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