VacTran 3 Manual (779748), страница 23
Текст из файла (страница 23)
Each curve underneath shows the delivered speed of theroots blower combined with a different diameter pipe, each fixed at 25 feet in length.© 2011 Professional Engineering Computations222VacTran 3Based on our requirement for backing speed of 400 CFM for the diffusion pump at 10-2 torr, we can see from theabove graph that the 10-inch diameter pipe will give us that performance. A conservative engineer might pick thenext larger pipe size, 12 inches, to be sure that the requirement is met with some safety margin.Step 6) Compare conductancesIf you were interested in further examining the differences between the different diameter pipes, you could select theVary button to show the following Conductance vs.
Pressure graph:Conductance vs Pressure7Diam=2.00E+00 InDiam=4.00E+00 InDiam=6.00E+00 InDiam=8.00E+00 InDiam=1.00E+01 InDiam=1.20E+01 InDiam=1.40E+01 InDiam=1.60E+01 In10C onduc tanc e ( C u F eet/M inute)61051041031021011001010-810-710-610-5101010-4-3-2Pressure (Torr)10-1100101© 2011 Professional Engineering ComputationsConductance Studies223Backing the roots blowerThe mechanical pump, which backs the roots blower, is required to provide a backing pump speed of 60 cfm. Atypical mechanical pump and roots blower combination is often sold as a turnkey package, with the two mounted toa common structural support.
In this example, assume that the mechanical pump is in close proximity to the rootsblower. We need to verify that the short length of pipe connecting them does not degrade the performance of themechanical pump below the required backing speed for the roots blower.step 1) Close the previous Conductance StudyClick on the close box (X) on the Conductance Study to close it.step 2) Start a new studyActivate a Series Conductance Study by clicking on thespeed button below the main menu.step 3) Add the mechanical pumpAs in the last example, add a pump by clicking once in the pump list and then on thebutton.
Select the file “mech.VTPMP” from the dialog.step 4) Add a pipeClick once on the conductance list. Add a pipe with a length of 24 inches.step 5) Set the diameter rangeNow set the From: variable field to 0.5 inches, the To: variable field to 3 inches, and the # Curves field to 6.© 2011 Professional Engineering Computations224VacTran 3step 6) Create Delivered Speed vs. Pressure curveClick on the Delivered Speed button to generate the following curve:Delivered Speed vs PressureSpeed at pump{s)Diam=5.00E-01 InDiam=1.00E+00 InDiam=2.00E+00 InDiam=2.50E+00 InDiam=3.00E+00 InDiam=1.50E+00 In100Delivered Speed (Cu Feet/Minute)908070605040302010010 -110 010 1Pressure (Torr)10 210 3The graph shows that the minimum pipe diameter required to achieve a pumping speed of 60 cfm at 10 torr is 1.0inches.See also:Activating a conductance studyCalculations in conductance studiesSummary of conductance study functionsHow to calculate backing pump speed11Raw data gas load modelsThis section explains how to create and edit raw data gas loads.
Most gas load models in VacTran are parametric;in other words, the gas load a particular time for an out gas model is calculated on the fly during pump downcalculations. In contrast, the raw data gas load model is a set of user defined data that defines gas load vs time asa set of data points.Gas loads are defined as any sources of gas, in addition to the initial vessel volume, which must be pumped inorder to achieve the target pressure.
The source can be from leaks, permeation, out gassing, or any combination.Any gas load will tend to increase the pump down time compared to an ideal system with no gas load, the extentof which will depend on the delivered speed of the pump station compared to the gas load. Some gas loads have aninsignificant affect on system performance, while others are large enough to prevent pump down to the targetpressure. If the gas load is greater than the pumping speed, more gas goes into the system than comes out, andpressure cannot decrease.© 2011 Professional Engineering ComputationsRaw data gas load models225A raw data gas load is a simple way to model a gas load mathematically by defining a curve, point by point, of totalgas load vs. time.See also:Modeling a Raw data gas loadWhy create a raw data model?Using Raw Data in system modelsDependence on gas typeCreating raw data gas load modelsRaw data dialog descriptionRaw data dialog commandsMenu commands for gas load modelsRight-click optionsOpening existing raw data modelsRaw data gas load exampleRaw data gas load file format11.1Modeling a Raw data gas loadDespite the variety of gas load sources common to vacuum systems, all can be characterized by a curverepresenting total gas load vs time.
Outgassing usually varies significantly with time, and is often the initialdominating phenomenon. A raw data gas load model is a two dimensional array of data representing gas load vstime. It contains no information on actual physical geometry or material characteristics other than the resultantoutgassing rate.The user, based on assumptions, published curves, measured data, or off-line calculations, creates the model.VacTran provides an efficient dialog for editing the model. Obtaining accurate gas load data is by far the mostdifficult and risky task in vacuum system modeling.
VacTran does not validate the data entered, other than ensurethat it contains positive numbers.© 2011 Professional Engineering Computations22611.2VacTran 3Why create a raw data model?There are other utilities for adding gas loads into a VacTran system model, as explained in System models. Theseallow direct entry of specific out gas and permeation geometry and material characteristics, and encourage use ofstored libraries of measures material data. These may appear to preclude the need for a raw data model, asomewhat redundant and less parametric method of data entry.The implementation of a raw data gas load model in VacTran is for those users who have measured data or theirown method of calculating gas load that may not be included in the program.
It is a catch-all for any gas loadsource not included the other categories.Gas loads are defined in units of pressure-volume/time. Common units for gas load are torr-liters/second, and atmcc/second.A VacTran raw data gas load model is an independent data file, saved on disk as a separate file, which can bereferenced by any number of system models. As with pump models, the actual data in the Raw data gas load fileis not stored with the system model.
Only the raw data file name and location on the disk is saved. Any changesto a particular gas load model will automatically update open system models when they are opened. (See Systemmodels)11.3Using Raw Data in system modelsIn a system model, an array of raw data gas load is accessed during pump down calculations. At each pressureincrement, the pump down time is calculated based on both the initial volume and the current gas load for theincrement.
This gas load is based on the elapsed time at the end of the previous increment. That time value isused to interpolate the gas load value from the raw data gas load array. If no data is found in the gas load model atthe current time increment, the gas load used will be zero. Therefore, it is recommended that a long time span bemodeled, so that the gas load may be used by many system models in different situations.A raw data can be entered in any order because it is automatically sorted.11.4Dependence on gas typeA raw data model contains gas load vs time data that has no information on the constituents of the gas. This isimportant to remember, because pump-down calculations are performed using the pump models that are gasdependent.
In other words, pump speed vs pressure for a pump model could be valid for one gas but much differentfor another. If the gas load curve being created is for a gas that would change the performance of a selected pump,the pump must be altered appropriately. As mentioned in pump models, there is no global, closed form solutionapplicable to all types of vacuum pumps for extrapolating a new pump performance curve for a different gas.Therefore, use caution when specifying a gas load that will be used later in a system model with pumps that arenot characterized for this gas.© 2011 Professional Engineering ComputationsRaw data gas load models11.5227Creating raw data gas load modelsTo create a new Raw Data Gas Load Model: Under the File menu, select the New...
command, or click on the iconas shown:In the New Document dialog that appears, select Raw data gas load and click on OK.(Click to expand)The following dialog allows raw data gas load model data entry and editing. Note the similarity to the pump modeldialog. Pump models and raw data gas load models are almost identical in nature, except that pump models storepump speed vs. pressure data, and gas load models store gas load vs.
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