Q.11 How to prevent back streaming of diffusion pump oil?
Diffusion pump diagram
Ans. To
prevent back streaming of Diffusion Pump oil it must be ensured that-
1. System
is "Roughed out" before Diffusion Pump is put in line.
2.
Rotary vacuum pump is maintaining a vacuum of at
least 10-2 Torr.
3.
If
process is causing release of a large amount of gas, then sometimes it
may be required to slow down the process.
4.
Cooling
water output volume and temperature must be checked. If this is not as per the
manufacturer's recommended values then descaling and increasing cooling
water circulation is required.
5.
If
problem of back streaming is frequent then increasing the capacity of backing
pump is required to solve it.
6.
Diffusion
pump must have a cryotrap or at least baffles on top to trap gasses trying to
flow out of pump into chamber.
7.
All
the valves should be checked from time to time.
8.
Strange
as it may sound, very low fore pressure can also cause back streaming.
Q.12 Why is diffusion pump water cooled?
Cooling
coils around diffusion pump
Ans. A diffusion pump has no moving parts and
therefore depends on diffusion pump oil for creation of vacuum by action of gas
diffusion.
For diffusion to occur the diffusion pump oil is
heated and vaporized using heaters. It is then released through jets at high
speed and in this way it traps air by diffusion process. Then it is cooled at
walls of diffusion pump. This liquefies the oil vapors and these vapors again
turn to liquid state. This completes the cycle and the process repeats
continuously.
Q.13 I have a Chinese diffusion pump (Not Varian) do I need to use
some different kind of diffusion pump oil?
Ans. Diffusion pumps as well as Diffusion pump oils are made as per a
standard engineering practice of interchangeability. So a good Diffusion pump
oil will be suitable for any Diffusion pump made anywhere in the world.
Just like same petrol works for cars made by Honda, Suzuki, Audi,
Toyota or any other brand.
Q.14 What is the difference between Vapor Jet Booster Pump (Ejector
Pump) and Diffusion Pump?
Ans. Although vapor jet booster or ejector pump works on the same
principle as diffusion pump but there are many differences, these are given
below:--
1. The jet design of diffusion pump and vapor jet booster pump are
different. In diffusion pump oil vapors are compressed to a higher compression
ratio compared to ejector pump.
2. Because of higher compression ratio diffusion pump is capable
of producing vacuum up to 1x10-10 Torr. Where as an ejector pump
cannot produce vacuum above 1x10-5 Torr.
3. Vapor jet booster pump creates vacuum in a shorter time than
diffusion pump.
4. In an ejector pump the working fluid i.e. vapors jet booster
pump oil has lower boiling temperature compared to diffusion pump oil used in
diffusion pump.
Q.15 When Should I check the diffusion pump oil level? In hot
condition or cold condition?
Ans. Level of diffusion pump oil should always be checked when pump is
cold. In hot condition some oil will be in vapor state so correct level will
not be indicated.
Q.16 While changing diffusion pump oil should we wait till pump
cools down or is this not required?
Ans. You should always wait for diffusion pump oil to cool down to room
temperature before taking it out of diffusion pump. Diffusion pump oil when
heated is at a temperature above 200oC. Trying to take out oil at
this temperature can be very dangerous.
Q.17 What is shelf life of silicone diffusion pump oil?
SV-SIGMA
Ans. If stored properly, shelf life of Silicone diffusion pump oil is 2
years.
Q.18 Which oil is more economical to use hydrocarbon DP oil or
silicone diffusion pump oil?
Ans. Under normal circumstances silicone diffusion pump oil works out
cheaper because of its long life and no requirement of maintenance. However if
plant is very old or process involves emission of corrosive gasses, then
hydrocarbon oil may work out cheaper.
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this please click here.
Q.19 Why is it important to check level of diffusion pump oil
regularly?
Ans. In case level of diffusion pump oil reduces then thermal
equilibrium in diffusion pump is disturbed. This causes Diffusion Pump Oil to
overheat, which results in oil loss, loss of vacuum and damage to diffusion
pump in extreme cases. It is therefore important to check the level of
diffusion pump oil regularly and top it up if required.
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Q.20 Can I reuse diffusion pump oil?
Used diffusion pump oil
Ans. No, diffusion pump oil cannot be reused. This is
because while creating vacuum diffusion pump oil is subjected to very high
temperature and extreme vacuum. This destroys the molecular structure of oil
which cannot be corrected by filtration or distillation.
About this read more here.
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Written by: Anshuman Punj.
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