VacTran 3 Manual (779748), страница 26
Текст из файла (страница 26)
Penetrations such as ports, doors, bellows, and windows areall sources of permeation. Some vacuum vessels are designed with metal seals, which have much lowerpermeation rates.Permeation is proportional to the pressure differential across the material, the permeability of thematerial, and the total area of the material normal to the pressure differential. The rate is inverselyproportional to the thickness of the material in the direction of the flow path. For many systems,permeation calculations can be simplified by assuming that the pressure differential is always 760 torr,or atmospheric pressure.Permeation consists of gas adsorption on the high pressure side of the material, diffusion through the material, andthen desorption on the low pressure side. After desorption, the gas contributes to the vessel pressure.
Therefore, ithas been surmised that there will be some initial period with no observed permeation, during which the firstmolecules are traveling through the material. This time lag is ignored in VacTran, and all permeation is consideredto start at the gas load start time. This is justified in most vacuum systems because during the initial pumping of asystem from atmospheric pressure, the starting gas volume in the vacuum vessel is orders of magnitude higher thanthe additional gas from permeation. Later in pumping, when vessel pressure is orders of magnitude less (such as10-6 torr), permeation becomes the ultimate limitation of attainable pressure for a given pumping system.In systems that operate at relatively high pressures, permeation can often be completely ignored, because it usuallyadds such a small gas load.
Before making this assumption however, it is prudent to do some simple handcalculations.See also:Why use a permeation library?Units of measure for permeationCaveatsUsing permeation data for leak detectionGeneral permeation formulaExample CalculationCreating permeation librariesPermeation dialog descriptionDialog commandsOpening permeation librariesPermeation library format13.1Why use a permeation library?Permeation libraries are convenient for storing data on a variety of candidate materials that would have potential usein a vacuum system.
When building a system model, one often adds permeation data to see the effect on pumpdown time. Since VacTran permeation rates are constant over time, one may find that they limit the total pumpdown of a system model to an ultimate pressure, which is greater than the target pressure.In other words, large permeation gas loads often prevent a system from achieving the desired pressure, because thedelivered speed of the pumping system is less than the gas added due to permeation.© 2011 Professional Engineering Computations24613.2VacTran 3Units of measure for permeationThe process of permeation of a gas through a material is commonly described in units ofFor a specified cross section area and section depth, the units becomePermeation data is stored in files called permeation libraries.
The data is stored as a rate with no associatedgeometry. When added to a system model, the dimensional information is added. Each library file can contain asmany material entries as desired. For convenience, separate files should be created for each general type ofmaterial, just to make them easier to find. For example, a permeation library called "Viton" could be created withentries for baked and unbaked viton, and for various gases.13.3CaveatsVacTran will calculate the permeation contribution to total gas load, by facilitating the definition of permeationmaterials.
Much published data exists for permeability of various materials. However, researchers using differenttest apparatus, material conditions, and reporting methods produce published data. As with out gassing, the usermust ultimately determine the validity of the data and results.• Note that permeation constants for materials are usually published for specific gases.
When using permeationmodels in a calculated gas load model, be sure you are using the permeation constants appropriate for the gas ofinterest.• According to Holland (1974) gas permeation can vary with temperature, purity of the gas, and the surface conditionof the material.• A given elastomer composition may be different than that for the same brand name formulated in previous years.• Elastomers can change with exposure to solvents and other materials.• Under conditions of ultra high vacuum, the vapor pressure of the permeation material can add to the system gasload. In other words, a vacuum seal can be an out gas sources as well as a permeation source.• It is a common technique to install some elastomer seals with a coating of vacuum grease.
This coating can alterthe rate of adsorption and desorption of the elastomer, as well as create an additional permeation path in series withthe elastomer.• Although common seals are circular in cross-section, other geometries are commercially available. In addition,the crushed geometry will vary with the fit and tolerances of the mating parts. These factors may alter the effectivecross section dimensions.13.4Using permeation data for leak detectionThe ability to leak test a vacuum vessel can be limited by the design of the vessel, particularly the materials used toseal openings. If the permeation rate of helium through a particular o-ring is relatively high, it may be incorrectlyinterpreted as a leak.
