Piston suction pump versus external gear pump as vacuum pump

[johnthekid]

Hey guys, which one creates the higher vacuum (lower pressure vacuum)? A piston suction pump (that kind of looks like a syringe) or an external gear pump design (that used two external gears)? IMO I think it is the former but I want more explanations.

 

[Baluncore]

Welcome to PF.

The answer will depend on the required pressure and the volume that must be evacuated. The best pump will be the one with the least leakage.

I think my first choice would be an eccentric vane pump, with graphite vanes. Next would be a piston pump, and lastly, an external gear pump, which will have the highest leakage and need the most oil to seal the gear tips against the housing.

 

[johnthekid]

Welcome to PF.

The answer will depend on the required pressure and the volume that must be evacuated. The best pump will be the one with the least leakage.

I think my first choice would be an eccentric vane pump, with graphite vanes. Next would be a piston pump, and lastly, an external gear pump, which will have the highest leakage and need the most oil to seal the gear tips against the housing.

Thank you. As the piston pump is better suited for a vacuum pump design than the external gear pump design, I will pick the piston pump design that looks like a syringe or a piston bellows. What other reasons that make external gear pumps to be insufficient as vacuum pumps?

 

[Lnewqban]

What other reasons that make external gear pumps to be insufficient as vacuum pumps?

Gear pumps are normally used for liquids only.

Please, see:
https://en.wikipedia.org/wiki/Vacuum_pump

https://en.wikipedia.org/wiki/Gear_pump

 

[johnthekid]

Gear pumps are normally used for liquids only.

Please, see:
https://en.wikipedia.org/wiki/Vacuum_pump

https://en.wikipedia.org/wiki/Gear_pump

Thank you so much.

 

[Averagesupernova]

Rotary lobe pumps typically used as superchargers have similarities to gear pumps. They are mostly used for non-liquids. I guess it depends on how wrapped up into definitions you want to get.
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This all sounds like homework questions to me.
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https://en.wikipedia.org/wiki/Roots_blower

 

[johnthekid]

Rotary lobe pumps typically used as superchargers have similarities to gear pumps. They are mostly used for non-liquids. I guess it depends on how wrapped up I to definitions you want to get.
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This all sounds like homework questions to me.
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https://en.wikipedia.org/wiki/Roots_blower

These questions are not related to homework. I'm curious to understand if using piston pumps as vacuum pumps is much better, turns out piston pumps are better so I pick them. These rotary lobe pumps have figure 8 shaped wheels, which one produce the higher vacuum between these rotary lobe pumps and piston suction pumps? Is it the same as the previous comparison between piston suction pumps and external gear pumps where piston suction pumps do better?

 

[Averagesupernova]

...turns out piston pumps are better so I pick them.

Pick them to do what? Many pertinent questions were asked and you have not answered. I suspect you don't really know what you need. You just want someone to tell you what you think you might want to hear.

 

[johnthekid]

Pick them to do what? Many pertinent questions were asked and you have not answered. I suspect you don't really know what you need. You just want someone to tell you what you think you might want to hear.

DIY vacuum pump. So back to the question, is it the same as the previous comparison between piston suction pumps and external gear pumps where piston suction pumps do better?

 

[Averagesupernova]

DIY vacuum pump. So back to the question, is it the same as the previous comparison between piston suction pumps and external gear pumps where piston suction pumps do better?

And back to my reply that the question is not answerable with any degree of reliability.
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Example:
I have a very large vessel to evacuate. It needs to be drawn down but not to the point that a typical refrigeration system needs to be prior to adding freon. A rotary lobe pump can move a lot of air in a hurry but will not achieve a high vacuum so that would be an ok choice. It would be a poor choice to evacuate a refrigeration system such as a residential central air unit. Smaller amount of gas to be evacuated but needs to be drawn down to a higher vacuum than a rotary vane can provide.

 

[johnthekid]

And back to my reply that the question is not answerable with any degree of reliability.
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Example:
I have a very large vessel to evacuate. It needs to be drawn down but not to the point that a typical refrigeration system needs to be prior to adding freon. A rotary lobe pump can move a lot of air in a hurry but will not achieve a high vacuum so that would be an ok choice. It would be a poor choice to evacuate a refrigeration system such as a residential central air unit. Smaller amount of gas to be evacuated but needs to be drawn down to a higher vacuum than a rotary vane can provide.

Multistage vacuum pumps or using multiple vacuum pumps are necessary if we want to get to much higher vacuum (lower pressure vacuum). But if we talk about using a standalone vacuum pump only that works independently, does a single piston suction pump works better as a vacuum pump than a single rotary lobe pump like it does when compare to a pump that used two external gears?

 

[Averagesupernova]

But if we talk about using a standalone vacuum pump only that works independently, does a single piston suction pump works better as a vacuum pump than a single rotary lobe pump like it does when compare to a pump that used two external gears?

You just don't get it. Rotary lobe pumps move a lot of air. One place they are used is to move dry granular material. The dry material never enters the pump. Rotary airlocks and centrifugal cyclone shaped containers are used to keep the dry material out of the pump. A piston pump is a very poor choice for this application. A centrifugal fan is a better choice than piston. Often times rotary lobe pumps are used. There is not a high vacuum requirement.
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So until you give us an application besides DIY vacuum pump the answer you are going to get is that anything will work until it doesn't.

