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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Know All About Diffusion Pump Part-1

Q.1 How does a diffusion pump work?

Ans. A diffusion pump works on the principle of gas diffusion to create vacuum. In a diffusion pump, diffusion pump oil is heated to its boiling temperature. Vapor of this oil is compressed in a vertically tapering hollow cone as it moves up. Simultaneously this oil vapor escapes from jets along the height of hollow cone. Oil vapor escaping at great speed traps air and then collides with water cooled wall of Diffusion Pump and moves down. Trapped air is released at bottom of Diffusion Pump. This creates differential pressure in diffusion pump. Pressure at top is minimum (high Vacuum) and at bottom is maximum (low Vacuum) from bottom of diffusion pump the air is sucked by rotary pump.

Q.2 Is a diffusion pump made only in cylindrical shape?

Ans. Diffusion Pump has been successfully made in rectangular shape also. In fact rectangular shaped Diffusion Pump has an advantage that its inlet connected to Vacuum Chamber can be of larger area and Diffusion Pump itself can be adjusted to occupy less space. This also works with Diffusion Pump Oil as a working medium just like cylindrical Diffusion Pump.

Q.3 I understand that Turbo molecular pump (TMP) is an alternative to diffusion pump. Is it better or worse than diffusion pump?

Turbo Molecular Pump

Ans. Turbo molecular pump works on the principal of positive displacement. In this pump a series of inclined blades are mounted on a shaft which rotates at very high speed. These are called Rotors. Fixed blades or stators are fixed in such a way that Rotors and Stators alternate. Because of rotation of shaft and incline rotors push air down to next row of Stators. Stators too are inclined in same direction and push this air to next row of Rotors and so on to exit, where a Rotary Pump draws away the air.

Though this pump creates good vacuum and can be used in place of Diffusion Pump but its applications are limited to critical applications like semiconductor coatings. This is because of its high cost, more requirement of maintenance and the fact that since this pump cannot be made of large size because of mechanical considerations, a diffusion pump can be replaced only by a couple of Turbo molecular pumps. Diffusion Pump on the other hand has no moving parts and requires almost no maintenance other than Diffusion Pump Oil change, is cheaper in cost and can be built in almost any size.

Q.4 Why is topping up required for Diffusion Pump Oil?

Ans. In a Diffusion Pump, DP oil evaporates to produce diffusion action. Since oil is in vapor state so part of it is lost by leakage, some of this oil vapor is also sucked away by Rotary Pump. It is therefore important that lost diffusion pump oil is leveled up by topping.

Not topping up will result in overheating of the remaining oil and loss of oil as explained above will further speed up, causing a drop in vacuum. In extreme cases Hydrocarbon Diffusion Pump Oil may burn to tar like substance and Silicone diffusion Pump oil may turn into grey colored crystals.

Read about diffusion pump maintenance here.

Q.5 Why can a diffusion pump not work alone and requires a rotary pump as backing pump?

 

Belt Driven Rotary Vacuum Pump

Ans. A diffusion pump merely creates a vacuum gradient where top of diffusion pump has a higher vacuum and bottom of diffusion pump has much lower vacuum. A diffusion pump has no moving parts to force air out, so though lower portion of Diffusion pump has a low vacuum but since it is still better than atmospheric pressure so air will not flow out.

This lower portion of diffusion pump is connected to rotary vacuum pump. Since vacuum in lower portion of Diffusion Pump is lower than what Rotary Vacuum Pump Oil is able to create so Rotary Pump evacuates the diffusion pump and diffusion pump is able to work.

Q.6 How can I get more life from my expensive Diffusion Pump Oil?

Ans. Diffusion pump oil is one of the costly consumables of the metallizer. With a few precautions, its life can be prolonged and money saved.

1. Never open the pump when the oil is still hot. Exposing the hot oil to air changes its viscosity, making it thick gel-like.

2. Take care to top up the oil in the pump in a timely way so as to avoid accidental over heating of the oil.

3. Keep a timely watch on the pressure and heater gauges.

4. Back streaming of vapors from the vacuum chamber and/or Rotary pump letting its vapors in to the Diffusion pump are two reasons for the DP oil to get contaminated. This can be prevented by installing filters above the chevron baffle of the DP and on the line in between the rotary and the diffusion pump. These filters must be cleaned from time to time.

5. Silicone DP oil decomposes on coming in contact with even trace elements of alkali metals e.g., sodium, potassium, cesium etc. Do not allow them and/or their vapors to come in contact with the DP oil.

6. Keep a check on the water flow through the cooling lines for proper cooling of the hot oil inside the diffusion pump.

Read more on this topic here.

Q.7 Do I need to open & clean jets of my diffusion pump every time I change oil in it?

Ans. If you are using a good quality silicone diffusion pump oil then there is no need of cleaning jets every time you change oil cleaning be done once in 4 to 5 years. However if you are using hydrocarbon diffusion pump oil then cleaning is a must every time you change oil.

Q.8 Is DC-702/DC-704/DC-705 Diffusion pump oil still available? If not then what is the alternative?

Ans. DC-702/DC-704/DC-705 - these oils are no longer available in the entire world as the manufacturer Dow corning stopped the production of these oils more than a year back.

Supervac Industries supplies SV-SIGMA, SV-SIGMA+ and SV-SIGMA FAST as alternatives of DC-704, DC-705 and DC-702 respectively. These oils are being used by a large number of customers in more than 20 countries.

Q.9 I observed whitish deposits in my diffusion pump. What could this be? It is serious?

Silicone crystals

Ans. On overheating silicone diffusion pump oil turns into whitish grey silicone crystals.

Yes, this is serious. You need to check cooling for diffusion pump and also make sure that level of diffusion pump oil never falls below minimum level. This requires topping up from time to time. Also a faulty temperature gauge not allowing heater cut-off to take place, can be responsible for the oil over heating.

Q.10 What is back streaming in diffusion pump?

Ans. If the pressure is too high in the diffusion pump then the emerging diffusion pump oil vapor from the jets trying to move down collides with air molecules in the pump. This results in no vapor going down the pump. Contrary to this, the vapor itself starts rising up the pump along with the air and raises pressure instead of reducing it. This undesirable process is called “back streaming”.

Because of back streaming Diffusion Pump Oil can travel to vacuum chamber or can even be removed by Rotary Vacuum pump. If it enters Rotary Pump then it contaminates Rotary Vacuum Pump Oil also.

(In the next part-2 of this series I shall explain how to prevent back streaming.)

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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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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