[Vacuum Suction] Relation between cup diameter and ejector flow rate? 1

[SudoHalt]

Hello,
I am trying to determine the vacuum ejector flow rate I need to carry a Gypsum block of 30kg. Based on my calculations for pressure and force, I was able to obtain a configuration of 4 suction cups, each one being 8cm in diameter.

I am planning to use one vacuum ejector per cup. We have a compressed air supply that can supply up to 1MPa (10bar).
The following document seems to give an example of calculations, including the appropriate cup height:

https://www.avs-yhtiot.fi/wp-content/uploads/2016/11/7.01.05_alipaine_ohje.pdf

To save your time, jump in to page 4. There is the table labeled: "Required suction rate as a function of the suction pad diameter". It basically recommends 12.7 liters/minute of flow rate for my configurations as I am planning 1 cup per ejector.

But based on what calculations are these suggested values obtained in the first place?

 

[EdStainless]

Have you read up on eductor theory?
Even with no leakage (say cup against glass plate)there is a flow required to maintain vacuum since there is a leak path back through the eductor itself. I am guessing that that is what this min flow is.
Now if you are trying to seal against a rougher surface or if your item has any moisture or porosity you will need more flow to overcome that contribution also.
If you are planing on using 4 cups will 3 actually lift it? Can you tolerate a failure? You might consider using two larger eductors and manifolding them to the set of lifting cups. It might be easier to maintain and troubleshoot.

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P.E. Metallurgy

 

[SudoHalt]

Hey there, thanks a lot for your reply. This is actually for a final year project, so yes, I did read about the theory behind them.

I have seen my friends obtain a vacuum lifting device that can lift a 100kg in theory, and it was able to lift the gypsum blocks I am attempting to lift. Interestingly, the vacuum ejector that they used was single stage, with a nozzle diameter of 2mm. The evacuation time was under 2 seconds, which is sufficient for my task really. The technical sheet I linked to in my first post effectively recommends a single stage vacuum ejector with a nozzle diameter of 2.5mm.

The total perimeter of the seal on the device my friends obtained was 160cm. Over the course of the last few weeks I came to determine it is better to use 8 suction cups instead of 4. This is partially to increase the circumference while keeping the surface area the same as I previously calculated. Even though my calculated surface area is much less than the vacuum device my friends obtained, I am able to obtain a larger circumference than their perimeter. I believe that means I have more sealing to reduce the vacuum leakage. This way I am able to justify a downgrade to the single-stage ejector with the 2.0mm nozzle diameter. If that fails, I will definitely keep the manifolding of two ejectors in mind as the cost difference isn't major even for my small scale project.

Based on what I understood from the technical sheets that would help me determine the correct setup, I believe the main factor by which they determined the required suction flow rate is the evacuation time needed. Maybe it's superficial how I concluded that, but in the sheet I linked to in my first post they said the following below the suggested values for flow rate: "The indicated values apply to all types of vacuum generators."

In addition, the following technical sheet (link) has a formula somewhere to calculate the evacuation time. One main factor of such a formula to calculate the evacuation time is the volume of the suction cup. This second observation convinced me that the selection according to the suction flow rate is being done for the evacuation time. However, at the same time, this paper also recommends using a safety factor of 8 for vertically aligned objects. This is the highest I have seen and it seems I will need to reconsider some of my decisions.

However, in another technical sheet (link), I saw a warning against using the suction flow rate as a primary assessment of the ejector's performance. But to be fair, they were mainly discussing against using the suction flow rate for multi-stage ejectors.

 

[EdStainless]

You only stage eductors for deeper vacuum, not more flow.
If you use more smaller cups you will have more edge, and that is where you are at risk of leaking.
There would be less leakage with one large cup, but no safety. You need to balance the two factors.
Lifting overhead requires high safety factors, 5 at a minimum.
And if these are spit into two groups each 1/2 alone needs to be able to lift the load.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[SudoHalt]

Oh wow, so what I am understanding is that to reduce leakage, I am not supposed to increase the circumference, but rather to reduce it. I am starting to understand that a single larger cup will have more leakage than a single smaller cup, even though the smaller cup has a smaller surface area. The smaller circumference of the single cup means there is less "failure points" through where the air can leak.

And since I have to keep the surface area the same, I need to reduce the circumference. I previously thought it is the exact opposite.

The beginning of page 2 of the following article seems to pretty much agree with what you just told me:

https://vacuforce.com/whitepapers/Vacuforce_Learn_Vacuum_Edition_47.pdf

Apparently I was a tad confused by the following technical sheet (link), which simply stated on Page 4 that you need to reduce the cup diameter to reduce leakage. I should have been smarter in my conclusions, but I am glad I happened to mention something about the suction cups. Thanks a lot!

 

[SudoHalt]

Mind me asking a different question this time, I was asking about the Ejector suction flow rate before. I would like to ask about compressed air flow rate right now. At the university I am in, they use pipes with 5mm internal diameter for the compressed air supply that connects to various apparatus. My friends have tried to use the 100-kg rated vacuum suction device they obtained, with the same 5mm diameter pipe, so I believe there is no need to worry about the pipe being too small.

I am personally curious about how I can calculate the maximum compressed air flow rate through a 5mm pipe. The pressure from the supply is 700 kPa. Let me make the naive assumption that the only limiting factor is the diameter of the pipe and not the huge air compressor that we got.