VacTran 3 Manual (779748), страница 20
Текст из файла (страница 20)
The three images below show the three different types of entrancesapplicable to pipe. Note that K factor and r/d ratios will only apply to viscous flow calculations, and not molecularflow 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 isused to determine 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. Formany designs where a pipe is welded into a vessel at a machined port location, sharp edged entrances arecommon.© 2011 Professional Engineering ComputationsSystem models185The 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 moredetail than can otherwise be gained from looking at conductance curves, and serves to allow the user additionalscrutiny into the basis for flow calculations.© 2011 Professional Engineering Computations186VacTran 3See also:Annular pipe entryCombination conductancesCone entryConstant entryElbow entryElliptical pipe entryMiter entryOrifice entryPipe bend entryPipe entryPump station modelsRaw data conductance modelsSlit entryTriangular pipe entry© 2011 Professional Engineering ComputationsSystem models1878.13.14 Slit entry dialogDefinitionA slit is a conductance element having a thin rectangular cross section and essentially zero length.
A slit can havean entrance corner radius, which by geometry creates a finite length, but this additional length is not included theslit calculations. The radius of the edge is used only in viscous and sonic flow calculations. Generally speaking, asharp edge (zero radius) will have a lower viscous flow conductance than a finite radius. Flow calculations for theradius edge are based on rules circular geometry.Entrance and exit lossesEntrance and exit losses are not added to slits, which are essentially an entrance or exit depending on the context.Where is it used?Slit conductance elements are often used flow limiters and metering devices in systems where a known flow rate isrequired.
Slits are a common component of some types of optical systems, such as spectrographs.See also:Annular pipe entry© 2011 Professional Engineering Computations188VacTran 3Combination conductancesCone entryConstant entryElbow entryElliptical pipe entryMiter entryOrifice entryPipe bend entryPipe entryPump station modelsRaw data conductance modelsRectangular pipe entryTriangular pipe entry© 2011 Professional Engineering ComputationsSystem models1898.13.15 Triangle entry dialogDefinitionA triangular pipe is a conductance element having an equilateral triangular cross section and non-zero length.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 hasbeen found for entrance and exit losses associated with triangular pipe, circular pipe formulas are used for endeffects. This will introduce some level of error that has not been quantified.
Use the entrance and exit functions forestimating purposes.Where is it used?Triangular pipe 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 a pipe.
You need not go any further than thistab if you know the dimensions, and end effects. Quantities greater than one will be calculated as conductances inseries. Note, however, that the entrance and exit loss value will also be multiplied. In most cases, a separate pipeentry will be appropriate for each length of pipe in the model, each with its own assessment of applicable endeffects.© 2011 Professional Engineering Computations190VacTran 3The Use Recent Entry tab allows selection of a previous conductance element that was entered, with the mostrecent entry shown at the top of the list.
This function is intended to be a time saver for frequently used geometries.© 2011 Professional Engineering ComputationsSystem models191The Entrance Detail tab contains more detail on the type of entrance present, and applies to the pipe calculationsonly if the entrance loss option is selected. The three images below show the three different types of entrancesapplicable to pipe. Note that K factor and r/d ratios will only apply to viscous flow calculations, and not molecularflow 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 isused to determine 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. Formany designs where a pipe is welded into a vessel at a machined port location, sharp edged entrances arecommon.The 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 moredetail than can otherwise be gained from looking at conductance curves, and serves to allow the user additionalscrutiny into the basis for flow calculations.© 2011 Professional Engineering Computations192VacTran 3See also:Annular pipe entryCombination conductancesCone entryConstant entryElbow entryElliptical pipe entryMiter entryOrifice entryPipe bend entryPipe entryPump station modelsRaw data conductance modelsRectangular pipe entrySlit entry© 2011 Professional Engineering ComputationsSystem models8.14193Adding gas loads to a modelIf the gas load list is active, gas load elements can be added.