By calculating the permeation rate, one can determine the maximum sensitivity of the leak© 2011 Professional Engineering ComputationsPermeation libraries247detection process, assuming the data for the particular seal material is correct.As previously mentioned, there is a time lag during which tracer gas molecules diffuse through the o-ring and adsorbfrom the low pressure surface. If the amount of time used for leak detection is short compared to this time lag, theeffect of permeation on leak detection will be minimized. The amount of time depends on material properties andgeometry, and is not covered in this text.13.5General permeation formulawhereQ is gas loadA is cross section area across direction of flowP is pressure differentiald is the length along direction of flow through the materialFor an o-ring of cross-section radius r and inside radius R,A = circumference x diameter= 2PR2randd = 2r13.6Example CalculationMaterial: Buna-N (rate for helium)permeability constant Kp: 5.8e-8 cm2/secondCross section area (A): 10 cm2thickness (d): 1 cmpressure differential P: 760 torror in more common units,© 2011 Professional Engineering Computations24813.7VacTran 3Creating permeation librariesTo create a new Permeation Library: Under the File menu, select the New...
command, or click on the icon asshown:In the New Document dialog that appears, select Permeation Library and click on OK.(click to expand)The following dialog appears for permeation model data entry and editing.Data listInput fields© 2011 Professional Engineering ComputationsPermeation libraries13.8249Permeation dialog descriptionTwo input fields are given at the top of the dialog, one for a material description and one for the permeation rate.Initially, the data list is empty.
The input fields are initially filled with default values. Pull down menus forpermeation units (pressure, volume, area and time) can be used at any time.The data list in the center of the dialog contains a scrolling window of out gas data. In a new permeation library,this list will be initially empty. Clicking on one row of the list will update the input fields at the top of the dialog. Tochange the value of this data, click in each input field and edit the number as you would with a word processor. Thenumber you edit in the input field will not change the list values below until you either add it to the list using the Addcommand, or replace the selected list value with the Replace 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.Buttons are provided for the basic editing functions. These Add, Replace, and Delete commands are alsoavailable under the Model menu at the top of the screen.See also:PermeationPermeation entry dialogPermeation calculations© 2011 Professional Engineering Computations25013.9VacTran 3Dialog 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.13.10 Opening permeation librariesTo open an existing Permeation Library: Under the File menu, select the Open... command, or click on the icon asshown:In the Open Document dialog that appears, select Permeation 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 thePermeation library file.© 2011 Professional Engineering ComputationsPermeation libraries25113.11 Permeation library formatThe permeation library file is stored as a text file, and can be edited with most word processors.
So if you prefer touse 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 "VTPER". When editingyour own files outside VacTran, be sure to follow the formatting guide for permeation libraries or VacTran will reportan error reading the file. A badly corrupted permeation library file may cause unpredictable results.The following example demonstrates the format is required for Permeation libraries.[General]Total=10[0]Description=NeopreneRate=1.8E-8[1]Description=NeopreneRate=2.3E-8[2]Description=NeopreneRate=2E-8[3]Description=NeopreneRate=2.1E-8[4]Description=NeopreneRate=7.5E-8[5]Description=NeopreneRate=1.1E-7[6]Description=NeopreneRate=7.2E-8[7]Description=NeopreneRate=7.5E-8[8]Description=NeopreneRate=1.9E-9[9]Description=NeopreneRate=6.8E-9(CS 2368B Oxygen)(CS 2367 Oxygen)(CS 2368B Argon)(CS 2367 Argon)(CS 2368B Hydrogen)(CS 2367 Hydrogen)(CS 2368B Helium)(CS 2367 Helium)(CS 2368B Nitrogen)(CS 2367 Nitrogen)© 2011 Professional Engineering Computations25214VacTran 3O-Ring LibrariesPermeation can be defined as the flow of gas across a vacuum boundary through a solid material from the highpressure side to the low pressure side.
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