Post on 20-Jul-2016
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) = P0 exp(-ΔHv/R(1/T – 1/T0))
– Applies to evaporation– Applies equally well to desorption– Replace ΔHv 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% SiO2) good to 550 C– Quartz or Vycor (96% SiO2) 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 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
Vacuum Valves
• Glass– Stopcocks– Ace valves– Neither is bakeable to very high temperature
Vacuum Valves: Glass Stopcock
Outlet
Mating ground glass surfacesMust be [heavily] greased
InletThrough hole aligns with inlet (open) or doesn’t (closed)
Vacuum Valves: “Ace Thred”
Inlet
Thread for stem drive
Stem seal O-rings
Outlet
Valve-sealingO-ring
Tapered Glass Seat
Vacuum Valves: Diaphragm valve
Valve seatDiaphragm!
Vacuum Valves: Bellows Valve
Valve sealO-ring
Actuator knobBellows
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) = P0 exp(-ΔHv/R(1/T – 1/T0))• 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