Vacuum System Design
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Transcript of Vacuum System Design
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Vacuum System Design ConsiderationsMaterialsPlumbingPumpingThroughputUltimate pressureDynamic equilibriumPumping speedLeaks
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LeaksRealHoles in the system!VirtualSurface adsorption OutgassingHubers ruleWater desorbs very slowly from all surfacesAlways backfill your vacuum system with dry nitrogenMinimize surface area
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Ultimate vacuum/limiting pressureSystem bakeoutP(T) = P0 exp(-Hv/R(1/T 1/T0))Applies to evaporationApplies equally well to desorptionReplace Hv with some desorption energy
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Vacuum System BakeoutHeating tapeGlassCopperBrassAluminum
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Materials for vacuum systems1. What is its vapor pressure?What is its specific surface area?Typical materials of choiceGlassHardnon-porous and structurally rigidSmoothminimum specific surface areaBakeablePyrex or Kimax (70% SiO2) good to 550 CQuartz or Vycor (96% SiO2) good to 1100 CChemically inert
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Materials for Vacuum ApplicationsCeramicsElectrical insulatorsThermal insulatorsBakeable to very high temperaturesCan be machinableLavaMust be fired after machiningExpands 2% on firingMacor
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Materials for Vacuum ApplicationsStainless steel304 and 316 are idealThe chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film. [Ditto for bumpers!] The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. Non-porous, impervious to infiltrationBakeable to high temperature
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Materials for Vacuum ApplicationsAluminumMuch easier to machine than stainlessAlso forms impervious oxideStrength-to-weight ratio is greater than steelOutgassing rate is 5-10X that of stainless
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Materials for Vacuum ApplicationsBrass and copperWhats brass?Easily machinedEasily joined with soft or silver solderFittings available from commercial plumbing suppliersVolatile zinc above 200 C
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Materials for Vacuum ApplicationsPlasticsEasily formedNot bakeable to very high temperatureNylon and Delrin are most stableOutgas water and airTeflon Bakes over 200 CSoft; poor mechanical strengthPolyimide (Kapton)Very low vapor pressureUsed for tape!
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O-ring sealsGroove design criteriahttp://www.oringsusa.com/html/gland_design.html
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Vacuum ValvesGlassStopcocksAce valvesNeither is bakeable to very high temperature
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Vacuum Valves: Glass StopcockOutletMating ground glass surfacesMust be [heavily] greasedInletThrough hole aligns with inlet (open) or doesnt (closed)
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Vacuum Valves: Ace ThredInletThread for stem driveStem seal O-ringsOutletValve-sealingO-ringTapered Glass Seat
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Vacuum Valves: Diaphragm valveValve seatDiaphragm!
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Vacuum Valves: Bellows ValveValve sealO-ringActuator knobBellowsValve seatStem seal O-ring
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Vacuum Valves: Gate ValveManual actuatorSealing plate, fully retractableMinimal reduction in throughput!
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Joinery: copper conflatsKnife edgesCopper gaskets (conflats)
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TrapsP(T) = P0 exp(-Hv/R(1/T 1/T0))At 275 K, pump oil has very low vapor pressureHence the water baffle
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At 77 K, water and many other contaminants have very low vapor pressure.Hence the LN2 trap.Diff pump trapGlass in-line trap and dewar