VacTran 3 Manual (779748), страница 39
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It demonstrate the relatively minor change in results for increments greater than500, and the rather dramatic changes that occur below 300 increments.This convergence sensitivity will vary depending on the type of problem.© 2011 Professional Engineering ComputationsHand calculating a pump down curveCalculation convergence based on number of increments400Pump down time (seconds)3503002502001501005000200400600800Number of increments© 2011 Professional Engineering Computations1000120039339420VacTran 3GlossaryActive pumpAvailable rangeBacking pump speedChoked flowConductance elementConductance listConductance modelConductance paletteDelivered speedFlow regimesGas modelGas loadGas load decay timeGas load start timeGas load stop timeIdeal systemIncrementsLeakMean free pathOperating rangeOrificeOut gas materialParallel conductance modelPermeationPump speedPump stationPump station modelPump station settingsPump throughputRate of riseRaw data conductance modelScreen captureSelected gasSelected RangeSeries conductance modelStart pressureSystem modelTarget pressureVacuum environmentVessel volumeVacuum pump© 2011 Professional Engineering ComputationsGlossary20.1395Active pump"Active pump" is a pump used in a system model that is not bypassed, or one that has data for the pressures usedin the current calculation.See alsoPump station definitionPump station settingsSystem model introduction© 2011 Professional Engineering Computations39620.2VacTran 3Available rangeThis term is applicable to system model pump stations.
On a given pump station, the available range is theexpressed as the maximum and minimum pressures that the pump station can be used. The Available range isalways wider than or equal to the Selected Range.The available start pressure for a pump station is determined by the highest pressure any individual pump in thepump station can operate.The available stop pressure for a pump station is determined by the lowest pressure any individual pump in thepump station can operate.Change the available range for a pump station by adding or deleting pumps from the pump station pump list.© 2011 Professional Engineering ComputationsGlossary20.3397Backing pump speedBacking pump speed is defined as the volumetric rate of gas flow required at the exhaust of a pump that cannot beexhausted to atmosphere, and is usually specified by the manufacturer at a specific operating pressure.
Thispumping speed is usually provided by a separate pump called a backing pump.Backing pump speed is applicable to some types of pumps, such as diffusion pumps, roots pumps (also known asroots blowers), and turbo pumps for which the rated pumping speed given by the manufacturer is dependent on aminimum backing pump speed. In other words, these types of pumps cannot operate without an additional pumpbetween them and the atmosphere.See also:Backing pump selectionDelivered speed© 2011 Professional Engineering Computations39820.4VacTran 3Choked flowChoked flow is a condition in which the rate of gas throughput in a conductance is limited by the speed of sound.Depending on gas properties, this will occur when the pressure ratio across the conductance is less than a criticalvalue. In a qualitative sense, if the pump speed at the inlet of the pump is much greater than the conductance athigher pressures, choked flow is the likely dominating phenomenon.For example, for air at room temperature, the critical pressure ratio is 0.525.Most vacuum systems are not designed to operate anywhere near the choked flow regime, so this is usually not anissue for most designers.
However, the vacuum industry covers a tremendous variety of applications, some ofwhich may require a high pressure ratio across an orifice. This can be the case where an orifice is used to limit theinitial pump down rate because of cleanliness concerns.VacTran calculates choked flow in conductance calculations for1) orifices and expanding cones2) pipe, elbows, miters, and bends with the exit loss option selected.© 2011 Professional Engineering ComputationsGlossaryVacTran does not calculate choked flow for1) constant conductance elements2) raw data conductance elementsSee also:Flow regimesSonic flow conductance© 2011 Professional Engineering Computations39940020.5VacTran 3Conductance elementA conductance element is a parametric representation of a physical resistance to gas flow, such as a pipe, orifice,or bend.A series of conductance elements are arranged into conductance lists.A constant conductance element is provided for cases where a constant conductance rate is specified by themanufacturer for a particular trap, valve, or fitting, or when a constant choked flow condition is simulated.
Note thatconstant conductance values are usually valid for only a limited pressure range, and are usually specified formolecular or choked flow only. For the other conductance elements listed above, the conductance rates, which arehighly variable, are calculated by VacTran at each pressure increment. for viscous, transition, and molecular flowregimes.A raw data conductance model allows the user to create a file of conductance versus pressure that can be used inany system model or conductance list.Models that use conductance elements:System modelsConductance studiesSee also:Basic concepts in vacuum technology© 2011 Professional Engineering ComputationsGlossary20.6401Conductance listA conductance list is a set of conductance elements connected in series, created for a pump station in a SystemModel.
