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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

Vacuum System Design Considerations

Materials

Plumbing

Pumping

Throughput

Ultimate pressure

Dynamic equilibrium

Pumping speed

Leaks


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
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
Vacuum System Bakeout

Heating tape

Copper

Brass

Glass

Aluminum


Materials for vacuum systems
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
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 applications1
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 applications2
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 applications3
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 applications4
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!


Vacuum system design considerations

O-ring

seals

Groove design criteria

http://www.oringsusa.com/html/gland_design.html


Vacuum valves
Vacuum Valves

  • Glass

    • Stopcocks

    • Ace valves

    • Neither is bakeable to very high temperature


Vacuum valves glass stopcock
Vacuum Valves: Glass Stopcock

Mating ground glass surfaces

Must be [heavily] greased

Outlet

Through hole aligns

with inlet (open) or

doesn’t (closed)

Inlet


Vacuum valves ace thred
Vacuum Valves: “Ace Thred”

Thread for stem drive

Stem seal O-rings

Inlet

Valve-sealing

O-ring

Tapered Glass Seat

Outlet


Vacuum valves diaphragm valve
Vacuum Valves: Diaphragm valve

Valve seat

Diaphragm!


Vacuum valves bellows valve
Vacuum Valves: Bellows Valve

Valve seal

O-ring

Valve

seat

Stem seal

O-ring

Bellows

Actuator knob


Vacuum valves gate valve
Vacuum Valves: Gate Valve

Sealing plate, fully retractable

Manual actuator

Minimal reduction in throughput!


Joinery copper conflats
Joinery: copper conflats

Knife edges

Copper gaskets (“conflats”)


Traps
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
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