Know All About Diffusion Pump Part-3

 

Q.21 What is the alternative of CVC4 diffusion pump oil.

Ans. Supervac Industries, New Delhi manufactures silicone diffusion pump oil SV-SIGMA. This oil is a perfect alternative of CVC4. Infact, SV-SIGMA has a much larger satisfied customer base and has been in use much before CVC4 started getting promoted.

 

Q.22 Can somebody supply me DC-702/DC-704/DC-705 diffusion pump oil?

 

 

Ans. Dow Corning the company that manufactured DC-702/DC-704/DC-705 has stopped production of this oil since last 2 years as such this oil is not available. Supervac Industries a leading supplier of vacuum oils worldwide supplies the following-

1) SV-SIGMA FAST which is an alternative of DC-702.

2) SV-SIGMA which is an alternative of DC-704.

3) SV-SIGMA PLUS which is an alternative of DC-705.

 

Even when DC-702/DC-704/DC-705 was available a large number of customers were using SV-SIGMA, SV-SIGMA+ and SV-SIGMA FAST because of considerable technological and cost advantage.

 

Q.23 Do I need to clean jets of my diffusion pump, every time I change diffusion pump oil in it?

Ans. If you are using good quality silicone diffusion pump oil then you need not waste precious machine time in cleaning of jets every time you change diffusion pump oil. Cleaning them once in about 5 to 7 years is enough.

Q.24 I am using silicone diffusion pump oil. Can I top up with hydrocarbon diffusion pump oil?

Ans. No, Silicone diffusion pump oil should never be mixed with hydrocarbon diffusion pump oil. So if you are using silicone diffusion pump oil then top up with silicone diffusion pump oil and if you are using hydrocarbon diffusion pump oil then top up with hydrocarbon diffusion pump oil.

Q.25 What should be the basic properties of silicone diffusion pump oil?

Ans. Silicone diffusion pump oil should have very low vapor pressure, should have high thermal stability and should be resistant to oxidation. These are very basic properties of silicone diffusion pump oil.

 

Q.26 What should the be basic properties of Hydrocarbon diffusion pump oil?

 

Supervac Hydrocarbon Diffusion Pump Oil

 

Ans. Hydrocarbon D.P. Oil should have very low vapor pressure, should have high thermal stability and should be resistant to oxidation. These are very basic properties of hydrocarbon D.P. oil. Apart from these qualities, hydrocarbon diffusion pump oil should not have aromatic compounds and artificial additives as these may turn to gel form or black tar like substance and harm the pump interiors.

Q.27 What is Holding Pump?

Ans. Holding pump is a simple rotary vacuum pump of small capacity. Its function is to create holding vacuum for diffusion pump, hence the name Holding Pump. It is invariably connected to diffusion pump and not to the vacuum chamber.

 

Q.28 Can I use diffusion pump without using a rotary pump in my vacuum system?

Ans. No, Diffusion pump cannot be used alone. Reasons are explained below.

A diffusion pump merely creates a vacuum gradient along its length where vacuum in top portion connected to vacuum chamber is very high and vacuum at the bottom of diffusion pump is low. Since even this low vacuum is better than atmospheric pressure so diffusion pump cannot discharge in atmosphere. Rotary pump therefore is added to evacuate the air from bottom portion of diffusion pump.

Q.29 What is stalling of Diffusion Pump?

Ans. A Diffusion pump works on the principle of gas diffusion. This pump requires Diffusion Pump Oil, which evaporates, gets compressed and then escapes through jets, trapping air on its path down to the bottom of Diffusion Pump and from there the air is evacuated by a Rotary Pump or a Booster Pump-Rotary Pump combination.

For this arrangement to work, vacuum of minimum 10^-1Torr in required to be maintained in the main body of Diffusion Pump. Otherwise the Diffusion Pump Oil escaping from jets in gaseous form collides with too much air available and further raises pressure in Diffusion Pump body. This stops creation of vacuum and is called Stalling of Diffusion Pump.

Q.30 What is the work of baffles in a diffusion pump?

Baffle in diffusion pump

 

Ans. Baffles are provided at the top of diffusion pump. They stop diffusion pump oil from escaping into vacuum chamber. Baffles should be properly water cooled to perform their function effectively.

 

 

 

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Know All About Diffusion Pump Part-2

Q.11 How to prevent back streaming of diffusion pump oil?