A conductance list is used to represent series conductances which connect a vacuum vessel to a pumpstation.Local connection losses between conductance elements, such as radical changes in cross section, are accountedfor using the entrance or exit loss options for the conductance element.Examples of conductance lists:System m odel conductance list© 2011 Professional Engineering Computations402VacTran 3Conductance m odel conductance listPum p station m odel conductance list© 2011 Professional Engineering ComputationsGlossary20.7403Conductance modelA conductance model is consists of a series of conductance elements arranged into a list (conductance list) thatrepresents the physical piping path between the pump and the vacuum vessel.
VacTran has series conductancemodels and parallel conductance models.Editing commands:oror Ctrl+A: Makes the Conductance Palette active so you can add a conductanceoror Ctrl-D Deletes the highlighted element from the list.elementor Double-click on an elem ent opens up an editing box for that elementGraphing commands:© 2011 Professional Engineering Computations404VacTran 3Station conductance vs pressureStation throughput vs pressureSee also:Choked flowConductance element definitionConductance list definitionConductance StudiesCreating conductance modelsDelivered speed definitionConstant conductance dialogElbow data dialogFixed rate leak dialogMiter data dialogEnter orifice data dialogEnter pipe bend data dialogPipe data dialogFlow regimesParallel Conductance StudySeries Conductance StudyRaw data conductance model© 2011 Professional Engineering ComputationsGlossary20.8405Conductance paletteA conductance palette is used to add conductance elements to other models, such as System models,Conductance models, Pump station models, and Conductance studies.The palette will be dimmed until an appropriate dialog is available to enter a conductance element.
For example, ona system model, click on the conductance list to activate the Conductance palette.Clicking on one of the buttons will activate the corresponding entry dialog for adding a conductance element to theactive model.Use Pipe Library optionIf the check box is checked, the library option will ask you to select a pipe library file. Pipe libraries enable you tostore and retrieve frequently used pipe diameters for future conductance element entries. A number of standardpipe sizes is provided with VacTran in the /Pipes directory.20.9Delivered speedDelivered speed can be defined as the gas flow rate, in volume/time units, as measured at the vacuum vessel conductance interface.
The delivered speed is a calculated value, and is always less than the pump speed, basedon the flow loss in the conductance model. VacTran calculates viscous , transition, and molecular flowconductance for all conductance elements.
At higher pressures, or in conductance elements of very small crosssection, choked flow may be encountered. Choked flow is not considered in this version of the program.A special case of delivered speed, called Backing pump speed, applies to the flow of a backing pump after losses ofthe foreline are calculated.See also:Basic concepts in vacuum technology© 2011 Professional Engineering Computations406VacTran 320.10 Flow regimesequations 3.8, 3.9, 3.10 Reference 2viscous flowtransition flowmolecular flowwhereD/l > 1101 < D/l < 1101 > D/lD = equivalent cylinder diameter of conductance element in centimetersl = mean free path for the gas at a given pressureMean free pathThe mean free path is generally defined as the distance a molecule of gas travels before it collides with anothermolecule.
When the mean free path of the gas is compared to the diameter of the conductance it is travelingthrough, the flow regime can be determined.Viscous flowAt the higher pressures during initial pumping, the flow of gas is limited by gas molecule-gas molecule interactions© 2011 Professional Engineering ComputationsGlossary407(viscosity). Under these conditions of viscous flow, the cross section dimensions of the conductance paths aremany times the mean free path.Molecular flowAt lower pressures, the mean free path may be many times the dimensions of the conductance paths.
Molecularflow then characterizes the movement of gas molecules, which is determined by gas molecule - pipe interactions.Transition flowAt some intermediate pressure, where the molecular mean free path is nearly the same as the conductance pathdimensions, flow is in a somewhat complicated transition regime which is neither viscous nor molecular incharacter, and is generally referred to as transition flow.Choked, or sonic flowUnder conditions of a high-pressure ratio, choked flow can occur, where the speed of the gas inside a pipeapproaches that of sound. Sonic flow is actually a special case of viscous flow, which can only occur at highenough pressures where gas molecules interact.
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