Vacuum System Design Considerations

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