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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Written by: Anshuman Punj.   

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Diffusion Pump Maintenance

The entire process of vacuum deposition rests on the foundation of efficient working of Diffusion pump. Good quality Diffusion pumps are like work horses: they are capable of functioning effortlessly for long periods of time provided they are fed nicely and taken care of. For diffusion pumps, the fodder is diffusion pump oil.

Diffusion pump oils are of two types – hydrocarbon and silicone.

Most diffusion pump users these days prefer silicone oil to hydrocarbon because of many reasons; one of them being – easy maintenance of the pump in case of silicone diffusion pump oil. Unlike hydrocarbon oil, silicone diffusion pump oil does not leave any deposits – carbon or otherwise – on the jets of the diffusion pump. Hydrocarbon oil is chosen only where cost cutting is required.

Supervac Industries manufactures both types of diffusion pump oils – Ultra high vacuum Silicone diffusion pump oil (SV-Sigma) and Hydrocarbon Diffusion Pump oil (SV-Delta).

Silicone oil should never be mixed with hydrocarbon oil. In case a pump has been running on hydrocarbon oil but now needs to be run on silicone oil, the diffusion pump chamber should be opened up, cleaned thoroughly with alcohol, other solvents, soapy water and de-ionized water. After that the opened chamber of the pump should be left to dry completely in a closed, clean environment for at least 3-4 days.   

WARNING: There should be no residual alcohol or moisture left in the pump. Reason?

1.     This can contaminate the oil that will be charged after the cleaning.

2.     Residual vapors increase the vacuum time.

In normal circumstances, if a good quality Silicone Diffusion Pump oil is being used then  diffusion pump needs to be opened up for cleaning only once every 4-5 years. Unnecessary opening up of the pump (as some freelance ‘maintenance experts’ tend to advise gullible customers) can damage the pump as they are not meant to be opened repeatedly. Also, this is an absolute waste of useful machine time and money.

All that is needed to be done for normal oil change is –

1.     Open the drain cock of the diffusion pump AFTER it has cooled down and let the oil flow out completely.

2.     Close the drain cock.

3.     Charge the new diffusion pump oil.

  

   

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                                                                                                          Written By: Anshuman Punj.

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Silicone Diffusion Pump Oil vs. Hydrocarbon Diffusion Pump Oil

Before the advent of Silicone Diffusion Pump Oils about 20-25 years ago. Hydrocarbon Diffusion Pump Oils ruled the Vacuum Industry. These oils are prepared from base oil that has been distilled from crude oil and purified by Dewaxing and Solvent extraction. This is further purified by additional vacuum distillations. Famous hydrocarbon oils brands are Supervac Hydrocarbon Diffusion Pump Oil(SV-DELTA), Apiezon C and Convoil 20. Silicone Diffusion Pump Oils on the other hand are made by controlled hydrolysis of Silanes, addition of Phenyl groups followed by Distillation. Famous Silicone Diffusion Pump oil brands are Supervac Ultra high Vacuum Silicone Diffusion Pump Oil (SV Sigma) and DC704.

 

 

SV-SIGMA

Table below gives the comparison of both these oils, this will help user decide which particular oil is better for a particular application.

Property	Hydrocarbon Diffusion Pump Oil	Silicone Diffusion Pump Oil
Ultimate Vacuum	1x 10 -6 Torr	1x 10 -8 Torr
Working Life	3-6 months	12 -18 months
Cost	Low	High
Pump Maintenance	Requires cleaning of Jets of Diffusion Pump every time oil is changed	No jet cleaning required
Vacuum Time	Fast	Very fast

 

 

 

SV-DELTA

 

As it is clear from above that Hydrocarbon Diffusion Pump Oil is suitable in following conditions------------

1)    Cost of operation has to be kept very low.

2)    Operation contaminates oil quickly.

3)    Equipment is very old therefore working life of oil and vacuum level will remain low irrespective of oil used.

For all other applications Silicone Diffusion Pump oil is suitable. Supervac Industries manufactured Silicone Diffusion Pump oil SV-Sigma is one of the best Silicone Diffusion Pump oils available in the world.

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Alternative/ Replacement of DC 704

Production of DC 704 has been discontinued. It served vacuum industry well for about 20 years. But in last 20 years technology has taken a big leap. To increase productivity Vacuum pumps need to work faster, higher level of vacuum is required by industry. Above all because of increasing competition Industry wants cost effective products. All these factors led to development of better Diffusion Pump Oils, ultimately wiping out DC 704.

An Indian company Supervac Industries has developed such a Diffusion Pump Oil. This oil is called High Vacuum Silicone Diffusion Pump Oil : SV-SIGMA.

This is next generation oil much ahead of now discontinued DC 704 in technology. Indian Vacuum Industry which is one of largest in world has been using this oil since last about 8 years. Supervac Industries also exports SV-SIGMA oil to 16 other countries.

SV-SIGMA has a very long life, fast vacuum time and gives vacuum of the order of 1 x 10^{-8 Torr}. Price is very competitive. If you are looking for an alternative to DC 704, look no further, get in touch with Supervac Industries.

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