How To Get Maximum Life From Your Diffusion Pump Oil

Diffusion Pump Oil is one of the costliest consumables required by vacuum users. In the present day scenario of cut throat competition, engineers are under a lot of pressure to reduce costs without compromising on quality. Based on my experience of almost 20 years in the vacuum line, I have compiled a list of do's and don'ts as to how one can get maximum life from the diffusion pump oil. For the benefit of our valued customers, the facts are given below -

1.   Topping up at regular intervals is most important factor in getting maximum life from Diffusion Pump Oil. Care should be taken to top up the oil in the pump in a timely manner. In the absence of topping up, volume of oil in the Diffusion Pump reduces, whereas heaters keep producing the same amount of heat. This causes overheating and accelerated loss of DP oil. This further reduces the volume of oil, which in turn causes more overheating and loss of DP oil is further accelerated and so on.

Diagram of Diffusion Pump

2.   Keep track of the water flow through the cooling lines. Check the inlet and outlet water temperature as well as the flow rate. Lines can become clogged or scaled over time. As Diffusion Pump oil vapor can only condense on coming in contact with cold walls of DP. Any reduction in cooling causes overheating and increase of pressure inside DP. This may result in loss of oil by back streaming.

3.   Strange as it may sound a very low fore- pressure can also cause loss of DP oil. This can be prevented by switching on heaters before fore pressure becomes too low.

4.   Faulty gauges for pressure/heater too, can cause accidental overheating of the oil or the oil not getting the required heating. Check that all the gauges are functioning properly.

5.   Silicone diffusion Pump Oils have a life of about 4 times that of Hydrocarbon Diffusion Pump Oils. Also Silicone Diffusion Pump oil does not blacken and clog the jets of DP so Silicone DP oil works out more economical in the long run.

6.   Poorly working Rotary Pump will put extra burden on diffusion pump. It must be ensured that Rotary pump is working properly to get full life from Diffusion Pump Oil.

7.   Do not expose the oil to air when it is heated. Doing so will affect the viscosity of the oil – it thickens and vacuum reduces. To avoid air exposure valves must be checked from time to time.

8.   To avoid having the oil migrate into the vacuum chamber, a cold cap should be fitted at the top of the nozzle assembly inside the diffusion pump. This cold cap condenses the vapor in that area.

9.   A concentric circular chevron baffle should be present at the mouth of the pump. It allows air molecules to wander in, but traps the heavier oil vapors as they try to escape.

10. There should be no gross vacuum leaks as this will result in deterioration of oil quickly. Vacuum leaks also cause the pressure to drop and in extreme cases may cause stalling of Diffusion Pump. Stalling will cause loss and deterioration of diffusion pump oil. Such leaks can be plugged by applying silicone vacuum grease to O rings and joints.

11. Sometimes back streaming of contaminants from the vacuum chamber takes place causing the oil to form sludge. This is preventable by installing filters above the chevron baffle of the diffusion pump.

12.    Silicone diffusion pump oil starts to decompose if it comes in contact with even trace elements of alkali metals (e.g. hydroxides  of sodium, potassium, cesium etc.). If process requires use of these then care should be exercised to prevent them reaching silicone DP oil. 

13.  In processes like PVD, coatings requiring plasma or transparent metalizing, gas load can cause vacuum of diffusion pump to fall below 10^-1Torr. This can result in back streaming of DP Oil or even stalling. This is preventable by reducing the process speed or increasing the capacity of backing pumps.

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Boiling Temperature of Diffusion Pump Oil Explained

A lot of confusion exists in the vacuum industry about certain things, for example-

·         What should the optimum temperature be set in diffusion pump heaters so as to get maximum level of vacuum in shortest possible time?

·         How can the temperature in diffusion pumps be set in such a way that the engineers are able to get maximum life from costly diffusion pump oil?

·         Is boiling temperature of silicone diffusion pump oil different from hydrocarbon diffusion pump oil?

Before attempting to understand optimum boiling temperature of diffusion pump oil, let us take a brief look about working of diffusion pump.

Since efficiency of diffusion pump depends solely on the working cycle of diffusion pump oil (diffusion pump has no moving parts) so this is evident that a higher diffusion pump temperature will mean a higher ultimate vacuum. Obviously a question comes to mind at this stage that why don’t we increase the boiling temperature to highest possible level. Answer to this is that increasing the temperature of diffusion pump oil involves financial implications (cost increase) because of the following reasons -

1.   Diffusion pump oil works in very difficult environment where there is vacuum and high temperature. So to increase the boiling temperature, superior (obviously costlier) oil will be required which is capable of withstanding these adverse conditions.

