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Q & A: slow vacuum of hot tank

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Most recent answer: 10/07/2012
Q:
In our machine vacuum pump creates vacuum in a tank upto 25 mbar within 5 min, but after raising the temperature of tank by blowing hot air in the tank we tried to do vacuum but to reach 25 mbar it is taking more than 35 minutes, why it is happening?
- Amit k (age 25)
mumbai,India
A:
I'm not sure, but there's a probable reason. The tank walls will "outgas". That means that gas molecules that are stuck inside the walls work their way to the surface and come out. That process is faster when the walls are hot.  So with the hot walls more outgassing is going on, making it harder to reach a good vacuum.

When one wants to reach a very good vacuum, it's common to intentionally heat the walls while the pump is on. This reduces the amount of trapped material. Then when the walls are allowed to cool down, you actually can get a better vacuum than you would have if the walls hadn't first been heated. The process is often referred to as a "bake-out".
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Mike W.

(published on 10/07/2012)

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