Vacuum Technology

VACUUM TECHNOLOGY-compiled by Benson .O Oderah.

Vacuum is any enclosure whose pressure is less than atmospheric pressure. (760mmHg)

APPLICATIONS OF VACUUM TECHNOLOGY

1. Electron microscopy

2. Sugar refilling

3. Vacuum filtration

4. Production of electric lamps/vacuum tubes

5. Vacuum cleaning

6. Semi conductors processing

7. Radio therapy and surgery

8. Freeze drying

9. Coating of glass

Terminologies

1. Mean free path-difference between one point to another in a vacuum system.

2. Throughput-^{pressure *volume} /_time

3. Conductance-reciprocal of resistance

4. Digass-_removal of any gas inside a pump.

5. Critical baking pressure-pressure which can be tolerated in diffusion pump to start pumping.

6. Leak route-amount of gas which enters or appears to enter the vessel in unit time.

7. Pump matching-ability of pump to work together with pressure from other pumps

8. Pumping speed-through put(^pv/_t)/^volume/_time

Types of vacuum systems

1. Static vacuum system-system pumps down, remove them and finally seal them. I.e. thermoflask and radio waves

2. Dynamic vacuum system-involves continuous evacuation. I.e. research institutions and industrial purposes

Equipment’s for evacuation/rotary vave pumps

A) ROTARY OIL SEALED PUMP.

Four stages in one cycle

Levels of vacuums

1. Low vacuum   760-25 torr

2. Rough vacuum 25-1 torr

3. High vacuum 1*10^-3-   10^-6 torr

4. Very high vacuum 10^-6-      10^-9 torr

5. Ultra high vacuum less than 10^-9 torr

Vacuum pumps

Two general classes

1. Gas transfer-physical removal of matter i.e. mechanical and diffusion pumps

2. Adsorption –entrapment of matter i.e. sublimation and ion pumps.

Rotary oil sealed pump-10^-3 torr (high vacuum)

Rotary + diffusion pump-10^-2 -    10^-6 torr

^Note. Rotary pump create vacuum, roughing the system and backing up diffusion pump.

Other classification of pumps

1. Roughing pumps i.e. rotary oil sealed pumps

2. Backing pumps i.e. diffusion pumps

3. Holding pumps

Rotary oil sealed pumps/mechanical pumps are used for;

1. Production of medium, high vacuum on their own

2. Roughing the system down to a pressure at which diffusion pump can start pumping

3. Backing up the diffusion pumps.

Diffusion pumps

Oil is boiled and vaporizes in a multistage jet assembly; oil vapour emerging from the nozzles impact momentum on the residual gas molecules and drive them towards the bottom of the pump; requires mechanical pump back streaming of vapour are some of the problems and can be min immersed with cooling coils used to condense the oil vapour before it enters the vacuum chamber.

Ion pumps

A cold cathode electric discharge creates an electron gas which is trapped by a small magnetic field. The electron gas ionizes residual gas particles in the chamber which are attracted to the cathode made of titanium.

The incident ions sputter off titanium which forms a thin film on neighboring surfaces and form a thin film on neighboring surface and form stable compound with residual gases in the chamber.

Minimizing condensing vapour in a vacuum system

1. Use of cold traps

2. Raising temperature

3. Using gas ballasting-system whereby gas is deliberated, introduced into vacuum before condensing value is formed, forcing back up trap cap to open so as to release the vapour.

Techniques used in classification of pumps

1. Positive displacement pumps-use mechanism to repeatedly expand a cavity, allow gases  to flow into the chamber ,seal the cavity and exhaust into the atmosphere.

2. Momentum transfer pumps/molecular pumps i.e. turbomolecular pumps- use high speed jets of dense fluid to knock the molecules out of the chamber.

3. Entrapment pumps- capture gases in a solid /adsorbed state (ion pumps/cryopumps)

Components of vacuum system

A)   Major components

-Vacuum vessels

-Vacuum pumps

-Connecting vessels /vacuum flanges

-Vacuum gauges

       B) Minor components

-Baffles/fore line traps

-Cold traps

-Sinks

-Vacuum system switches

-Protective devices.

