vacuum system design considerations (1)
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Vacuum System Design Considerations
•Materials•Plumbing
–Pumping–Throughput
•Ultimate pressure–Dynamic equilibrium–Pumping speed–Leaks
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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
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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”
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Vacuum System Bakeout
Heating tape
Glass
Copper
Brass
Aluminum
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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
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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”
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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
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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
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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
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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!
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Cross Section Width Depth
(mm) (W) (L)
1 1.3 0.8
1.5 1.8 1.1
2 2.6 1.5
2.5 3.2 1.9
3 3.9 2.3
3.5 4.5 2.7
4 5.2 3.15
4.5 5.8 3.6
5 5.5 4.3
5.5 6 4.7
6 6.5 5
6.5 7 5.5
7 7.5 5.7
7.5 8 6.6
8 8.5 6.8
8.5 9 7.23
9 9.5 7.65
9.5 10 8.08
10 10.5 8.5
O-ring seals
Groove design criteria
http://www.oringsusa.com/html/gland_design.html
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Vacuum Valves
• Glass– Stopcocks– Ace valves– Neither is bakeable to very high temperature
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Vacuum Valves: Glass Stopcock
Outlet
Mating ground glass surfacesMust be [heavily] greased
InletThrough hole aligns with inlet (open) or doesn’t (closed)
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Vacuum Valves: “Ace Thred”
Inlet
Thread for stem drive
Stem seal O-rings
Outlet
Valve-sealingO-ring
Tapered Glass Seat
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Vacuum Valves: Diaphragm valve
Valve seatDiaphragm!
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Vacuum Valves: Bellows Valve
Valve sealO-ring
Actuator knobBellows
Valve seat
Stem seal O-ring
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Vacuum Valves: Gate Valve
Manual actuator
Sealing plate, fully retractable
Minimal reduction in throughput!
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Joinery: copper conflats
Knife edges
Copper gaskets (“conflats”)
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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
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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