Learn all about boiling temperature of diffusion pump oil

Hi friends. My name is Anshuman Punj and I am CEO of Supervac Industries. Supervac Industries is a leading manufacturer of vacuum pump oils and supplies them throughout the world. I am a Mechanical Engineer by qualification.

Today we are going to talk about a very interesting topic about the boiling temperature of diffusion pump oil and why it is so important. Before we understand about the boiling temperature of diffusion pump oil let us take a look at the diffusion pump itself.

This is a cross-section of diffusion pump. The oil gets heated over here with heaters, moves up a vertical column and in the meantime gets released from the jets. Since the oil is carrying a lot of energy, it gets released with a lot of velocity. The speed of the oil is sometimes even above 1 mach. It comes and strikes the cold walls which are water cooled and liquefies once again. As it is coming out with a great velocity it hits the air molecules and traps them and releases them somewhere here. Where it gets released and it gets liquefied.

So basically in a diffusion pump we have a pressure gradient. Pressure vacuum over here will be 10-8 Torr or even above that and vacuum at this point will be 10-1 Torr. Over here a rotary pump comes in handy and pulls out the air molecules that have been released by the oil. This is how a diffusion pump works. As we can see oil is the only working medium. There are no moving parts in a diffusion pump.

Now the boiling temperature of diffusion pump oil assumes extreme significance. In fact it is ‘the most important’ specification of the oil. Now what decides this boiling temperature? If we choose an oil with a very low boiling temperature we will have the advantage that we’ll be able to heat it very quickly. But at the same time since the boiling temperature will be very very low it won’t be able to carry enough energy to complete the action. Similarly if we choose an oil with a very high boiling temperature we will get a very good vacuum. But at the same time we’ll require enormous amount of energy in heating the oil and the cost of oil will go up very much because the oil will have to withstand such a high temperature and under very adverse conditions of extremely high vacuum. So a trade off is struck in between; some kind of a balancing act. All over the world studies have been done and engineers have now finally arrived at a conclusion that the boiling temperature of a diffusion pump oil for normal applications is somewhere about 220oC. As we can see over here in this table, the boiling temperature of 220oC gives us a vacuum of 1×10-7 Torr which is more than sufficient for most of the applications that take place in the industry.

However we if we require vacuum even above this, like for research purpose then we could always go for an oil with higher boiling temperature, say above 255oC. Supervac manufactures SV-Sigma+ which meets this criteria. Or sometimes vacuum time is extremely important. Vacuum level is not that important. So we go for an oil with even lower boiling temperature of 180oC which gives a vacuum of 1×10-6 Torr. But this doesn’t matter because we’re getting the vacuum very fast.

Now world over, diffusion pump oils are manufactured with these 3 boiling temperatures.

This understand of the boiling temperature gives us another advantage. Suppose manufacturer is charging you very high for a diffusion pump oil or the diffusion pump oil recommended by the machine manufacturer is not available in your country or region. You can always choose an equivalent of that diffusion pump oil based on the boiling temperature of the oil being recommended to you.

For example, if you are using any of these oils- DC-704, CVC4, Hivac F4, Ulvoil D31 or Indomet Tetra, you could use SV-Sigma manufactured by Supervac Industries which comes at a fraction of the cost.

Similarly if you are using DC-705 or Hivac F5, you could use SV-Sigma+ or if you are using DC-702, you could use SV-Sigma Fast. These oils will give you the same working parameters- vacuum level, working life, vacuum…everything. But they will be somewhat cheaper.

If you want to know more about diffusion pump oil or about diffusion pumps, please visit www.supervacoils.com

Related Post: What is Vapor Pressure of Diffusion Pump oil

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