2.   At higher temperature O-rings and oil seals may give way so better quality sealing material is required (Costly again).

3.   For handling higher temperature, construction of diffusion pump also has to be stronger.

4.   For attaining higher temperature higher energy charges will be incurred.

5.   For obtaining higher temperature longer time is required so vacuum time increases.

Decision about boiling temperature is basically a balance between level of vacuum required and cost incurred. Our guideline in line with the established practice in the industry is as below -

S. No.	Vacuum Required	Temperature in Diffusion Pump	Supervac Products	Equivalent Products
1.	10-9 to 10-10 Torr	245oC	SV-SIGMA+	DC-705
				Hivac F5
2.	10-7 to 10-8 Torr	220oC	SV-SIGMA	DC-704
				CVC4
				Hivac F4
				Ulvoil D31
3.	10-5 to 10-6 Torr	180oC	SV-SIGMA FAST	DC-702

Some myths are prevalent in the industry about diffusion pump oils. These are clarified as under-

Myth no.1: Boiling temperature of hydrocarbon diffusion pump oil is different from boiling temperature of silicone diffusion pump oil- This myth is baseless as boiling temperature of both hydrocarbon and silicone diffusion pump oil is same. For example- Well known hydrocarbon diffusion pump oils like Supervac SV-DELTA, Lion S, Ulvoil D11 and CVP300, all have boiling temperature of 220^oC.

Myth no.2: My diffusion pump has been designed for hydrocarbon/silicone diffusion pump oil only- This is also false. You can use any diffusion pump oil (hydrocarbon or silicone) in any diffusion pump based on your requirement. For details about merits of hydrocarbon and silicone diffusion pump oil please read my blog post-http://supervacindustries.blogspot.in/2013/04/silicone-diffusion-pump-oil-vs.html

Myth no.3: My equipment manufacturer asks me to use only a particular brand of diffusion pump oil which is exorbitantly priced- Your equipment manufacturer is trying to fleece you. For details about equivalent vacuum oils manufactured by reputed and established manufacturers anywhere in the world please read our blog post at- http://supervacindustries.blogspot.in/2014/03/vacuum-oil-equivalents.html. In case you don’t find it there then please contact us. We will help you find an equivalent product.

Myth no.4: There is no need of topping up- Absolutely false and dangerous myth prevailing in the industry. Topping up at regular intervals is of utmost importance to get proper vacuum and avoiding damage to the equipment.

Myth no.5: Charging less diffusion pump oil in diffusion pump reduces vacuum time- Absolutely false. Charging less oil than required will cause back streaming, burning of diffusion pump oil and damage to costly equipment.

Myth no.6: My diffusion pump oil works at 170^oC- Diffusion pumps do not work at temperature below 180^oC. Your oil supplier is supplying you cheaper vapor jet booster pump oil.

Conclusion:-

Based on the information given above I am sure that you will be able to choose the right diffusion pump oil as per your requirement. For example- if level of vacuum required is only up to 10^-5 Torr and equipment is old then hydrocarbon diffusion pump oil will work more economical. Similarly if vacuum level required is 10^-7 Torr then there is no point in using SV-SIGMA+. Furthermore by using simple precautions like frequent topping up you can get a much longer life and value for money from your diffusion pump oil.

For more helpful tips about how to get your money’s worth from your diffusion pump oil, you can read my blog post- http://supervacindustries.blogspot.in/2013/09/how-to-get-maximum-life-from-your.html

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Why is Cleaning of Vacuum Chamber Required?

Vacuum Chambers

It is but natural that during the process, material being coated on substrate will fall off in the vacuum chamber. For example, Aluminum heated and evaporated to be coated on BOPP film (film metallizing) or on some object (object metallizing) will fall off on vacuum chamber and shields. Being in molten state this will weld to the chamber surface. (PHOTO-1)

Metallizing in the Vacuum Chamber (Photo-1) 

After a number of work cycles there will be a buildup of this welded metal which will have large gaps capable of trapping a lot of air (PHOTO-2).

 

Photo-2

As it is evident the porosity holding air will be zigzag as it has been formed randomly. Pumping system will find it very difficult to remove this trapped air. Hence ultimate vacuum will drop and vacuum time will increase. It is important therefore that this build up of molten metal is removed from time to time so that trapped air inside vacuum chamber remains to minimum level.

