VacTran 3 Manual (779748), страница 40
Текст из файла (страница 40)
VacTran addresses choked flow by including a choked exit optionfor pipes, elbows, miters, and bends.see also:Choked flow20.11 Gas modelThis is a file containing data for a gas, including name, dynamic viscosity, molecular diameter, molecular weight,and temperature. Temperature is used in all conductance calculations to determine mean free path.CAUTIONUse caution in making significant changes to the temperature value without checking if the selected gas parametersare valid. For example, dynamic viscosity of a gas will change significantly with temperature.
VacTran does notautomatically calculate this change, because there is no general formula that is valid for all possible gas types.Therefore, when changing the temperature value, be sure to change the selected gas properties as well.20.12 Gas loadA gas load is any source of additional gas to a vacuum system, expressed in pressure * volume / time units. Gasloads are often intentionally introduces as part of a vacuum process, such as reactive sputtering. In addition,additional gas sources include outgassing from materials exposed to the vacuum system, permeation of gasesthrough seals, and leaks.
These are typically undesirable from the user's perspective, and can bound theperformance of the vacuum system.See also:Exponential out gas calculationsGas load decay timeGas load start and stop timeGas load throughput calculationsLeakLeak entry dialogOut gas calculationsOut gas librariesOut gas materialPermeationPermeation calculationsRaw data gas load models© 2011 Professional Engineering Computations408VacTran 320.13 Gas load rateA gas load rate is an expression of the gas load of a material per unit surface area.example: torr-liters/second/cm2See also:Exponential out gas calculationsGas load decay timeGas load start and stop timeGas load throughput calculationsLeakLeak entry dialogOut gas calculationsOut gas librariesOut gas materialPermeationPermeation calculationsRaw data gas load models20.14 Gas load decay timeThe classical out gas formula approaches infinite gas load as time approaches zero.
Real world systems do notbehave this way. Use the decay time to ignore the theoretical gas load curve values at earlier start times. Forexample, for a decay time of 10 seconds, the calculated gas load curve will be based on an initial time of 10seconds. The curve will then be applied to the system model at the Gas load start time.The following graph illustrates the effect of varying decay times without varying the Gas Load start time.
Because ofthe decay in out gassing, the gas load curve is reduced as the decay time increases.1 s q meter of s tainles s s teelS tart time s et to 0.01 s ec ondsS top time = 1e5 s ec ondsG as load vs timeVarying G as L oad D ec ay T ime1.E + 02G as load (T orr-liters /s ec ond)1.E + 011.E + 001.E -011.E -021.E -031 s ec ond dec ay time1.E -04100 s ec ond dec ay time1.E -051.E -061.E -071.E -081.E -021.E -011.E + 001.E + 011.E + 021.E + 031.E + 041.E + 05T im e (se c onds)See also:Exponential out gas calculations© 2011 Professional Engineering ComputationsGlossaryGas load start and stop timeGas load throughput calculationsLeakLeak entry dialogOut gas calculationsOut gas librariesOut gas materialPermeationPermeation calculationsRaw data gas load models© 2011 Professional Engineering Computations409410VacTran 320.15 Gas load start timeStart time delays the addition of gas loads to the system during pump down calculations.
For example, if gas loadstart time is one hour, the calculated gas load curve will start being applied to the system after one hour.This value can be changed by clicking on the button shown, which will activate the Environment Dialog.The following graph illustrates the effect of varying start times without varying the Gas load decay time.
The samegas load curve is essentially shifted later in time.© 2011 Professional Engineering ComputationsGlossaryG as load vs timeVarying G as L oad S tart T ime1.E + 044111 s q meter of s tainles s s teelD ec ay time s et to 0.01 s ec ondsS top time = 1e5 s ec ondsG as load (T orr-liters /s ec ond)1.E + 031.E + 021.E + 011 s ec ond s tart time1.E + 00100 s ec ond s tart time1.E -011.E -021.E -031.E + 001.E + 011.E + 021.E + 03T im e (se c onds)See also:Exponential out gas calculationsGas load decay timeGas load start and stop timeGas load throughput calculationsLeakLeak entry dialogOut gas calculationsOut gas librariesOut gas materialPermeationPermeation calculationsRaw data gas load models© 2011 Professional Engineering Computations1.E + 041.E + 05412VacTran 320.16 Gas load stop timeGas load stop time is allotted time limit for all gas load calculations.
This time limit should be set by the user to begreater than the maximum pump down time for the system. If stop time is excessively high, the temporalresolution (and accuracy) of the gas load calculations will be reduced. If the stop time is less than the pump downtime for the system, it will shut off the gas load before a complete pump down curve can be generated.
