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’ Rings Metal Seals Feedthroughs Electrical Thermocouples Vacuum Fittings and Accessories

Vacuum Fundamentals Valves Manual Pneumatic Electromagnetic 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

Vacuum Fundamentals Throughput varies with temperature and is generally specified at 20 o C 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)

Vacuum Fundamentals Conductance So if gas flows through a pipe from a pressure P1 to a pressure P2 then the pumping speeds will be:

Vacuum Fundamentals Conductance The Conductance between two points can be expressed as the quantity rate of flow divided by the pressure drop: So – What are the units of Conductance?

Vacuum Fundamentals Conductance The Conductance between two points can also be expressed in terms of Pumping Speed:

Vacuum Fundamentals Conductance If a pump of Speed S p is connected to a system through a pipe of Conductance C then then the effective Speed S c is given by:

Vacuum Fundamentals Conductance Conductance varies as the mode of flow changes Account must be taken of this when calculating conductance values

Vacuum Fundamentals

Conductance of Pipelines Continuous flow: Molecular flow:

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

Conductance of Fittings For fittings in series:

Vacuum Fundamentals Conductance of Fittings For fittings in parallel:

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: 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