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Vacuum Flow Meter for Compressed Air

10/09/2018 6:49 AM

Hello,

I just came across an application where I have to measure the flow rate of compressed air that is being sucked out of tank and we have to install a flow meter on a line coming out of tank.

Thermal Mass flow meter is little expensive, so I am looking for best suitable flow meter which could measure the flow rate on this vacuum application.

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#1

Re: Vacuum Flow meter for compressed Air

10/09/2018 6:57 AM

If the air is compressed it suggests it's above atmospheric pressure, so why do you need to "suck" it out?

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#4
In reply to #1

Re: Vacuum Flow meter for compressed Air

10/09/2018 7:49 AM

Following are details what i got; Fluid is air but when they to empty the tank, they make sure to suck all the air so they want to install a flow meter for this

Line Sizes: 8”

Max. Flow rate: 6000 m3/h

Max. Temperature: 50 C
Max. Pressure: 10 bar g
Fluid: Compressed Air

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#17
In reply to #4

Re: Vacuum Flow meter for compressed Air

10/10/2018 5:49 AM

Whats wrong with air in tank?

They never get all air out, but check vacuum pumping for information.

Install a flow meter on the outlet of the pump and you see how much comes out.

John is right, no suction needed if container has higher than atmospheric pressure.

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#18
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Re: Vacuum Flow meter for compressed Air

10/10/2018 9:26 AM

We need more detail about what you're trying to achieve.

If the object is just to get the tank to as close as possible to 0bara, and it starts at 10barg or anything above atmospheric ~ 1bara, first thing is to vent it to atmosphere before starting any vacuum pumps.

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#2

Re: Vacuum Flow meter for compressed Air

10/09/2018 7:04 AM

Another one of those non-numerical posts. It's time to put it right.

Questions:

  • Flowrates?
  • Pipe diameters?
  • Pipe materials?
  • Temperatures?
  • Pressures?

All the usual stuff. Plus:

  • Why is air <...flow rate...> of such interest, when the pressure, or absence of it, in the <...tank...> is far easier to measure?
  • Please describe the pressure rating and the volume of the <...tank...>?

because this quest sounds like bullshine at the moment.

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#3
In reply to #2

Re: Vacuum Flow meter for compressed Air

10/09/2018 7:48 AM

Following are details what i got; Fluid is air but when they to empty the tank, they make sure to suck all the air so they want to install a flow meter for this

Line Sizes: 8”

Max. Flow rate: 6000 m3/h

Max. Temperature: 50 C
Max. Pressure: 10 bar g
Fluid: Compressed Air

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#5
In reply to #3

Re: Vacuum Flow meter for compressed Air

10/09/2018 8:21 AM

Actually, <...they...> need to install a pressure transmitter, so that the tank is empty when the reading approaches 0bara. The flowrate of a fluid that varies in density is not terribly useful as a way of ensuring emptiness.

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#7
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Re: Vacuum Flow meter for compressed Air

10/09/2018 9:59 AM

It would seem that measuring the flow is not the correct approach. If you start with 10 bar and your flow meter has a 1 percent error, you could have 1.5 psia left in the tank. Measuring the tank pressure would be much more accurate and probably cheaper.

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#6

Re: Vacuum Flow Meter for Compressed Air

10/09/2018 9:38 AM

Why do you need to add another meter to this at all? There is an orifice at the junction of your tank and evacuating device. Just measure or look up the size of this orifice. The pressure differential across this orifice will allow you to calculate the flow.

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#8

Re: Vacuum Flow Meter for Compressed Air

10/09/2018 10:30 AM

If you "suck all the air out of a tank", this will happen!

Railroad tank car vacuum implosion

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#10
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Re: Vacuum Flow Meter for Compressed Air

10/09/2018 3:12 PM

...if it is not rated for full vacuum, perhaps.

Perhaps the <...they...> have already thought of this, though at the moment it's impossible to tell; perhaps the original poster has disappeared up the vacuum hose never to return...

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#11
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Re: Vacuum Flow Meter for Compressed Air

10/09/2018 10:42 PM

Perhaps only a small part of the original poster has been subjected to vacuum, in which case his motives are a whole new subject.

