VacTran 3 Manual (779748), страница 12
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pressure. No differentiation is madebetween cryogenic pumps, turbo pumps, roots blowers, or any other type of pump. Mathematically, all pumpswill have a characteristic pump speed vs. pressure curve that will affect pump down calculations.VacTran makes no judgment about the pump during calculations other than the range of its performance curve.Considerations such as regeneration cycles for cryogenic pumps, or trapping of mechanical pumps are up to thejudgment of the user. Since VacTran pump models are raw data, they are essentially ignorant of operationalrequirements for selecting vacuum pumps.A pump model contains the characteristic pump speed vs. pressure data for a given pump, pumping a given gasor gas mixture.
This is important to remember, because manufacturers usually publish pump data for pumpingnitrogen or air only. If such data, usually from a catalog, is modeled in VacTran pumping some other gas, someerror will be introduced into the results. The significance of this depends on the difference in gas propertiesbetween the desired gas and the rated gas, and the technology of the pump. If there was a reliable, closed formsolution applicable to all types of vacuum pumps for extrapolating a new pump performance curve for a differentgas, it would be part of the program.
We have found no such convenience, so we recommend only usingmeasured pump data from the manufacturer or other reliable source.When pumping a gas other than that specified for a pump, consult the manufacturer for guidance. Performancewill likely vary with different gases.© 2011 Professional Engineering ComputationsIntroduction to system models791Introduction to system modelsA VacTran system model is a mathematical representation of a vacuum system. A minimum system model has avessel, a pump, and a conductance path between the pump and the vessel.The above flow diagram illustrates the concept.
With the pump operating, and valves open, gas will flow from thevessel, through the conductance, through the pump, and finally to the atmosphere. In capture type pumps, such ascryogenic pumps, the gas accumulates in the pump itself. VacTran associates specific information with the vacuumvessel, such as its volume and type of gas being pumped. The pump contains a performance curve representingpumping speed at different pressures. This curve can be a straight line, but is more often a highly variable functionof pump inlet pressure.
Finally, the conductance path can consist of various types of flow elements such as pipes,elbows, and orifices. The simplest place to start is to assume the conductance element is a pipe, and ignore thegate valves. For the purpose of this discussion, the gate valves shown in the figure have no conductance loss andare conceptual in nature.The example just described shows no gas load. Calculations based on systems with no gas load are "ideal"systems, for which pump down time is calculated using only the initial volume in the vacuum vessel. This volume isconstant, making pump down time straightforward calculation. When “real” pump down time is calculated (with agas load), the total volume being pumped will include the vacuum vessel plus additional gas sources which mayvary over time.
Since this total volume is variable, prediction of pump down time is somewhat more complicated.© 2011 Professional Engineering Computations927.1VacTran 3System model componentsSystem models are built from Pump Stations, Gas loads, and Environment SettingsSystem models provides detail on how to put these components together.Pump stations are built from one or more pumps working in parallel, plus one or more conductance elementsin series. A system model can have up to six pump stations. A pump station can have an unlimited numberof pumps and conductances.Pumps are independent model files referenced by the pump station. Each pump model contains pump speedvs pressure information.Conductance elements are piping components that have specific flow characteristics associated with theirgeometry.A conductance model is a special case of a conductance element that is actually a list of conductanceelements, either in parallel or series.A raw data conductance model is another special case of a conductance element that packagesconductance vs pressure data into a separate file.Gas Loads...
built from one or more gas load elements that add together.Gas Load elements are either parametric (calculated) or raw data (provided by user)Examples of parametric gas loads are permeation and out gas modelsA system model can have unlimited gas load elementsA raw data gas load element is a special gas of a gas load element that packages gas load vs time data intoa separate file.Environment Settings for a system model provide bounding parameters for pressure, time, volume, and theresolution of calculations (increments). More increments means higher resolution calculations.© 2011 Professional Engineering ComputationsIntroduction to system modelsThe following figure shows the hierarchical relationship of system model components.© 2011 Professional Engineering Computations93947.2VacTran 3Simple System studyThe Simple System Study was developed to allow rapid calculations for the minimum vacuum system.
This studyhas only one pump station consisting of a single pipe and a single, constant speed pump. These are connected toa vacuum vessel with no gas load. On the Study menu, select Simple System……or use the speed button as shown:… to display the dialog shown:© 2011 Professional Engineering ComputationsIntroduction to system models95The Simple System Study is a five-way solver that allows you to select the variable you are interest in, and alter allother parameters.
For example, you can determine the pump speed required to achieve a given pressure in a giventime period. In the example below, solve for pipe length.Change any variable, then click on Solve calculated value to generate a new pump down curve.Click on the tab marked Pipe Length.Change any variable, and then click on Solve calculated value to generate a new pump down curve. This time,the pipe length will be calculated based on the other variables.Additional tips:1)Change any variable in the dialog, then click on Solve calculated value to create a pump down curve.
Thecurve will be shown in the Main Graph Window.2) Change any units using the pull down menus. Note that the volume unit for the vessel volume is independent ofthe volume unit for the pump speed.3) Click on Create system model to generate a system model from the data you have entered. You will also beasked to name the new pump that is generated as follows:4) If you right-click, the following pop-up menu appears, this has additional graphing options.These same options area available under the Graph menu.
Write system report creates a text summary of thestudy variables in the Main Text Window.5) The five tabs at the bottom of the window let you select the variable to solve. For example, click on the volumetab, then click on Solve calculated value to solve for the vessel volume that meets all other criteria.6) There is one case for which the solver will not converge. If you solve for a pipe dimension or pump speed, theremay be no solution because the model may be conductance limited. For example, if you enter a small pipediameter, and then solve for pump speed you may get an error message if the conductance is too small to achievethe target pressure in the desired pump down time.© 2011 Professional Engineering Computations968VacTran 3System modelsThis section is a guide to the interactive dialogs used to create and modify VacTran system models.
Using systemmodels, shows how to obtain a multitude of analytical results from the model. The basic concepts of systemmodels are explained in Introduction to system models.A system model contains the data required to calculate the pump down time of a vacuum system.Click on an area of the dialog for m ore helpSee also:System models and pump dataCreating system modelsSystem model dialog commandsOutput from system modelsSystem model valvesPump, conductance, and gas load listsDialog check boxesVacuum vesselWorking with environment settingsSystem model right-click options© 2011 Professional Engineering ComputationsSystem models8.197System models and pump dataA system model can contain references to many different pumps, which are created using the techniques describedin Pump Models.
The system model data file does not archive the actual pump data. It maintains pointers to thefiles of these models. Therefore, when recalling a system model, the file names of pumps in that model will besearched on the disk at the location which they were first referenced. If a pump model name has been changed, orthe file has been moved to a different folder, the system model won't know where it is, and will prompt the user forits location.Vacuum pump models can be referenced by any number of system models.
Any changes to open pump modelswill update all the system models referencing them.8.2Creating system modelsTo create a new System Model: Under the File menu, select the New... command, or click on the icon as shown:In the New Document dialog that appears, select System Model and click on OK.(click to expand)Alternatively, use the system model speed button as shown below:© 2011 Professional Engineering Computations98VacTran 3The following dialog allows system model data entry and editing.Pumps in this stationConductances in this stationGas loads in this modelThe system model dialog is a working functional flow diagram of a vacuum system. Gas flows from the vessel,through the conductance element(s), then through the pump(s). Additional gas loads may flow into the vessel,represented by the source list at the bottom of the dialog.A system model can have up to 6 pump stations.
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