This valuecan be changed by clicking on the button shown, which will activate the Environment Dialog.See also:Exponential out gas calculationsGas load decay timeGas load start and stop timeGas load throughput calculationsLeakLeak entry dialogOut gas calculationsOut gas librariesOut gas materialPermeationPermeation calculationsRaw data gas load models© 2011 Professional Engineering ComputationsGlossary41320.17 Ideal systemAn ideal vacuum system model contains no gas load. Pump down time for an ideal system will be calculatedbased on pumping only the initial volume of gas.
An ideal system will always have the best possible pump downtime, but may not necessarily be realistic because of gas load considerations.20.18 IncrementsIncrements are the number of steps to divide up a calculation. For a sequential set of calculations such as pumpdown time, this is the number of increments of pressure between the start and target pressures. More incrementsgenerally mean a more accurate calculation, but also require more time to perform. About 50 to 500 increments areusually required to get an adequately smooth curve.20.19 LeakA leak is a constant gas load that results from the entrance of gas from a small hole or crack. Leaks can bemodeled in VacTran as a constant gas load.
Real leaks are the unintentional passage of gas from outside thevacuum boundary, usually as a result of cracks in welds or faulty sealing mechanisms. Virtual leaks come fromsources other than outside the vessel boundary, and are often caused by a design flaw. For example, a bolt that isinstalled within the vacuum boundary into a blind hole will trap a small volume of gas at the base of the hole whenthe vessel is pumped.
This small volume of gas will slowly leak past the bolt threads into the vacuum system,adding to the gas load.© 2011 Professional Engineering Computations414VacTran 3See also:Exponential out gas calculationsGas load decay timeGas load start and stop timeGas load throughput calculationsLeak entry dialogOut gas calculationsOut gas librariesOut gas materialPermeationPermeation calculationsRaw data gas load models20.20 Mean free pathGas molecules collide with each other and with vessel walls as they move about.
The average distance a gasmolecule travels before it collides with something is the mean free path. If the mean free path is much less than thesmallest dimension of the pipe or vessel, intermolecular collisions will dominate flow characteristics (viscous flow). Ifthe mean free path is much greater than the smallest dimension of the pipe or vessel, molecule-wall collisions willdominate flow characteristics (molecular flow).20.21 Operating rangeOperating range usually refers to the range of pressures for which a particular pump model has data. In VacTran,which models pumps as raw data, the operating range is the maximum and minimum pressure values in the pumpmodel.
Double clicking on any pump in a system model will show the operating range of that pump.20.22 OrificeAn orifice is a conductance element consisting of a circular opening having zero depth. An orifice can have an edgeradius that affects viscous flow calculations. Orifices have a transmission probability of 1.© 2011 Professional Engineering ComputationsGlossary41520.23 Out gas materialA set of coefficients used to calculate surface desorption and evaporation. The resulting model can be combinedwith other outgassing materials and stored in an out gas library.See also:Exponential out gas calculationsGas load decay timeGas load start and stop timeGas load throughput calculationsLeakLeak entry dialogOut gas calculationsOut gas librariesPermeationPermeation calculationsRaw data gas load models20.24 Parallel conductance modelA parallel conductance model is a conductance list that can be saved as a separate file.
The individual elements inthe list are combined in parallel using the ideal parallel combination formula. Losses associated with geometrytransitions between group of parallel conductance elements and upstream or down stream conductance elementsare not accounted for in the combination formula. These losses have to be included in each conductance elementas an exit or entrance loss.20.25 PermeationPermeation, as defined in VacTran, is the flow of gas across a vacuum boundary, through solid material, from thehigh pressure to low pressure side. Unlike a Leak, which is essentially a small conductance path, permeation is amechanism of gas traveling through a solid material. In real vacuum systems, permeation can contributesignificantly to the total gas load at lower pressures.Most of the permeation contribution to a vacuum chamber constructed of common structural metals such asstainless steel or aluminum will be through elastomeric seals.
Therefore, the more penetrations, ports, doors,bellows, and windows on the vacuum vessel, the more sources of permeation. This is why some vacuum vesselsare designed with metal seals, which have much higher permeation rates than elastomer seals.The rate of permeation through a material is proportional to the pressure differential across the material, thepermeability of the material, and the total area of the material normal to the pressure differential.
The rate isinversely proportional to the thickness of the material in the direction of the flow path. A pressure differential of 760torr is most often calculated, because permeation usually doesn't significantly affect pump down time until thesystem achieves pressures below 1 torr. Therefore, for many systems, permeation calculations can be somewhatsimplified by assuming that the differential pressure is always 760 torr, or atmospheric pressure.The process of permeation consists of gas adsorption on the high pressure side of the material, diffusion throughthe material, and then desorption on the low pressure side.
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