vacuum fundamentals high-vacuum technology course week 8 paul nash he subject leader (engineering)
Post on 17-Dec-2015
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Vacuum Fundamentals
Conductance
• Resistance to gas flow of the components has an influence on pumping speed and ultimate pressure obtainable
• Every component in the system has a volume to be pumped and in addition gives some resistance to gas flow
Vacuum Fundamentals
Conductance
• Components can include (in addition to the process chamber itself)– Valves– Gauge head fittings– Pipelines and fittings
• Each of these has a ‘Conductance’ and is generally in manufacturers data– This is the inverse of resistance
Vacuum Fundamentals
Conductance
• Based on the equation we saw earlier: CS
CSS
P
Pc
121.116.11300
6.11300
lsSc
161.116.1130000
6.1130000
lsSc
Pump A
Pump B
Vacuum Fundamentals
Volume• The volume of the system is really the sum of
the parts – not just the chamber– Pipelines can have a significant impact– Some valves may have long flow paths– Extended tubulation should be avoided – remember
gauge head mounting?
Vacuum Fundamentals
• System Design• Gas and Vapour Load• Outgassing• Baking• Pump down time
Vacuum Technology
Vacuum Fundamentals
Gas and Vapour Load
• A significant factor in the selection of a pumping system
• The total gas and vapour load is affected by the following:
Vacuum Fundamentals
Gas and Vapour Load
1. Volume of System
2. Materials of construction and surface finish
3. Leakage into the system
4. Permeation and diffusion through walls and seals
5. Outgassing of process material
6. Back migration from pump
Vacuum Fundamentals
Gas and Vapour LoadLeakage Outgassing
Residual gas
To pumping system
Process load (vapour, trapped volumes etc.)
Back streaming
Vacuum Fundamentals
Gas and Vapour Load
• Vapour pressure of materials used must be much less than the required ultimate pressure
• Surfaces should produce minimum outgassing by being cleaned and preferably polished to reduce surface area
• Surfaces should not be affected by exposure to atmosphere (rust etc.)
Vacuum Fundamentals
Gas and Vapour Load
• Most common substance on surfaces is water vapour. Sytems under vacuum can be raised to atmosphere using an inert gas such as dry nitrogen. This helps subsequent pumpdown
• Baking will speed up the outgassing process – essential to achieve UHV
Vacuum Fundamentals
Outgassing
• The spontaneous evolution of gas from a material – becomes a significant factor as pressure reduces
• The outgassing rate is the quantity of gas given off per unit time by every unit of surface area in the system
• Usually expressed as mbar litres per second per cm2 mbar l s-1 cm-2
Vacuum Fundamentals
Outgassing
• Outgassing rate decreases with time and eventually reaches a constant value (typically after 4 hours)
• Polymers and elastomers outgas at rates 100s of times higher than most metals and glass
• Typical outgassing rates are shown on the next slide:
Vacuum Fundamentals
Outgassing
• Outgassing rate decreases with time and eventually reaches a constant value (typically after 4 hours)
• Polymers and elastomers outgas at rates 100s of times higher than most metals and glass
• Typical outgassing rates are shown on the next slide:
Vacuum Fundamentals
Outgassing
p
totult S
QP
..........2211 AqAqQtot • Total Gas Load
• Pult = Ultimate pressure
• Sp = Pump speed
• q = Outgassing rate of material 1, 2 etc.• A = Area of material 1, 2 etc.
Vacuum Fundamentals
Outgassing
• Examples:Process chamber of mild steel has surface area
50000 cm2
Seals made of Viton with surface are 250 cm2
Pump speed 280 ls-1
What is the pressure after 4 hours – (assuming no leakage)?
Vacuum Fundamentals
Outgassing
• Examples:Now assume the same chamber is made of
polished Stainless Steel
What is the pressure after 4 hours – (assuming no leakage)?
Vacuum Fundamentals
UHV System Design
• Eliminate elastomers, hydrocarbons and greases
• Avoid use of poor outgassing materials – eg: mild steel, porous materials, poor surface finishes
• Bake the system
Vacuum Fundamentals
UHV System Design
• Eliminate materials that cannot be baked
• Use clean techniques – gloves, clean atmosphere
• Stainless steel is popular– Low outgassing– Doesn’t readily corrode– Can be baked to 750oC
Vacuum Fundamentals
Baking
• Typical bakeout temperatures between 250oC and 450oC
• Maximum baking temperature may be limited by materials
• All components should be baked to similar temperatures – gases will transfer from hot areas to cooler areas
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