Fore line traps/bafflen back streaming

-Used to prevent hydrocarbons and water vapour from back streaming

Vacuum flanges

-Used to connect vacuum chambers, tubing and vacuum pumps to each other

Vacuum valves

-Used for isolation, flow control and enabling access

Valve parts

-Valve body

-Flappers

-Actuators

Valve types

-Gate valve

-Angle and inline valve

-Ball valve

-Butterfly valve

-Leak valves

Types of vacuum oil .

-Mineral oil

-White oil

Properties of vacuum oil

1. Should be at low pressure

2. Should have a suitable viscosity

3. Should have a adequate lubricating property

4. Should be chemically suitable at pump working temperature

5. chemical/oil should not be hazardous in vacuum system.

6. Mechanically inert /unreactive pumping fluid.

Defects of diffusion pumps

1. Back migration-transfer of vapour to higher vacuum side by evaporation of molecuoles which cling to the surface of the pump.

2. Back streaming-movement of vapour molecules back into vacuum chamber.

3. leak-continuous appearance of gas in vacuum system

4. Insufficient cooling system

5. Contamination of pumping oil

6. High pressure.

Measurement of vacuum using gauges

1. Thermal conductivity gauges i.e. thermos couple gauge or pirani gauges

- pirani gauges -operation is based on thermal conductivity; a filament is heated and its temperature is measured ,temperature depends on heat lost to the environment which in turn depends on vacuum level. Works between   10-  10^-3 torr

2. Ionizing gauges i.e. cold cathode and hot ionizing gauges

-ion gauges-A filament is used to emit electrons which are attracted to a positively charged grid, inside s negatively charged collector. Electrons collide with gas molecules a round the grid and ionizes them, positively charged ions are attracted to the collector and create ionic current; works between

 10^-3 – 10 -¹⁰ torr.

Leak and leak detection

Leak refers to continuous appearance of a gas in a vacuum system

Types of leak

True/real leak-occurs due to opening in system and it is repairable

Virtual leak-caused by outgassing and it is irreparable.

Leak detection

Over pressure methods

1.wet testing –vessel is immersed in water or liquid at low surface tension such as alcohol whereby leak is detected by stream of bubbles i.e. the system vessel must be pressurized prior to application of liquid.

2. sealing compounds-in this, suspected soldered joint are covered with plasticine or a compound, if leak exist, it is them sealed and pressure reduced.

3. Sniffer technique-in this, gas issuing from leak in a pressurized system is sampled by being drawn through a flexible tube into a suitable detector, electrons ionizes gas molecules and then measured.

4. Spark coil (testa coil)-used in detecting /locating leak in glass vacuum system

The tip of coil is passed over the surface at the glass and when it comes near leak point, high frequency spark lump from the tip of coin to the leaking area.

Low pressure methods

1. discharge tubes-use change in colour when different probe gases are introduced

2. pirani and thermocouple detectors-hydrogen gas is used as probe gas

3. Ionizing detectors-butane gas is used as probe gas

4. Palladium leak detector- hot filament ionizing gauge which is sensitive only to hydrogen gas, hydrogen gas will enter the system via leak, diffuses through hot palladium and is ionized by collision with electron stream .resulting into positively charged ions being collected by electron stream which is at negative potential.

5. Halogen leak detector- halogen gas which gets into the system through the point of leakage, detected by palladium node electrons.

How vapour behave when compressed in a vacuum system

When vapour is compressed at a constant temperature; the pressure and volume decreases until saturation point is reached; further compression may cause condensation of oil vapour; any condensation of such gases/vapour may produce deteriorations of the vacuum pumps.

Difference between

1.       Fractionating pumps-have individual jets chimney for each nozzle and uses oil.

2.       Non fractionating pumps-only have one chimney and uses mercury.