Vacuum limitations 2

[wkellen]

I have a question on vacuum. I have searched every reference I can find on it and can't seem to get what I am looking for.

I have a chamber of sorts that is .25 CF. I need to draw it down to as near perfect vacuum as possible. My Vacuum Pump specs are 98.0ACFM and 29.9"HgV. There is a 5" diameter pipe leading to the chamber. At the chamber it splits to four 3/4" diameter holes. The other side of each hole has a channel that has half of a 3/4" diameter (basically a "U" shape). From there at each "U" shape, the vacuum has to pull through a section that is basically a rectangle. They are .050" tall and 5" wide over a length of about 6.5" each. This is where the .25 Cubic Foot chamber starts.

I need to pull this vacuum down in about .025 to .045 seconds. Does the CFM just increase through the rectangular section to achieve this, or is there a point where it reaches a "terminal velocity" and can only pull so much vacuum in the given time?

 

[EdStainless]

How fast?
A drawing of the system would help.
The pump is capable of moving 98 CFM at standard conditions, there are pumping curves for these from the manufacturers.
And it sounds like just a roughing pump, not high vacuum.
First of all with a mechanical pump you will only get to 100 microns or so, is that good enough or are you looking for real vacuum?
If you want this fast and repeatable you will want a reservoir and a master isolation valve.
that way you can trip the valve and have the two chambers reach equilibrium pressure quickly.



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P.E. Metallurgy, Plymouth Tube

 

[MacGyverS2000]

Yeah, at 25-45ms, I'm expecting a reservoir setup is the only reasonable method here.

If you truly want to get to pure vacuum, as Ed says, you will need a diffusion pump, turbo pump, or even an ion pump to get those last few molecules. But I doubt you'll ever do that level of vacuum in such a short period of time, even with a giganto reservoir...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[wkellen]

The vacuum pump is this:

Quincy Compressor Model QSVI-75 - HP Rotary Screw Vacuum Unit rated for 980.0 ACFM and 29.9" HgV ultimate vacuum.

Product Features
• Voltage: 460/3/60
• Control Type: Auto Dual
• Full Voltage Motor Starter - Mounted & Wired
• EPACT High Efficiency Drive Motor
• Full Instrumentation including Vacuum Gauges
• UL Listed Control Panel
• Unique Pressure Lubrication System
• Direct Drive - No Gears or Belts
• Water Cooled
• Full-Range Field-Adjustable Modulating Inlet Valve
• Mounted Inlet Filter (25-50 HP)
• 2 Year Airend Warranty
• 1 Year Package Warranty
• 8,000 Hour Fluid of Choice (PG,Plus)
• Fluid Type Selected: QuinSyn-PG


I divided the ACFM by 10 because it is used for 10 machines at the same time. Yes, it will depend on what the other machines are doing as to what is available to this one machine. They may all be open to the atmosphere at the same time or they may all be closed at the same time. I am just looking for "If it was a single pump on a single machine" situation. I cant control the pump conditions at a given time. I am more looking to see if the chamber it is drawing down is capable given that it is going through all the different restrictions. That is assuming the full vacuum is available throughout the 5" pipe at all times.

I do agree there needs to be an accumulator bottle type of device at full vacuum close by with a valve to equalize the 2 chambers. If there were, would it be capable of drawing the chamber down to the point they were of equal vacuum almost instantly or is there a "terminal velocity" to CFM pulling vacuum?

Thanks!

 

[TheTick]

There is a limit to how much vacuum any pump can draw. After that, other means nay be necessary.

E.g. extreme vacuums are achieved by devices that capture and sequester individual gas molecules.

 

[handleman]

Your time of milliseconds is not reasonable.
Your spec of "near perfect as possible" is not a spec. You have to have a number.
A vacuum reservoir is not going to help you at all with a single vacuum pump.
As soon as you open the vacuum valve, pressure will equalize from your 0.25CF chamber into your reservoir. If you have a 25CF reservoir at absolute 0 psi, then you open the valve, you will have a 25.25CF volume at 0.15psia. Your pumping speed is much lower at this vacuum level, therefore (1) your 0.25CF chamber will not reach (whatever your nonexistent spec is) until your pump can evacuate that tiny pressure from that huge volume.

The only way the reservoir helps is if you can use it to rapidly suck the chamber down to decimals of a PSIA, then close the valve to the chamber and suck it down the rest of the way with the pump.
Your 45ms time is barely enough for the valve to open, much less open, flow, close, then another valve open and finish vacuum. Plus you need to have another pump running all the time to renew vacuum in your reservoir..

