vacuum fundamentals high-vacuum technology course week 7 paul nash he subject leader (engineering)
TRANSCRIPT
Vacuum Fundamentals
High-Vacuum Technology Course
Week 7
Paul Nash
HE Subject Leader (Engineering)
Vacuum Fundamentals
• Recap on last session
• Progress on assignments to date
Vacuum Technology
Vacuum Fundamentals
Joints and Seals‘O’ RingsMetal Seals
FeedthroughsElectricalThermocouples
Vacuum Fittings and Accessories
Vacuum Fundamentals
ValvesManualPneumaticElectromagnetic
Vacuum Fittings and Accessories
Vacuum Fundamentals
• System Design Considerations
Learning Objectives
Vacuum Fundamentals
System Design Considerations
Vacuum Fundamentals
The volume of a gas crossing a given point in a given period of time
This is referred to as ‘Pumping Speed’ and is normally measured in litres/second (l/s). Other units may also be encountered as below:
Volume Flow Rate
Vacuum Fundamentals
By stating the flow in terms of pressure times volume flow rate, the variation in gas density with pressure is allowed for
This gives a flow unit that relates directly to the actual quantity of gas in the flow:
Throughput
time
volumepressureThroughput
Vacuum Fundamentals
Throughput varies with temperature and is generally specified at 20oC
The standard unit is the millibar litre per second (mbar l / s), but other units are also in use as below:
Throughput
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
• The pumping speed at any point is:
• Where– Q is the Throughput (Torr l s-1 for example)– P is the Pressure at that point (Torr for example)– S is the Pumping Speed (l s-1 in this case)
P
QS
Vacuum Fundamentals
Conductance
• So if gas flows through a pipe from a pressure P1 to a pressure P2 then the pumping speeds will be:
11P
QS
22
P
QS
Vacuum Fundamentals
Conductance
• The Conductance between two points can be expressed as the quantity rate of flow divided by the pressure drop:
21 PP
QC
So – What are the units of Conductance?
Vacuum Fundamentals
Conductance
• The Conductance between two points can also be expressed in terms of Pumping Speed:
2
1
1
11
SSC
Vacuum Fundamentals
Conductance
• If a pump of Speed Sp is connected to a system through a pipe of Conductance C then then the effective Speed Sc is given by:
CS
CSS
P
Pc
Vacuum Fundamentals
Conductance
• Conductance varies as the mode of flow changes
• Account must be taken of this when calculating conductance values
Vacuum Fundamentals
Vacuum Fundamentals
Conductance of Pipelines
• Continuous flow:
• Molecular flow:
L
PDCc
45.136
L
DCm
31.12
Vacuum Fundamentals
Conductance of Pipelines
• D = pipe bore in cm (assuming circular cross-section)
• P = average pressure in pipe in mbar • L = pipe length in cm• C = conductance in ls-1
• A graph can be used to estimate conductances
Vacuum Fundamentals
Vacuum Fundamentals
Conductance of Fittings
• For fittings in series:
..........1111
321 CCCCTOTAL
Vacuum Fundamentals
Conductance of Fittings
• For fittings in parallel:
..........321 CCCCTOTAL
Vacuum Fundamentals
Conductance
• Effect of an orifice on pumping speed:
How much faster is pump B?
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
Conductance
• Conductance varies dependant on:– Pressure region– Straight or bent pipe– Type of gas (molecular weight)– Temperature of gas– Length of pipe or fitting– Surface finish– Diameter
Vacuum Fundamentals
System layout
Poor
Good
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 layout
Vacuum Fundamentals
System layout