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