 

[ProEpro]

Read the manuals and other data from someone like Pfeifer or at least Wikipedia. Like has already been said you are completely ignorant about what you are trying to do.

Even if the pump did get you to 29.9"hgV there is still a long ways to go you reach high vacuum. The only way to get those results is to put it into outer space and open the valve.

Even a 25ml volume can take an hour to finish outgassing at ultra high vacuum.

 

[wkellen]

All of these answers make sense. Thanks!

Lets say I have this pump listed above. Then the 5" pipe running out to a 6.68 cubic feet tank at each machine. On both sides of the tank is a valve to close it off. The tank is vacuumed down to -29 inHg while the valve between the tank and the chamber is closed. Then the valve between the pump and the tank closes and holds the tank at -29 inHg. The chamber then closes with no pressure in it and opens the valve between the tank and the chamber. Now I have 6.93 cubic feet of space (tank and chamber combined) at -27.95 inHg. Now, is there any way to figure out how long it will take to equalize to -27.95 inHg through a rectangular passage that measures .050" x 5.0" or .25 in^2?

 

[EdStainless]

If you have 10 stations you will 10 reservoirs.
Perhaps also an 11th one backing all of them up, this one would be continuously pumped by your large pump.
the others would either be connected the larger reservoir, or isolated from it just before the valve to the chamber was opened.

Contact someone like KJ Lesker and ask how fast a gate valve actuator can open a 4" gate valve.
I suspect that it is nearly 1sec.

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P.E. Metallurgy, Plymouth Tube

 

[wkellen]

@ProEpro "Like has already been said you are completely ignorant about what you are trying to do." Well I don't think it had been put quite like that already.

If I already knew how to do the calculations or knew about vacuum that much, I wouldn't be here on a forum asking questions.

Thanks!

I understand the need of the extra reservoirs. I was also concerned about the gate valve timing. Still not really the question I am asking. The timing I need is the draw down to equilibrium time after the valve opens.

 

[Ron]

For fast operation, don't use a gate valve. Use a butterfly or ball valve.

 

[tygerdawg]

Suggest you contact some of the major industrial vacuum pump manufacturers and start talking to their Applications Engineering teams to help you with the necessary system design guidelines. Busch, Becker, Gardner-Denver, Atlas Copco, many others. Achieving high levels of vacuum in a short amount of time requires significant effort.


It also appears you have not fully developed your required process specifications yet, either. Any discussion without that will be a waste of everyone's time.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[racookpe1978]

You are at the point of near-sonic velocities if you are trying to "equalize" that low a pressure at that fast a speed through those small openings.

But at a near-vacuum state, "near sonic" is also very, very few molecules per second on a "gas basis" - but obviously many trillions on a molecular basis. "Standard gas laws begin failing at that low a pressure, so be very careful if you extrapolate from "usual" pressures and temperatures.

You have a transient state from start-of-valve opening, through end-of-valve-opening through keep-valve-open-while gas-flows towards pump through start-of-valve-close through end-of-valve-closing. Look at the four volumes (valve closed, valve partially open, valve fully open and gas flowing, valve partially closed. At near-sonic speeds, do you not have to have to have enough time open for the "last molecule" at the back of the high-pressure volume to get past the valve outlet to the high-vacuum point before molecules begin traveling backwards?

 

[rb1957]

can you take your equipment to the ISS ? they have a ready supply of vacuum that'll help depressurise your tanks.

another day in paradise, or is paradise one day closer ?

 

[EdStainless]

don't talk to the vacuum pump makers, go to people like Lesker who are accustomed to building systems.
This is really a systems proble.

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P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

The basic math isn't that complicated and you are calculating some odd thing that doesn't make sense. Consider that your 0.25 cf is filled with 1 atm air, and you have an empty tank that's 6.68 cf.

You now have 0.25 cf /(0.25 cf + 6.68ft) = 0.0361, so, regardless of the speed, you got down to 0.036 atm of air, that's not even close to what a roughing pump can do, which is more like a millionth of an atm. The converse is that you need a vacuum reservoir that's 250,000 cf to just get into the range of a roughing pump's best pressure, which is still a long ways from high vacuum.

Now, we look at the physical size of the reservoir, which would be on the order of a 38-m radius sphere (this is already ludicrous, but...) In order to equalize the pressure to get this vacuum, let's assume that your 0.25 cf box is in the center of the sphere, so the farthest any air molecule has travel is 17 m. 17 m / 0.045 seconds = 388 m/s, which is higher than the speed of sound. So, you would need your air molecules to travel at the speed of sound to get barely what a roughing pump can do, which is still about 1000 times higher than anything resembling high vacuum.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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