Diffusion pump diagram 

Ans. To prevent back streaming of Diffusion Pump oil it must be ensured that-

1.     System is "Roughed out" before Diffusion Pump is put in line.

2.     Rotary vacuum pump is maintaining a vacuum of at least 10^-2 Torr.

3.     If process is causing release of a large amount of gas, then sometimes it     may be required to slow down the process.

4.     Cooling water output volume and temperature must be checked. If this is not as per the manufacturer's recommended values then descaling and increasing cooling water circulation is required.

5.     If problem of back streaming is frequent then increasing the capacity of backing pump is required to solve it.

6.     Diffusion pump must have a cryotrap or at least baffles on top to trap gasses trying to flow out of pump into chamber. 

7.     All the valves should be checked from time to time.

8.     Strange as it may sound, very low fore pressure can also cause back streaming.

Q.12 Why is diffusion pump water cooled?

Cooling coils around diffusion pump

Ans. A diffusion pump has no moving parts and therefore depends on diffusion pump oil for creation of vacuum by action of gas diffusion.

For diffusion to occur the diffusion pump oil is heated and vaporized using heaters. It is then released through jets at high speed and in this way it traps air by diffusion process. Then it is cooled at walls of diffusion pump. This liquefies the oil vapors and these vapors again turn to liquid state. This completes the cycle and the process repeats continuously.

Q.13 I have a Chinese diffusion pump (Not Varian) do I need to use some different kind of diffusion pump oil?

Ans. Diffusion pumps as well as Diffusion pump oils are made as per a standard engineering practice of interchangeability. So a good Diffusion pump oil will be suitable for any Diffusion pump made anywhere in the world.

Just like same petrol works for cars made by Honda, Suzuki, Audi, Toyota or any other brand.

Q.14 What is the difference between Vapor Jet Booster Pump (Ejector Pump) and Diffusion Pump?

Ans. Although vapor jet booster or ejector pump works on the same principle as diffusion pump but there are many differences, these are given below:--

1. The jet design of diffusion pump and vapor jet booster pump are different. In diffusion pump oil vapors are compressed to a higher compression ratio compared to ejector pump.

2. Because of higher compression ratio diffusion pump is capable of producing vacuum up to 1x10^-10 Torr. Where as an ejector pump cannot produce vacuum above 1x10^-5 Torr.

3. Vapor jet booster pump creates vacuum in a shorter time than diffusion pump.

4. In an ejector pump the working fluid i.e. vapors jet booster pump oil has lower boiling temperature compared to diffusion pump oil used in diffusion pump.

                                                                                                             

Q.15 When Should I check the diffusion pump oil level? In hot condition or cold condition?

Ans. Level of diffusion pump oil should always be checked when pump is cold. In hot condition some oil will be in vapor state so correct level will not be indicated.

Q.16 While changing diffusion pump oil should we wait till pump cools down or is this not required?

Ans. You should always wait for diffusion pump oil to cool down to room temperature before taking it out of diffusion pump. Diffusion pump oil when heated is at a temperature above 200^oC. Trying to take out oil at this temperature can be very dangerous.

Q.17 What is shelf life of silicone diffusion pump oil?

SV-SIGMA 

Ans. If stored properly, shelf life of Silicone diffusion pump oil is 2 years.

Q.18 Which oil is more economical to use hydrocarbon DP oil or silicone diffusion pump oil?

Ans. Under normal circumstances silicone diffusion pump oil works out cheaper because of its long life and no requirement of maintenance. However if plant is very old or process involves emission of corrosive gasses, then hydrocarbon oil may work out cheaper. 

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Q.19 Why is it important to check level of diffusion pump oil regularly?

Ans. In case level of diffusion pump oil reduces then thermal equilibrium in diffusion pump is disturbed. This causes Diffusion Pump Oil to overheat, which results in oil loss, loss of vacuum and damage to diffusion pump in extreme cases. It is therefore important to check the level of diffusion pump oil regularly and top it up if required.

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Q.20 Can I reuse diffusion pump oil?

Used diffusion pump oil

Ans. No, diffusion pump oil cannot be reused. This is because while creating vacuum diffusion pump oil is subjected to very high temperature and extreme vacuum. This destroys the molecular structure of oil which cannot be corrected by filtration or distillation.

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