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Vacuum System Design Considerations

Vacuum System Design Considerations. Materials Plumbing Pumping Throughput Ultimate pressure Dynamic equilibrium Pumping speed Leaks. Leaks. Real Holes in the system! Virtual Surface adsorption Outgassing Huber’s rule Water desorbs very slowly from all surfaces

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Vacuum System Design Considerations

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  1. Vacuum System Design Considerations Materials Plumbing Pumping Throughput Ultimate pressure Dynamic equilibrium Pumping speed Leaks

  2. Leaks • Real • Holes in the system! • Virtual • Surface adsorption • Outgassing • Huber’s rule • Water desorbs very slowly from all surfaces • Always backfill your vacuum system with dry nitrogen • Minimize surface area

  3. Ultimate vacuum/limiting pressure • System bakeout • P(T) = P0 exp(-ΔHv/R(1/T – 1/T0)) • Applies to evaporation • Applies equally well to desorption • Replace ΔHv with “some desorption energy”

  4. Vacuum System Bakeout Heating tape Copper Brass Glass Aluminum

  5. Materials for vacuum systems • 1. What is its vapor pressure? • What is its specific surface area? • Typical materials of choice • Glass • Hard…non-porous and structurally rigid • Smooth…minimum specific surface area • Bakeable • Pyrex or Kimax (70% SiO2) good to 550 C • Quartz or Vycor (96% SiO2) good to 1100 C • Chemically inert

  6. Materials for Vacuum Applications • Ceramics • Electrical insulators • Thermal insulators • Bakeable to very high temperatures • Can be machinable • “Lava” • Must be fired after machining • Expands 2% on firing • “Macor”

  7. Materials for Vacuum Applications • Stainless steel • 304 and 316 are ideal • “The 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 infiltration • Bakeable to high temperature

  8. Materials for Vacuum Applications • Aluminum • Much easier to machine than stainless • Also forms impervious oxide • Strength-to-weight ratio is greater than steel • Outgassing rate is 5-10X that of stainless

  9. Materials for Vacuum Applications • Brass and copper • What’s brass? • Easily machined • Easily joined with soft or silver solder • Fittings available from commercial plumbing suppliers • Volatile zinc above 200 C

  10. Materials for Vacuum Applications • Plastics • Easily formed • Not bakeable to very high temperature • Nylon and Delrin are most stable • Outgas water and air • Teflon • Bakes over 200 C • Soft; poor mechanical strength • Polyimide (Kapton) • Very low vapor pressure • Used for tape!

  11. O-ring seals Groove design criteria http://www.oringsusa.com/html/gland_design.html

  12. Vacuum Valves • Glass • Stopcocks • Ace valves • Neither is bakeable to very high temperature

  13. Vacuum Valves: Glass Stopcock Mating ground glass surfaces Must be [heavily] greased Outlet Through hole aligns with inlet (open) or doesn’t (closed) Inlet

  14. Vacuum Valves: “Ace Thred” Thread for stem drive Stem seal O-rings Inlet Valve-sealing O-ring Tapered Glass Seat Outlet

  15. Vacuum Valves: Diaphragm valve Valve seat Diaphragm!

  16. Vacuum Valves: Bellows Valve Valve seal O-ring Valve seat Stem seal O-ring Bellows Actuator knob

  17. Vacuum Valves: Gate Valve Sealing plate, fully retractable Manual actuator Minimal reduction in throughput!

  18. Joinery: copper conflats Knife edges Copper gaskets (“conflats”)

  19. Traps • P(T) = P0 exp(-ΔHv/R(1/T – 1/T0)) • At 275 K, pump oil has very low vapor pressure • Hence the water baffle

  20. At 77 K, water and many other contaminants have very low vapor pressure.Hence the LN2 trap. Diff pump trap Glass in-line trap and dewar

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