# How does a vacuum pump work?

## Main Question or Discussion Point

I understand that the purpose of a vacuum pump is to remove the gas molecules from a sealed volume to create a partial vacuum, but just what is actually happening to the gas molecules throughout this process? I have been researching online, but have not found any guides that clearly explain the process. Many thanks, Ryan.

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Here are the basic stages of a vacuum pumps operation with a general description of how it works using an example:

1. You have a 1m3 container filled with a gas.
2. You expand that container to 2m3.
3. The gas volume will increase with it so the gas occupies the entire space.
4. Once you've expanded it you divide it in half so you have two 1m3 containers with half the gas in each.
5. You empty one container by reducing its volume to zero and forcing the gas out.
6. You remove the divider and allow the gas to expand to 2m3 again.
7. You then repeat the process over and over, each time the amount of gas reduces.

That's all a vacuum pump does (but not as efficiently). It creates a low pressure area which allows the gas to expand into it (2 & 3), then 'cuts off' that gas from the rest (4) and then forces that gas out (5). It then repeats the process over and over (6 & 7).

For a better description and animations, see this site: http://www.absolute-vacuum.com/resources_whatisavacuumpump.php [Broken]
Any device which can induce a pressure difference between the two regions in the space is called a pump. The pump which creates the vacuum in the certain system is called a vacuum pump.
This is another vane pump animation, demonstrating the principle I outline above: http://www.mekanizmalar.com/vanepump.html

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To overcome mechanical inadequacies it is very common to use several vacuum pumps in series. One pump feeds the next and the next... To get really high vacuum they use different types of pumps in stages.

Here are the basic stages of a vacuum pumps operation with a general description of how it works using an example:

1. You have a 1m3 container filled with a gas.
2. You expand that container to 2m3.
3. The gas volume will increase with it so the gas occupies the entire space.
4. Once you've expanded it you divide it in half so you have two 1m3 containers with half the gas in each.
5. You empty one container by reducing its volume to zero and forcing the gas out.
6. You remove the divider and allow the gas to expand to 2m3 again.
7. You then repeat the process over and over, each time the amount of gas reduces.

That's all a vacuum pump does (but not as efficiently). It creates a low pressure area which allows the gas to expand into it (2 & 3), then 'cuts off' that gas from the rest (4) and then forces that gas out (5). It then repeats the process over and over (6 & 7).

For a better description and animations, see this site: http://www.absolute-vacuum.com/resources_whatisavacuumpump.php [Broken]

This is another vane pump animation, demonstrating the principle I outline above: http://www.mekanizmalar.com/vanepump.html
This is how a really simple pump works, but it'll take forever to get anything close to a vacuum. Ion pumps are a lot more efficient and a bit more complicated.

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This is how a really simple pump works, but it'll take forever to get anything close to a vacuum. Ion pumps are a lot more efficient and a bit more complicated.
Really? It's the basic concept behind the hand held pumps I can empty my bell jar in about 30 seconds (well enough to stop hearing the sound anyway).

It's not a bad pumping system, it's the basics of a vane pump.

sophiecentaur
Gold Member
OK as far as it goes but that sort of 'vacuum' is chock full of molecules. To get rid of them you need to work a lot harder and be much more inventive than a simple 'pump' pump.
There is a basic limit to how well you can do with a conventional pump, based on the ratio of max to min volumes of the cylinder and its other spaces during the pumping cycle.

Well with all due respect, we can sit here criticising my description all day, or, we could provide a better one.

What I described is a basic way (as I mentioned) of generating a vacuum - not a perfect vacuum.

sophiecentaur
Gold Member
OK. But I didn't want anyone to think that a conventional pump would produce the sort of vacuum which is needed for 'serious' experiments. Great for the sound demo and excellent value for School. And, to be fair, you still need that sort of pump to produce a 'starter vacuum' on which diffusion pumps etc. can work.

The word vacuum is a bit like the length of a piece of string. - it's not an absolute - - - -ever.

1. You have a 1m3 container filled with a gas.
2. You expand that container to 2m3.
3. The gas volume will increase with it so the gas occupies the entire space.
4. Once you've expanded it you divide it in half so you have two 1m3 containers with half the gas in each.
5. You empty one container by reducing its volume to zero and forcing the gas out.
6. You remove the divider and allow the gas to expand to 2m3 again.
7. You then repeat the process over and over, each time the amount of gas reduces.
That's a great description. And as far as I know, this type of pump is the first stage for most, if not all, scientific pumping systems. For "serious" vacuums, this type of mechanical system will pump the next, finer pumps, that are designed to work only when the pressure is already that of a partial vacuum.

