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

Connect With Us

Name

Email *

Message *

Written by: Anshuman Punj. 

  

Go to Supervac Industries Website

 

   Click Here  

Procedure for Safe Running of Vacuum Furnaces

A furnace is typically run at temperature between 500 ^o C- 1200 ^o C. Because of such elevated temperatures, furnaces require a lot of safety precautions in their normal operations. Because of requirement of better products many times it is essential that heating inside furnace be done under vacuum. Such a furnace is called a vacuum furnace.

Advantages of vacuum heat treatment of components-

1.   No unwanted carburization takes place.

2.   Bright and smooth surface finish because of non-oxidation of components.

3.   Low distortion due to high temperature uniformity inside the vacuum furnace.

4.   Results are easily reproducible because of defined temperature guidance by load thermocouples.

5.   Complete documentation possible.

6.   Completely automated procedure.

Vacuum in furnace not only prevents oxidation of heated material but also allows controlled cooling under neutral atmosphere. At the same time addition of vacuum also mandates additional safety precautions to be observed so that there is no damage to equipment or chance of injury to people operating the equipment.

Because furnaces have been in operations since very long time so most engineers are aware of basic precautions for safe running of the furnaces. About vacuum system however not much literature is readily available. This blog post primarily addresses issues related to vacuum system for running the vacuum furnace safely.

Major precautions are given below-------

1.   In a vacuum system having diffusion pumps, the diffusion pump must never be touched or opened till it has cooled down to room temperature because during operations diffusion pump oil is heated to a temperature of about 250 ^o C. Add to it the latent heat of vaporization and we have a dangerous situation at hand if mishandling occurs.

2.   Leakage in vacuum systems can result in cold air suddenly rushing into the heated furnace. To prevent this, following needs to be done.

a)   O Rings and oil seals must be replaced from time to time.

b)   Oil seals and o rings must be smeared with high vacuum silicone grease which prevents damage to these and also provides additional vacuum sealing.

c)   O rings and oil seals should always be placed properly in the groove provided for them. Otherwise they can get sucked in under vacuum.

d)   Flanges where o ring are fitted must have good surface finish. A coarse surface finish may result in gas leaking through the seal.

e)   All joints must be tightened so that o rings /oil seals are squished to about 80% of their original cross section. Less than this will result in vacuum leak and greater than this will result in damage to o ring.

f)    Good quality neoprene, viton or silicone o rings should be used.

g)   O rings must never be removed with a screw driver or any other sharp object as this can damage the groove of o ring.

3.   At the start of the cycle, for a minute or two, outlet of rotary pump throws out oil mist. Either provision should be made to exhaust this mist outside or a mist eliminator must be installed.

4.   Care should be taken to change the vacuum oils periodically.

5.   Diffusion pump oil must be topped up as soon as it falls below the minimum level. This prevents over heating of the diffusion pump.

6.   In case leakage is observed then proper leak detection procedure should be followed. For details on leakage detection please see http://supervacoils.com/leakage-detection-and-prevention-in-vacuum-systems/ .

7.   During heating in a furnace out gassing increases so vacuum pump of sufficient capacity must be selected. Inadequate pumping capacity of vacuum system will not be able to remove gases.

8.   At least once in a month furnace must be run on a dry run where empty furnace is heated to a temperature of 1000 ^o C for at least 30 minutes under vacuum.

9.   Faulty vacuum reading can result in accident. So vacuum gauges must be checked from time to time.

10. To prevent back streaming of rotary vacuum pump oil, rotary pumps must be fitted with solenoid valves.

Solenoid Valve

11.      Sequencing valves in vacuum system must be properly maintained.

12.      Moisture is detrimental to creation and maintenance of vacuum. When furnace is not in operation, as far as possible furnace doors must be kept closed to prevent moist air entering the chamber.

13.  Using good quality silicone diffusion pump oil rather than hydrocarbon diffusion pump should be used to minimize the risk of explosion.

14.  Outlet water temperature from diffusion pump cooling coils must be monitored and never allowed to go beyond for 40 ^o C.

15.  If a human is present inside the chamber and vacuum is switched on the person will die almost instantly. Proper interlocking and fool proofing must be observed to prevent this at all costs.

16. Always use silicone diffusion pump oils instead of hydrocarbon/mineral oils for your diffusion pump as silicone oils are inert and more resistant to process gases present in the vacuum furnace. Mineral oils are likely to deteriorate or catch fire when in contact with certain process gases e.g., oxygen.

I am sure that by observing these precautions (which are primarily about the vacuum system of the furnace) you will get safe and trouble free operation of your vacuum furnace.

Connect With Us

Name
Email *
Message *

Written by: Anshuman Punj. 

  

Go to Supervac Industries Website

 

   Click Here