Naturally a question comes to mind that why don’t plant manufacturers provide shields all around the target so that the extra metal dropping and welding to vacuum chamber is eliminated. The problem is that excessive shielding will in itself become an air trap and will result in what is called a virtual leak (details at http://supervacoils.com/leakage-detection-and-prevention-in-vacuum-systems/). This virtual leak itself is capable of slowing down the vacuum time considerably. Therefore this is impractical.

Production engineers therefore are left with no choice but to clean the vacuum chamber periodically.

Various means of achieving this cleaning are discussed below----------------------

1.    Boron Nitride Suspension: -

In this method Boron Nitride Suspension/Paste is applied in vacuum chamber on parts where stray metal can fall. Boron Nitride Suspension adheres to applied surface and forms a protective coating. This coating acts as a physical barrier and prevents welding of stray metal to vacuum chamber. At the time of cleaning it is very easy to remove stray metal that has fallen in the chamber; it is just swept off!

Alternatively some users mix Boron Nitride Powder with water and then apply it to the required surface.

Advantages: - Advantage of Boron Nitride Suspension/Paste is that it is easy to use, neat in use, has very good lubricity and is therefore very effective (Fig.- a). It has good adhesion properties so it easily sticks to the applied surface. Boron Nitride Suspension is also inert and is very easy to remove (normally removed with a wet cloth Fig.- b).

 

Disadvantages: - Boron Nitride Suspension is expensive and mildly hygroscopic which causes it to absorb moisture.

2.    Graphite Suspension: -

Application of graphite suspension/paste is very similar to BN suspension/paste. In this method Graphite Suspension/Paste is applied in vacuum chamber on parts where stray metal can fall. Just like BN suspension, it adheres to the applied surface and forms a protective coating. This coating acts as a physical barrier and prevents welding of stray metal to the vacuum chamber. At the time of cleaning it is very easy to remove the stray metal that has fallen in the chamber, it is just swept off!

Advantages:- Advantage of Graphite Suspension are that it is easy to use, has very good lubricity and is therefore very effective; has good an adhesion properties so it easily sticks to the applied surface (Fig.-c). Graphite Suspension is inexpensive and is very easy to remove (normally removed with a wet cloth Fig.-d).

 

Disadvantages: - Graphite Suspension/Paste is somewhat messy to use and mildly hygroscopic which causes it to absorb moisture.

3.   Caustic Soda (NaOH):-

In this method, after vacuum is broken, the chamber is opened and Aqueous Caustic soda is applied to remove deposited Aluminum. Caustic soda reacts with aluminum and therefore removes it from chamber surface by chemical reaction.

2Al + 2NaOH + 2H₂O ------------> 2NaAlO₂ + 3H₂

Advantages: - Main advantage of caustic soda is that it is applied after operation is over and machine is idle. So it does not interfere in the vacuum process.

Disadvantages: - Caustic soda is polluting and a very hazardous chemical and needs to be handled very carefully.

 

Reaction of aluminum with caustic soda is exothermic and produces a lot of heat. Cleaning with caustic soda requires chemical reaction so it is slow. Cost of cleaning is also high as special facility needs to be created to store and dispose of caustic soda.

4.  Removing by Abrasives: -

 

In this method, a small hand held grinding machine is employed to remove deposited metal by abrasion. Care must be exercised to polish/ buff the ground surface afterwards otherwise rough edges can trap air and moisture.  

Advantages: - Main advantage of this process is that it is applied after operation is over and machine is idle. So it does not interfere in the vacuum process.

Disadvantages: - Disadvantages are many. Fine abrasive particles fly during the process and can contaminate the chamber. This process is time consuming and requires expertise on the part of operator. Excessive grinding because of repetitive use can weaken the walls of chamber and cause vacuum leakages. Also it is impossible to be able to remove deposited metal cleanly with this process.

5.  Hammer and chisel method: - 

 

Crudest of all methods, can damage plant in a matter of days. Must not be attempted.

Conclusion

In my opinion, application of Boron Nitride Suspension/Paste continues to be the most effective and practical way of keeping the vacuum chamber clean of the deposited metal. If cost is a major consideration, for example, if the surface to be protected is very large then Graphite Suspension/Paste can be used as the next best thing. For comparison between Boron Nitride Suspension and Graphite Suspension detailed blog on the subject http://supervacoils.com/graphite-suspension-vs-boron-nitride-suspension/ can be read.

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Written by: Anshuman Punj.   

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