 

[johnthekid]

You just don't get it. Rotary lobe pumps move a lot of air. One place they are used is to move dry granular material. The dry material never enters the pump. Rotary airlocks and centrifugal cyclone shaped containers are used to keep the dry material out of the pump. A piston pump is a very poor choice for this application. A centrifugal fan is a better choice than piston. Often times rotary lobe pumps are used. There is not a high vacuum requirement.
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So until you give us an application besides DIY vacuum pump the answer you are going to get is that anything will work until it doesn't.

You mentioned centrifugal fan and I have no idea what it is because I'm only familiar with rotary vane, pistons and gears. Thank you for the mention. I only want to evacuate a small chamber, say a small metal (for example, aluminium or steel) chamber with a single vacuum pump made from cheaper alternatives instead of buying expensive piston suction vacuum pumps. If possible, me and my friends want to compare which pump does better as a vacuum pump when used singularly. But rotary lobe pump works only when multiple vacuum pumps are used, is that right? What if we used it alone without a backing pump or anything?

 

[johnthekid]

A centrifugal fan is a better choice than piston

Wait, is it centrifugal fan or centrifugal pump? Because the original discussion is about vacuum pumps. Are you talking about turbomolecular pumps?

 

[Averagesupernova]

Wait, is it centrifugal fan or centrifugal pump? Because the original discussion is about vacuum pumps. Are you talking about turbomolecular pumps?

For the application I was describing what is typically referred to as a centrifugal fan. Many similarities to a centrifugal pump.

 

[johnthekid]

For the application I was describing what is typically referred to as a centrifugal fan. Many similarities to a centrifugal pump.

I believe you're talking about a complicated vacuum system which is way over my head.

 

[Averagesupernova]

I believe you're talking about a complicated vacuum system which is way over my head.

So a household vacuum cleaner is over your head?

 

[johnthekid]

So a household vacuum cleaner is over your head?

They share the same working mechanism? Ok, so just in general from the best to worst as a vacuum pump, the best will be eccentric rotary vane pumps, then piston suction pumps and finally gear pumps which are external gear pumps and internal gears.

Does a single rotary lobe vacuum pump do better than a single piston vacuum pump as a vacuum pump when used independently?

Edit: sorry for my sheer ignorance about the working mechanism of vacuum cleaners, you're clearly describing it while I was confused

 

[Averagesupernova]

They share the same working mechanism? Ok, so just in general from the best to worst as a vacuum pump, the best will be eccentric rotary vane pumps, then piston suction pumps and finally gear pumps which are external gear pumps and internal gears.

Does a single rotary lobe vacuum pump do better than a single piston suction pump as a vacuum pump?

Do you seriously think that all the companies out there making various types pumps and only one is the best? Each pump is the best for a specific application. I've been trying to get you to understand this.
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You say you want to evacuate a small volume and are wanting to compare pumps? You want us to tell you which pump will make your choice the best? What are you using for a vacuum vessel? I expect a real answer. Specifics.

 

[johnthekid]

Do you seriously think that all the companies out there making various types pumps and only one is the best? Each pump is the best for a specific application. I've been trying to get you to understand this.
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You say you want to evacuate a small volume and are wanting to compare pumps? You want us to tell you which pump will make your choice the best? What are you using for a vacuum vessel? I expect a real answer. Specifics.

What if we are talking about widespread application especially when used independently?

Let's say a 12 inches long, 12 inches tall and 12 inches wide steel box or container with thick walls. I am not sure whether to buy a piston suction pump or a external gear pump. This is more like a fun experiment like others making DIY vacuum pumps from syringes and etc. But based on others responses, it looks like a piston suction pump does better than an external gear pump when it comes to act as a vacuum pump.

 

[johnthekid]

Sorry, change the 12 inches dimensions to 20 inches dimensions.

 

[Averagesupernova]

Sorry, change the 12 inches dimensions to 20 inches dimensions.

Do you realize the dangerous territory you are playing in?

 

[Baluncore]

But based on others responses, it looks like a piston suction pump does better than an external gear pump when it comes to act as a vacuum pump.

A piston pump needs valves, a gear pump does not. If the valves are spring loaded, one way check valves, they will reduce the vacuum you can pull by the valve operating pressure. You will need automatic cam operated valves with your piston pump.

 

[johnthekid]

Do you realize the dangerous territory you are playing in?

Atmospheric pressure crushing things?

 

[Averagesupernova]

Atmospheric pressure crushing things?

Yes.

 

[johnthekid]

A piston pump needs valves, a gear pump does not. If the valves are spring loaded, one way check valves, they will reduce the vacuum you can pull by the valve operating pressure. You will need automatic cam operated valves with your piston pump.

If we are still going with the former, does the piston pump still generate higher vacuum than the external gear pump?

 

[johnthekid]

Yes.

Is a cube shaped container appropriate?

 

[Baluncore]

A piston pump with pressure operated check valves will not do as well as a vane or an external gear pump.

 

[Averagesupernova]

Is a cube shaped container appropriate?

What do you think? Have you thought about the pressures on each side? And have you thought about how it will be built to withstand those pressures and what it takes to seal the joints?

 

[johnthekid]

A piston pump with pressure operated check valves will not do as well as a vane or an external gear pump.

Even if both are designed as simple as possible? Now I'm curious about the Torr range that a piston pump with pressure operated check valves and an external gear pump can produce.