Diffusion Pump -Overview, Parts, Working, Pumping Speed Equation.
A diffusion Pump is a type of kinetic vacuum pump.
Diffusion pumps are the first type of vacuum pump operating in the regime of free molecular flow.
The diffusion pump was invented by Wolfgang Gaede in the year 1915.
The design of the diffusion pump was based on the finding that gas cannot diffuse against a vapor stream but will move in the direction of the stream.
The diffusion pump is also known as a momentum transfer pump in some modern textbooks.
For a wide pressure range, the diffusion pump has a high pumping speed.
But when the inlet pressure is high, the pumping speed decreases until, at about 10-1 bar, it becomes very small.
This pump is widely used all over the world for industrial and research applications.
Parts Of Diffusion Pump
Pumping Chamber:
This chamber is used to decrease the pressure in the diffusion pump before diffusion pump starts working.
Body of Diffusion Pump:
The diffusion pump consists of a cylindrical body fitted with a flanged inlet.
Electrical Heater:
At the bottom of the cylindrical body, there is an electrical heater for heating the pump oil which is used to produce the high-speed pump vapors.
Pump fluid or oil:
Now pump oil is usually fluids like dc704 and dc705 which are phenyl siloxane compounds, phenyl ethers as well as fluorinated polyphenol ethers are also used as pump fluids.
The ideal pumping fluid used in a diffusion pump will be thermally stable, non-volatile, chemically inert, and non-toxic.
Foreline Baffle:
A foreline baffle or outlet duct is provided at the side of the lower pump body through which the gases are pumped out are discharged outside by means of a mechanical fore pump.
A baffle valve:
The chamber to be evacuated is separated from the diffusion pump by means of a baffle valve.
Jet forming Structure:
The jet-forming structure is the most important part of the diffusion pump.
The jet-forming structure consists of a concentric cylinder as you can see in the diagram. These concentric cylinders are partially capped.
They have flared ends all along. These flared ends help them to form an umbrella-like jet through which the pump fluid vapors or oil vapors come out at supersonic speed.
Cooling Cap:
A cooling Cap is used to cool the oil vapor inside the jet chamber.
Nozzles:
Nozzles help to convert the high-pressure steam into high-velocity steam of oil vapor.
Cold Trap:
The cold trap is an optional component that can be used to capture any backflow or residual vapor that is not pumped out by the diffusion pump. It is typically cooled with liquid nitrogen or a similar coolant.
Cooling Coils:
Cooling coils help to cool the whole system of the diffusion pump by the circulation of water. It sucks the heat from the diffusion pump.
The upper 2/3rd of the cylinder is provided with water-cooling coils or these walls of the cylinders are cooled
Maybe extra:
. There is a duct line or fore line
Working Of Diffusion Pump:
1) Before starting the pump, the pressure in both the chamber and the diffusion pump is brought down to nearly 0.1 Torr before the diffusion pump starts working.
After the initial pumping or reduction of pressure on both sides i.e at the vacuum chamber and at the pump side, the valve to the fore pump is closed and the baffle valve is opened so that only the vacuum chamber is in connection with the diffusion pump.
2) At the start, the pump fluid is heated by means of the electric heating coil and the vapor steam rises up.
3) The vapor steam rises up towards the jet-forming structure and as the heat provided by the electric heater increases, the pressure in the jet-forming structure or jet chamber increases and the oil vapor comes out along the flared ends or nozzles at supersonic speeds.
4) At the same time, the gas molecules from our vacuum chamber which is to be evacuated will come down from the inlet of the pump and they get trapped with these oil vapors.
5) The combination of gravity and the downward direction of the vapor moves the gas molecules toward the bottom of the pump.
6) Any gas molecule that tries to go upward will be pulled down or forced downward again by the oil vapors and the oil vapor together with the gas molecule coming from the vacuum chamber will come out in the form of jets of vapors and they will strike the body of the cylinder which is already cooled by the water cooling coils.
The gas molecules gain additional momentum as they get trapped with the oil vapors coming in form of a jet.
7) The cooling coils will condense the oil vapors and the oil vapors will return to the reservoir where the pump oil is kept and the gas molecules will be pumped out by using the backing pump which will suck the gas molecules.
Pumping Speed Of Diffusion Pump:
The pumping speed of the diffusion pump depends on the following factors:-
1) The position and dimension of the high vacuum stage.
2) The velocity of the pump fluid vapor.
3) The mean molecular velocity of the gas which is to be pumped (C).
C is calculated as:
The maximum specific pumping speed at room temperature on pumping air is calculated as:
The diffusion pump has a high pumping speed for lighter gases as compared to heavier gases.
Application Of Diffusion Pump:
1) It is used to create a high vacuum in industrial vacuum processing.
2) It is also used in mass spectrometry, research and development, analytical instrumentation, and nanotechnology.
3) It is also used in purification degassing and vacuum distillation.
Advantages of Diffusion Pump:
1) Since there are no moving parts in diffusion pumps operate practically without noise.
2) Diffusion pumps are relatively low costs to operate.
3) Diffusion pumps can create very high vacuum levels up to 10-9 torr which is much lower than other types of vacuum pumps.
4) Diffusion pumps have low backstreaming rates which means the vacuum chamber in the diffusion pump is less likely to contaminate.
5) Diffusion pump can handle reactive gases such as oxygen and nitrogen without damaging the pump.
Disadvantages of Diffusion Pump:
1) Diffusion cannot be used to pump some gases such as hydrogen and helium.
2) If the oil in the diffusion pump gets degraded, it can lead to the contamination of the vacuum chamber.
3) It requires a high temperature from 150°C to 250°C to operate properly, which is hard to attain in some situations.