To activate the list, click on it.To activate the Gas load palette, press thebutton, click on thebutton at the top of the screen, usethe Ctrl+A short cut, or select the Add command from the Model menu. You may select the Use library to selectmaterial properties from a stored file.A following dialog will let you enter the specific geometry information.If the Gas load palette is not visible, you can activate it by selecting it under the Window menu. If it is visible butnot active, it will look like this:The Gas load palette will only be active if there is System Model on the screen with an active gas load list. Forexample, if you want to add a gas load to the gas load list in a System Model, bring a System Model window to thefront by clicking on it, then click on the gas load list to make it the focus.
The Gas Load palette will always activatewhen a gas load list has the focus of the program.See also:Exponential out gas entryLeak entryO-ring entryOut gas entryPermeation entryRaw data gas loads© 2011 Professional Engineering Computations194VacTran 38.14.1 Exponential out gas entry dialogMaterial out gassing is a phenomenon that is often characterized by an exponential decay curve. The outgassingrate is usually dependent on time exposed to vacuum. It drops rapidly in early time, and transitions to a very slowdecay rate at later time periods.An out gassing curve usually has a reference time at which a known out gassing rate is applied.
For theexponential out gas model, a gas load (Q1) at 1 second is used.is the slope on a log-log plot. More about theformula for exponential out gassing is given in Exponential out gas calculationsThe exponential entry is very similar to the out gas entry, except that it only has one slope and no separate field forsurface area.
This assumes the user has data in this form, which may be more convenient to use than the out gasentry. There are no library options associated with the exponential entry dialog.See also:Leak entryO-ring entryOut gas entryPermeation entryRaw data gas loads© 2011 Professional Engineering ComputationsSystem models1958.14.2 Leak entry dialogA fixed rate leak is simply a flat-line gas load vs. time model.Use this dialog to enter the gas load value. Use this type of element if accurate data is available and you are notconcerned with time-dependent gas loads.
Otherwise, use other elements such as out gas sources, and letVacTran calculate the gas load.See also:Exponential out gas entryO-ring entryOut gas entryPermeation entryRaw data gas loads© 2011 Professional Engineering Computations196VacTran 38.14.3 O-ring entry dialogAn o-ring is a special case of a permeation element, and is described by its compression ratio, inside diameter andsection diameter. The inside diameter assumes that the o-ring is in a circular o-ring groove. It is also common foro-rings to seal rectangular openings. In these cases or other geometries, determine the inside perimeter of yourinstalled geometry, and calculated the equivalent inside diameter as if it were installed in a circular groove.For example, if the perimeter dimension is 6.28 inches, then the equivalent diameter should be calculated byD = 6.28/ = 2 inches.O-ring compression for common face-seal elastomers is usually 20%.The permeation rate, found in material handbooks, completes the definition.The Quantity value is simply used as a multiplier for total gas load.Use the above dialog to enter the O-ring geometry.
The gas load curve for an o-ring will be a horizontal line at aconstant gas load value.Enter O-ring data using permeation and o-ring libraries© 2011 Professional Engineering ComputationsSystem modelsSee also:Exponential out gas entryLeak entryOut gas entryPermeation entryRaw data gas loads© 2011 Professional Engineering Computations197198VacTran 38.14.4 Out gas entry dialogAn out gas source is described by:surface area: area of the material that is exposed to the vacuum environmentAlpha 1 and 10: the slope of the out gas curve at 1 and 10 hoursQ1 and 10: the out gas rate at 1 and 10 hoursAn out gas source decays with time. See also: Out gas calculationsEnter out gas data using a gas load library© 2011 Professional Engineering ComputationsSystem models199Click once on the material in the scrolling list, and enter the surface area in the entry field.
Click OK to add the newgas load to your model.See also:Exponential out gas entryLeak entryO-ring entryPermeation entryRaw data gas loads© 2011 Professional Engineering Computations200VacTran 38.14.5 Permeation entry dialogA permeation source is described by:section area: projected area of the permeation material through which gas will flow from outside the vacuumboundary to the inside.thickness: the path length gas must travel through the material to flow into the vacuum vessel.permeation rate: characteristic steady state constant based on the particular material under specific conditions oftemperature and operating history, and gas that the rate is based on.A permeation source is treated as constant with time.
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