VacTran 3 Manual (779748), страница 41
Текст из файла (страница 41)
After desorption, the gas contributes to the vesselpressure. Therefore, it has been surmised that there will be some initial period during which no permeation will beobserved, during which the first molecules are traveling through the material. This time lag is ignored in VacTran,and all permeation is considered to start at time zero. This is justified in most vacuum systems because during theinitial pumping of a system from atmospheric pressure, the starting gas volume in the vacuum vessel is orders ofmagnitude higher than the additional gas from permeation. Later in pumping, when vessel pressure is orders ofmagnitude less (such as 10e-6 torr), permeation becomes the ultimate limitation of attainable pressure for a given© 2011 Professional Engineering Computations416VacTran 3pumping system.Therefore, in some vacuum systems which operate at relatively high pressures, permeation can be completelyignored, because it adds such a small gas load.
In other situations of lower target pressures or huge numbers of oring seals, permeation will be a critical factor in system performance.Since the effects of permeation are not usually significant above pressures of 1 torr, VacTran calculates theconstant rate of permeation from time zero using a constant pressure differential equal to the global Start Pressure.See also:Permeation entry dialogPermeation dialog descriptionPermeation calculations© 2011 Professional Engineering ComputationsGlossary41720.26 Pump speedPump speed is the gas flow rate, volume/time, as measured at the inlet to the vacuum pump.
This data is obtainedfrom the pump manufacturer and used to create vacuum pump models.See also:Modeling a pump as raw dataModeling a pump, caveatsPump modelsOpening existing pump modelsImporting DOS pump files20.27 Pump stationA pump station consists of one or more vacuum pumps connected by one or more conductance elements to thevacuum vessel in a system model. Conductance elements can be pipes, elbows, bends, orifices, or constantvalues.
A system model has to have at least one active pump station in order to calculation pump down time.20.28 Pump station modelA pump station model is a stand-alone file that can be saved under a unique name and recalled for use in systemmodels.20.29 Pump station settingsActivated from a System Model, the Pump station settings dialog allows you to change several characteristics ofeach pump station.20.30 Pump throughputThe gas flow rate, pressure-volume/time, as measured at the inlet to the vacuum pump.technologists prefer this unit of pump performance rather than pump speed.Some vacuum20.31 Rate of riseWhen a gas load is present in a system model, the pressure rate of rise is a graph can be generated to show theevolution of vessel pressure increase as additional gas is added.
The pressure rise profile depends on the vesselvolume, the gas loads, and the starting pressure.20.32 Raw data conductance modelThis type of model is a set of data containing conductance vs. pressure in a two dimensional array. This can beused in a system model when parametric models such as pipes or elbows are no appropriate.© 2011 Professional Engineering Computations418VacTran 320.33 Screen captureUse the screen capture button as shown to grab an image of the screen. A new window will open up with the image inside.The image can then be saved or copied for use in other applications.20.34 Selected gasThe selected gas is a set of data representing a specific gas or gas mixture to be pumped, which includesmolecular diameter, dynamic viscosity, and molecular weight. In many systems, gas composition can changeduring pump down due to outgassing, leaks, permeation, back streaming, and purging. Consider the significance ofthese effects on gas composition when choosing the selected gas.Note also that temperature can affect the dynamic viscosity of the gas, which affects viscous flow calculations.Make sure the data for the gas you are using is applicable to the temperature.© 2011 Professional Engineering ComputationsGlossary41920.35 Selected RangeThis term is applicable to system model pump stations.
On a given pump station, the selected range is theexpressed as the maximum and minimum pressures that the user has selected for operating the pump. Theselected range is always narrower than or equal to the Available range.The selected start pressure for a pump station is determined by the user, and is less than or equal to the highestpressure any individual pump in the pump station can operate.The selected stop pressure for a pump station is determined by the user, and is greater than or equal to the lowestpressure any individual pump in the pump station can operate.Change the selected range by clicking on the Station Settings button.20.36 Series conductance modelA series conductance model is a conductance list that can be saved as a separate file.