Thanks to jarednjames for the example and to everyone else for replying.:)

Am I am on the right track here...

Vacuum cleaner example: When the fan is switched on a partial vacuum is created due to the lower pressure created behind the fan and dirt/debris is pushed up the vacuum cleaner due to the higher pressure outside.

Diaphragm muscle: This muscle expands the chest cavity causing the lungs to increase in volume. This expansion reduces the pressure and causes a partial vacuum as the molecules spread, but this is soon filled with air from higher pressure outside.

The vacuum pump: And so a fan or rotor attracts either gas or a fluid into the inlet (alike the vacuum cleaner) and gas/fluid is collected in a small cavity that expands. This itself causes lower pressure that continues to attract molecules as it expands until the desirable point. This is sealed and the gas/fluid is then evacuated out of the outlet (similar to the diaphragm muscle with the exception of having been sealed). The tank now has the desired vacuum or it can continue the process until the desired amount is attained (if this is a fluid then it gets to where it needs to go). Many thanks.

sophiecentaur
Gold Member
Just one proviso. A fan or any other pump doesn't (cannot) "attract" a gas. There is no attractive force between well separated molecules. It is the excess pressure outside that pushes the fluid into it.

I understand that the purpose of a vacuum pump is to remove the gas molecules from a sealed volume to create a partial vacuum, but just what is actually happening to the gas molecules throughout this process? I have been researching online, but have not found any guides that clearly explain the process. Many thanks, Ryan.
In my estimate Ryan wants to know how the molecules of a fluid behave at reduced pressure not much different than STP.

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sophiecentaur
Gold Member
Oh yes. Always a good idea to read the OP before launching out and ranting!

The molecules just become more and more spaced out as the pressure drops. Their speed just depends upon the temperature - which is actually the average kinetic energy of the molecules. Things actually get a bit different as pressures become higher, when the mutual attraction of adjacent molecules comes into it. But, at low pressures, the molecules tend to behave like ideal little 'ball bearings', bouncing off each other and the container sides.
BTW, in deep space, the density of molecules is about one per metre cube - even that's not a perfect vacuum.

Just one proviso. A fan or any other pump doesn't (cannot) "attract" a gas. There is no attractive force between well separated molecules. It is the excess pressure outside that pushes the fluid into it.
Sorry for my wording, I meant attract gas molecules (accelerate/push them).

After learning more about positive displacement, then there's Momentum transfer and Entrapment for me to get through!!:)

I was just looking at vacuum in outer space after reading your post and found this info interesting from Wikipedia:
"Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth (10−12) of atmospheric pressure, and can reach ≈100 particles/cm3. Outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average. However, even if every single atom and particle could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy, and other phenomena in quantum physics."

sophiecentaur
Gold Member
Sorry for my wording, I meant attract gas molecules (accelerate/push them).

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I still don't agree with the word "attract". Any net movement of gas molecules is caused by pressure from 'behind'.
The word "suck" sucks in this application.

It's amazing how dirty a 'good' vacuum is. It must be a real nightmare to get the LHC vacuum good enough throughout the whole volume of the loop.

I still don't agree with the word "attract". Any net movement of gas molecules is caused by pressure from 'behind'.
The word "suck" sucks in this application.

It's amazing how dirty a 'good' vacuum is. It must be a real nightmare to get the LHC vacuum good enough throughout the whole volume of the loop.
I have a better understanding now. Thanks for your help!:)

Youall have inspired me to look up the workings of a turbo molecular pump that some labs use at my place of business. I see that they are capable of pressures down to 10^-10 Torr, or about 1.3 * 10^-10 mbar.

At these pressures the mean free path length is about 1000 kilometers according to wikipedia. So molecules of air don't act like a gas at all, at the dimensions of the pump, but as objects that are far more likely to hit elements of the pump than other molecules.

In an image search with Google for turbo molecular pumps you can see that the pump veins are very strangely arranged--not at all like the blades of a turbine jet engine that are arranged to maintain laminar flow.