Designing and Operating High Vacuum Systems

[jumbogala]

Hi, I'm curious about how vacuum systems work... I was advised to try posting this here, hopefully it's the right place.

So if you're designing a high vacuum system, how do you actually operate it?

For example if you have a chamber connected to an oil diffusion pump connected to a rotary pump, do you have to open the valves first and bring it up to atmospheric pressure? Then close the valves and start the pumps? Once you reach your lowest pressure, do you turn off the pumps and then open the valves? I'm confused about the order of these things.

 

[mgb_phys]

It depends on the pump technology.
Each pump concentrates the air on one side to a higher pressure on the other.
Each pump may only work at a certain pressure range.

Typically you remove most of the air with a rotary pump, these can work at high pressures but not reach a very high vacuum.
Then you switch to a high vac pump, an oil diffusion pump or more likely these days a turbo-molecular.
You still need a rotary pump on the output of these, called a backing pump - the diff/turbo pump concentrates the air from the high vac to a medium vac which the backing pump can then compress to atmospheric pressure and exhaust.

With oil diffusion pumps you have to be very careful about the design of the valves and order of operation, if air at too high a pressure is allowed to reach the hot diffusion oil while it is running the pump will be damaged. Modern turbo pumps have smart controllers that will safely shut themselves down.

Once the vacuum is acheived you might have to keep some pump running to handle any leakage or outgassing. If the system is cooled you might have cryopumps inside the vacuum to handle this.

 

[dr dodge]

if this is for pressure measurement, the system will be of a differential nature, as is the case with our system in house. you really need 1 turbo and 2 "backing pumps". you pull the whole system down with both pumps and the turbo, then close off one side, leaving the turbo on your reference side, and a backing pump on each side. then you use a calibrated leak to ease the pressure up. This is who supplies our stuff

http://www.oerlikon.com/leyboldvacuum/

dr