VacTran 3 Manual (779748), страница 42
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Forexample, performance of momentum transfer pumps such as turbos are affected by gas molecularweight, cryogenic pump speed depends on condensation, and ion pump speeds depend on breakingintermolecular bonds.If you need to model a pump for a new gas, contact the manufacturer of the pump for the performancecurve, or measure the performance directly.When I try to save pump data, there does not appear to be an option to save it as .VTPMP. Theoptions are all graphics formats, such as JPG.When you tried to save the file, the graph was probably the front-most window. The software thinksthat you want to save the graphic image, so all the options were associated with graphics formats.Click on the pump window (the one with the data) to bring it to the front, then try saving it. You shouldsee the save dialog as you would expect with the correct pump file extension.How do I model two pumps in series?It is common practice to build systems with roots blowers in series with other roots blowers or amechanical pump.
Roots blowers in series have a combined compression ratio, which is ofteneffective for certain situations. The combined pumping speed of these combinations is beyond thescope of this program because of the number of geometry related variables specific to each pump.© 2011 Professional Engineering Computations424VacTran 3While two pumps in parallel can simple be added to a pump station in a system model, two pumps inseries require consultation with the manufacturer or testing for accurate performance information.Do I have to enter all the pump data? Why don’t you supply pump curves with your program?Some vendor pump curves are included with the program; these are in subdirectories named bycompany. Most major pump manufacturers use VacTran, and most likely have all of their pumpsmodeled.
These companies can be contacted directly for access to the data.How do I model a valve?The type of valve will determine the challenge in modeling. A fully open gate valve can be accuratelymodeled as an aperture. A fully open poppet valve, often used for diffusion pumps, can be modeled asa 90-degree miter.
More complicated situations included metering valves, throttling valves, and otherpartially open geometries.My catalog quotes a single conductance value for a trap. Can I model the trap as constantconductance element?Constant conductance is only valid under two circumstances: molecular flow and choked flow. Thevalue quoted in vacuum catalogs is usually only valid for molecular flow. If the trap is only used in thisflow regime, a constant conductance element will work just fine. For viscous flow, the constant valuewill typically understate the real conductance value, which varies by orders of magnitude with pressure.The conductance list in my system model seems to have frozen up. How do I get it to respond?You have bypassed the conductance list by clicking on one of the bypass valves, which tells theprogram to ignore all conductance losses.
To reactivate the conductance list, simply click on one ofthe bypass valves.How do I change the start and stop pressure for a pump in a system model?Click on the button called “Station settings” in the system model.My system pumps the entire roughing line with the vessel. How do I model my isolation valvenear the pump instead of near the vessel?Click on “Add cond volume” in your System model.
This will include the volume of the conductance tothe vessel volume for pump down calculations.Can I use VacTran data in my own spreadsheet or another graphing program?Yes. All graph data is shown in the text window after each calculation. You can highlight the data andcopy it to the clipboard, then paste it into your other application. If you past into a spreadsheet, set thedefault Text Window options to “No header”, “Tab delimited”, and “No item numbers”.How do I add special annotations to the graph?Given the number of high quality presentation graphics programs on the market, PEC has chosen notto duplicate these functions.
VacTran’s graphs can easily be copied and pasted into any suchprogram, such as Microsoft Powerpoint, and then annotated with a variety of graphics tools. Use theExport button on the graph window to copy the graph to the Clipboard. If a pixel-based paint program isused, select the bitmap export function. If an object oriented draw program is used, select theWindows metafile (WMF) export option. Windows metafile format has the advantage of being scalablein other applications without degrading print quality.How do I know which gas loads in the VacTran library are most like my system?Sometimes, you are starting a new system design and have no idea what the gas loads will be.
Usethe library values as a starting point to bound your calculations. For greater accuracy, try to find data© 2011 Professional Engineering ComputationsFrequently asked questions425for your particular situation from existing systems or test samples. The variability of out gas data isenormous, and will have profound affects on your results.Can I turn up the temperature to see the affect on outgassing?In the real world, temperature has a large affect on outgassing. However, this is a highly complexrelationship that depends on the gases being emitted.
VacTran does not have the ability to model this.Therefore, if you change the system temperature in the program, the only calculation affected ismolecular flow conductance. The only valid way to model a higher temperature system is to getmeasured data from out gas materials at that temperature.Is there a way to zoom in on a graph to see more detail?Yes. Hold down the left mouse button on the graph and drag a zoom window. To restore the originalscale of the graph, use the right click button over the graph, and select "Unzoom graph" from the popup menu.
Caution: the graph will stay zoomed in until you unzoom it.In my system model, I cannot add any new conductance elements. The conductance list doesrespond, and seems to be inactive.The conductance list was probably bypassed. In this mode, the conductance list is inactive, and allsubsequent conductance calculations ignore the conductance losses for that pump station. To restorethe conductance list to active mode, simply click on any of the gate valves.I have a roughing line that tees into two identical conductance paths to two identical vacuumvessels.
How do I model this?Step 1) Create a new system model using the roughing pump and the conductance path up to the tee.Use the Generate pump model command under the Graphs menu to create a new pump thatincorporates the conductance loss up to the tee. Save this new pump under a new name.Step 2) Open the new pump, and use the multiply button to reduce the whole pumping curve by 0.5,because this pump will be shared by two vacuum vessels. Save the pump.Step 3) Create a new system model with the new pump, the path from the tee to one vessel, and thevolume of one vessel. If gas loads are modeled, use the values for one vessel.Step 4) Now you have a valid symmetry model which can be used to calculated pump down time,delivered speed, or any other calculation related to both vacuum vessels.I seem to have a system model that has changed its conductance losses significantly… and Idon’t know what happened.Here are two actions that would change conductance losses for a given set of conductance elements:1) Clicking on the virtual "gate valves" on the system model that bypass the conductance list.
Thebypass option essentially ignores everything in the conductance list and does subsequent calculationswith zero losses.2) Changing the exit or entrance loss for a conductance element. For example, a pipe with an exit losswill have an a conductance at atmospheric pressure that is many orders of magnitude lower than pipewithout this loss.I am generating some pumping curves for the various pumps we have, but when I try to savethe data, there does not appear to be an option to save it as .VTPMPWhen you tried to save the file, the graph was probably the front-most window. The software thinks that© 2011 Professional Engineering Computations426VacTran 3you want to save the graphic image, so all the options were associated with graphics formats.Click on the pump window (the one with the data) to bring it to the front, then try saving it.
You shouldsee the save dialog as you would expect with the correct pump file extension.Can VacTran can be used to calculate the conductance of a valve?A gate valve can often be modeled as an aperture, and a poppet valve can be modeled as a miter.VacTran cannot calculate conductance for free-form geometries. It uses published, closed-formparametric equations for pipes, elbows, apertures, etc. Where data is available for a unique geometry,VacTran provides a raw data entry option for conductance vs pressure information for the newconductance element.Where is the help file? VacTran does not seem to find it!The help functions may have been installed in an unexpected location on your computer.
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