of vacuum vesselGeometry of the out gassing materialRelative humidity before vessel is sealedAmount of time vessel is up to airSurface finish of materialsCleanliness of materialsProcesses which deposit materials on surfacesTemperatureMaterial vapor pressureCorrosion or other chemical degradationNote that outgassing constants for materials are usually published for specific conditions or finishes.When using out gas models in a system model, be sure you are using the out gas constantsappropriate for the conditions you are interested in.Data provided with VacTran is from published references and subject to interpretation.Use this data carefully.© 2011 Professional Engineering ComputationsOut gas libraries12.4237Definition of out gas dataThe mathematical representation of outgassing used in VacTran is widely accepted and published in references 1and 7.Two periods are considered when defining the outgassing constant, or specific outgassing rate of amaterial.
An initial period of about one hour is defined by one set of constants, and a secondary period of up to tenhours or greater is defined by secondary constants. A change in log-log slope, out gas value, or both usuallycharacterize the transition to the secondary period.The initial period has been described as a time during which mostly surface outgassing of contaminants such aswater occurs.
The second period is said to consist of mainly bulk outgassing from the interior of the material. Theexact transition time between these two periods varies depending on the specific conditions present when the datawas measured. VacTran uses one and ten hours as the transition boundary between the two outgassingprocesses, because a large body of data has been published for this range.The transition period between one and ten hours is assumed a combination of the primary and secondaryconstants. Some sources of data give these constants for times slightly different from one and ten hours.
Sincethe process of calculating out gassing should be considered one of estimation, substituting an outgassing constantrepresenting 4 hours to infinity for 10 hours to infinity may not add considerable error. This judgment is ultimatelyleft to the user.© 2011 Professional Engineering Computations23812.5VacTran 3General formula for out gassingOutgassing rate (Q)qnt nwhere n = number of hours (1 or 10)t = timen= slope constant at time nq = outgassing constant at time nMultiply the outgassing rate by the surface area to obtain out gas throughput.Therefore, the following constants are associated with a given VacTran outgassing material:q1 and a1 for time up to 1 hourq10 and a10 for 10 hours or greaterFor time values less than one hour, VacTran uses the following formula:Note that this degenerates to Q1 at time = 3600 seconds, or one hour.For time values greater than one hour, but less than 10 hours, VacTran uses the following formula:This degenerates to Q1 at time = 3600 seconds, or one hourThis degenerates to Q10 at time = 36000 seconds, 10 hoursFor time greater than 10 hours, VacTran usesAt time = 36000 seconds, this degenerates to Q10.© 2011 Professional Engineering ComputationsOut gas libraries12.6Example CalculationMaterial: Silicone1 = 1.0710 = 1.1Q1 = 7.000E-06 torr - liters/secondQ10 = 1.700E-06 torr - liters/secondSurface area (A): 10 cm2At time (t) = 100 secondstorr-liters/second/cm2Using 10 cm2, Q = 3.238 x 10-3 torr-liters/secondAt time (t) = 10,000 secondstorr-liters/second/cm2Using 10 cm2, Q = 3.737 x 10-5 torr-liters/secondAt time (t) = 1,000,000 secondstorr-liters/second/cm2Using 10 cm2, Q = 4.39 x 10-7 torr-liters/second© 2011 Professional Engineering Computations23924012.7VacTran 3Gas load start and stop timeClassical out gas formulas can be inaccurate for very low time values.
As “t” in the termqnt an approaches zero, the calculated gas load approaches infinity. For short duration pump downcalculations in which low time values can be generated, unrealistically large gas load values can becalculated. This is simply an artifact of the equation, which is widely used in the industry. Tocompensate, use the Decay gas load to set the starting time for out gas calculations. This can be setfrom the Environment dialog.Click on the button sown below to activate this dialog.© 2011 Professional Engineering ComputationsOut gas libraries12.8241Creating out gas librariesTo create a new Out Gas Library: Under the File menu, select the New... command, or click on the icon as shown:In the New Document dialog that appears, select Out Gas Library and click on OK.(Click to expand)The following dialog appears, allowing for out gas library creation and editing:© 2011 Professional Engineering Computations24212.9VacTran 3Out gas library dialog DescriptionFive input fields are given at the top of the dialog, one for a material description and four for the out gas data.Initially, the data list is empty, and input fields are filled with default values.
At the bottom of the dialog are pulldown menus for out gas units (pressure, volume, area and time), which can be used at any time.The data list in the center of the dialog contains a scrolling window of out gas data. Clicking on one row of the listwill update the input fields at the top of the dialog. To change the value of this data, click in each input field and editthe number as you would with a word processor. The number you edit in the input field will not change the listvalues below until you either add it to the list using the Add command, or replace the selected list value with theReplace command.Underneath the data list, a pull down menu is provided to change the number format of the displayed data.
Theformat menu can be changed at any time, an all data in the list will be immediately updated.Buttons are provided for the basic editing functions.available under the Model menu.These Add, Replace, and Delete commands are also12.10 Out gas library dialog commandsAdd (Ctrl+A): Adds the data in the input fields to the list.
Since this is the default command, pressing the Returnkey has the same effect as clicking on Add.The Replace, and Delete 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.resorted according to the current sort option. This option will be dimmed if the model is empty.The list isDelete (Ctrl+D): Delete the currently highlighted list selection. This option will be dimmed if the model is empty.© 2011 Professional Engineering ComputationsOut gas libraries24312.11 Opening out gas librariesTo open an existing Out Gas Library: Under the File menu, select the Open...
command, or click on the icon asshown:In the Open Document dialog that appears, select Out Gas Library 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 the Outgas library file.© 2011 Professional Engineering Computations244VacTran 312.12 Out gas library formatThe out gas library file itself is stored as a text file, and can be edited with most word processors. So if you preferto use a word processor to enter all the data, save the file as "TEXT". VacTran will not read word processor files intheir native formats, but will read text files.
Change the extension of the file from "TXT" to "VTOUT". When editingyour own files outside VacTran, be sure to follow the format for out gas libraries or VacTran will report an errorreading the file. A corrupted out gas library file may cause unpredictable results.The following example demonstrates the file format for Out Gas Libraries.
Note that A1 and A10 are shorthand foralpha at 1 hour and alpha at 10 hours. Q1 and Q10 are shorthand for gas load rate a t 1 and 10 hours. Q is in unitsof torr-liters/second/cm2[General]Total=2[0]Description=Viton A (fresh)A1=0.8A10=0.8Q1=1.14E-6Q10=1.14E-6[1]Description=Viton A (bake 12h at 200 C)A1=1A10=1Q1=2.025E-10Q10=2.025E-10© 2011 Professional Engineering ComputationsPermeation libraries13245Permeation librariesPermeation is the flow of gas through a vacuum boundary from the high pressure to low pressure side. Unlike aleak, which is essentially a small conductance path, permeation is gas traveling through a solid material.Permeation can dominate total gas load after long periods.Most of the permeation contribution to a vacuum vessel constructed of common structural metals such as stainlesssteel or aluminum will be through elastomeric seals.
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