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#14
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Re: Vacuum Flow Meter for Compressed Air

10/09/2018 11:08 PM

This post is illogical and not realistic. The OP is clueless. Whoever, "they" is has no idea either. "All of the air" means a hard vacuum, and the result is pictured already. Unless this imaginary tank is built to withstand 14.7 PSI external pressure.

In any case a flow meter is useless for this folly.

Until "they" explain what "they" are really trying to do this is a waste of time.

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#9

Re: Vacuum Flow Meter for Compressed Air

10/09/2018 12:19 PM

I would put a valve in the main line with a metered bypass....when the tank approaches 0 psi close the valve and measure the flow in the bypass line...

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#12

Re: Vacuum Flow Meter for Compressed Air

10/09/2018 10:48 PM

A rotameter. (Plus maybe a pressure gauge and some computation.)

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#13

Re: Vacuum Flow Meter for Compressed Air

10/09/2018 11:03 PM

Weigh the tank. You can get digital personal scales for around $10. Good for maybe 180 - 200 kg. Use a flexible hose.

When the weight stops going down, the tank is empty.

Though you may not always get the same 'empty weight', if there is condensation in the tank.

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#15
In reply to #13

Re: Vacuum Flow Meter for Compressed Air

10/09/2018 11:31 PM

Stuart, I suspect the change in total tank weight from full air to empty air will be less than the display resolution of almost any commercial weighing device.

If your post is humor, I applaud. Along similar lines, in our main factory we have a large volume central vacuum tank and a relatively small vacuum pump, this combination supporting several isostatic molding stations. The vacuum-rated tank came from the food processing industry initially and has two sightglasses built in. You would faint at the number of visitors who ask if the sightglasses allow us to see how much vacuum is in the tank. The calculated answer commonly is "Not unless we switch on the tank's internal lights".

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#16
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Re: Vacuum Flow Meter for Compressed Air

10/10/2018 4:22 AM

Almost right, Rel PL.

I have a medium size 2 cylinder compressor (bought 20 years ago for $95 - & running well now, as I write this. It has had an oil change, though - IIRC)

It looks to me to be about 2 cu ft volume - 0.07 kg air, at STP. The resolution on my (consumer) digital personal scales is 0.1 kg. So, more likely than not to register.

P.S. Where can I buy those lights?

P.P.S. How do you handle the pain when the sight of the vacuum hits the back of the retina?

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#22
In reply to #15

Re: Vacuum Flow Meter for Compressed Air

10/11/2018 5:53 PM

Air at [loosely defined] ambient temperature and pressure has a weight of around 1.2kg/m3. Its effect is slight and usually swamped by other effects in everyday occurrence. However:

  • for precision weighing as an extreme example, the partial buoyancy of the test weights in air has to be taken into account, as the weights will give a different outcome in a pressurised environment, in ambience, in a partial, or in a total vacuum.
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#19

Re: Vacuum Flow Meter for Compressed Air

10/10/2018 11:36 AM

If they fill the tank, shut off the fill and then empty it then you could use pressure and volume of the tank as well as exit pipe size (as this will ultimately determine flow rate if tank is large enough) to calculate the flow rate. That would just require a very inexpensive digital pressure transducer and display with minor mathematical scaling option.

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#20
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Re: Vacuum Flow Meter for Compressed Air

10/10/2018 1:30 PM

"If they fill the tank." How will anyone know when the tank is "full" of air? Air is compressible. Is there a sight glass? A level indicator?

A tank with one molecule of air, still has "air" in it. How full is full?

Did you catch this?

Line Sizes: 8”

Max. Flow rate: 6000 m3/h

Max. Temperature: 50 C
Max. Pressure: 10 bar g
Fluid: Compressed Air

If they want all the air out of the tank they can just fill it completely full of water!

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#21

Re: Vacuum Flow Meter for Compressed Air

10/10/2018 4:53 PM

This thread would start better with a description of the process and an explanation of why <...they...> need all the air out of this tank and why it can't just be filled with water instead. Get writing!

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#23

Re: Vacuum Flow Meter for Compressed Air

10/25/2018 8:55 AM

I agree that the op should restate the question giving details of the process involved.

If the tank must be purged of all oxygen before refilling,then an inert gas should be used to flush the system.

Unambiguous answers require clear questions.

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#24
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Re: Vacuum Flow Meter for Compressed Air

10/25/2018 9:47 AM

No, they don't.

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