The individual elements inthe list are combined in series using the ideal series combination formula. Losses associated with geometrytransitions between conductance elements are not accounted for in the combination formula. These losses have tobe included in each conductance element as an exit or entrance loss.© 2011 Professional Engineering Computations420VacTran 320.37 Start pressureThe start pressure is the gas pressure that a vacuum system has before a pump station begins removing gas.Usually, this is atmospheric pressure, 14.7 psi, or 760 Torr. In VacTran, it is the pressure at which pump downanalysis starts, measured at the vacuum chamber.
This must be greater than the target pressure, and within thepumping capacity of at least one of the pump stations.20.38 System modelA system model is a simulation of a complete vacuum system, which includes pumps, conductances, gas loads,volume and gas property information. It contains the current vacuum environment and up to three pump stations.Total pump down time is calculated from the combined delivered speeds of the three pump stations, or individualpump stations if they are set to turn on at different pressures.Click on an area of the dialog for m ore help20.39 Target pressureThe target pressure is the gas pressure that you are intending to achieve using some combination of pumps andconductances.In VacTran, it is pressure at which the pump down analysis ends, measured at the vacuum chamber.
This mustalso be in the defined range of at least one of the selected pump models.© 2011 Professional Engineering ComputationsGlossary42120.40 Vacuum environmentThe vacuum environment is a set of information that includes vessel volume, selected gas, start pressure, targetpressure, and number of calculation increments. This data is stored and retrieved with each system model. Thelast vacuum environment before exiting VacTran is saved for the next session.Click to see m ore on Environm ent Settings© 2011 Professional Engineering Computations422VacTran 320.41 Vessel volumeTotal volume of the chamber to be evacuated, not including the conductance elements connecting to the vacuumpumps.
System models have the option to include or exclude the volume of conductance elements from the totalpump down volume.20.42 Vacuum pumpA vacuum pump is the key active vacuum system element tasked with removing or capturing gas from a vacuumsystem. In VacTran, a vacuum pump is modeled as a data set of pump speed vs. pressure. No differentiation ismade between cryogenic pumps, turbo pumps, roots blowers, or any other type of pump.
Mathematically, allpumps will have a characteristic pumps speed vs. pressure curve that will affect pump down calculations.see also:Pump modelsOpening existing pump modelsImporting DOS pump files© 2011 Professional Engineering ComputationsFrequently asked questions21423Frequently asked questionsThrough many years of benchmarking VacTran against a wide variety of industrial andlaboratory systems, we have found it to be reliable and reasonably accurate.
However,users may encounter unexpected results with the program if basic engineeringprincipals are violated, or simplifying assumptions over–extend the boundaries of areasonable model. The following list of common errors and questions has beencompiled based on customer support over many years of use. If you find somethingstrange that isn’t described here, don’t hesitate to contact us for further support.How often do you update VacTran?VacTran has been updated with new functions for nearly 25 years. Minor updates are distributed whenavailable via www.vactran.com and are available for free download to existing users of the same majorversion.
Occasionally, a major update is published that requires an upgrade fee. PEC determineswhen a substantial increase in functionality justifies a major update.How do you decide what new features to include in VacTran?Since we are always seeking to improve VacTran, customer feedback is an extremely important factorin prioritizing new development efforts. A new feature has be stable, intuitive, and based on soundengineering and software design principals with the widest possible benefit to the user community.Can I load VacTran on more than one computer?Many people have more than one computer (desktop and laptop, for example).
VacTran can be loadedon more than one computer, but the license only allow one instance of the software to be operating at atime. You will have to move the hardware key between computers to do this.Does VacTran automatically adjust pumping speed for different gases?VacTran does not make this adjustment if you change the system gas or temperature. The mainreason for this is that there is no universal conversion formula that covers every type of pump.
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