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 Always backfill your vacuum system with dry nitrogen –Minimize surface area

Ultimate vacuum/limiting pressure System bakeout P(T) = P 0 exp(-ΔH v /R(1/T – 1/T 0 )) –Applies to evaporation –Applies equally well to desorption –Replace ΔH v with “some desorption energy”

Vacuum System Bakeout Heating tape Glass Copper Brass Aluminum

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% SiO 2 ) good to 550 C –Quartz or Vycor (96% SiO 2 ) good to 1100 C Chemically inert

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”

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

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

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

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!

Cross SectionWidth Depth (mm)(W)(L) O-ring seals Groove design criteria

Vacuum Valves Glass –Stopcocks –Ace valves –Neither is bakeable to very high temperature

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

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

Vacuum Valves: Diaphragm valve Valve seat Diaphragm!

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

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

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

Traps P(T) = P 0 exp(-ΔH v /R(1/T – 1/T 0 )) At 275 K, pump oil has very low vapor pressure –Hence the water baffle

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