Vacuum test

[Peter_1]

I have a problem with a vacuum-pump test.
I was showing my son the behavior of water when it is vacuumed with a 2-stage pump.
So, I pored a glass of water in a drinking glass bottle and attached the vacuum to it.

There was also an electronic vacuum gage connected to it to let him show what happens with vacuum pressure if you stops th vacuum and the pressure rise to the corresponding saturation pressure and remains then stable.

We started the pump - vacuum hose of 3/8 - and the 5 cm (2 inches) water in the bottle started to boil really heavy after +/- 20 seconds.
Waw.

But during 2 minutes, you saw that the boiling became lesser and lesser and after 2, 3 minutes the boiling stopped completely although the vacuum pressure was still the same.
We were then well below the saturation pressure of water at 20 °C.

We changed oil in the pump- it was new oil the 1st time - but same result.

So we let it ran for 2 hours and there was perhaps every 4 to 5 minutes one bubble coming out of the water. But the level didn't drop at all.
We also tried to heat the bottle but this gave no result.

We repeated the test with 2 other pumps and always the same bad result.

The first heavy boiling we saw was probably the enclosed gasses that evaporated. But as soon those were gone, the boiling process stopped.

Why is the process stopping at a sudden? I can make a small movie of this but for the moment, it doesn't work at all like I had hoped.

So my son made of course the right observation: if there's water in the system, then it isn't removed by the pump :confused:

PS is it gage or gauge pressure?

[Brian_UK]

gauge.

I'm thinking on the main part....

[Argus]

.

Are you using a vacuum pump with gas ballast for this exercise?

If not, I have an idea that the vapour will partially condense in the oil in the pump. As soon as the oil in the pump becomes saturated with water, the pump efficiency falls away rapidly as the pump is now trying to clear it own water-logged system.

You may prove this with either preserving the vacuum in the subject chamber and doing an immediate oil change or substituting another pump without breaking the vacuum and noting any changes.


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Citation (http://www.chevy-wiki.com/wiki/Chevrolet_Citation)

[Brian_UK]

Peter, from what I've read this morning on the web about vacuum the concensus is that "Water is not a good thing to have inside a vacuum chamber".

I think that we all knew that however ;)

Some items from my reading are...

Quality of the vacuum seal on your test chamber.
Back leakage through the oil seal of the pump.
Low pumping duty as the pressure falls.

As you say the vapour pressure should have kept it bubbling in your minds eye but with the low pumping effect on a small test chamber perhaps it shouldn't.

How much heat did your add to the bottle and what was the water temperature ?

A couple of sites which have some interesting technical discussion docs are:-
http://www.belljar.net
http://www.fnrf.science,cmu.ac.th/theory/vacuum/index.html

[NoNickName]

Argus is right: as soon as the oil is saturated with water, the vacuum pump will not work properly.
You will never be able to vacuum a bootle with water. At 25°C the saturated water vapour pressure is 23.76mmHg or 31 mbar or 3100 Pa. As soon as pressure drops below 3100Pa new water will evaporate to keep the system in balance, until no water remains.

The boiling you saw in the early stages is just air diluted in water.

But this opens my mind to another question: what oil are you guys using in your vacuum pumps? I don't want to be ripped off by Edwards or Galileo, and would like to buy the appropriate oil from the oil producers.

[Brian_UK]

The other thing with water is the surface tension which has to be broken to allow the molecules to jump about towards the vacuum pump inlet hose.

Under 'normal' conditions you need 500 microns just to break that surface tension.

[US Iceman]

That's interesting Brian. If an ultrasonic device was used to aggitate the water would that help?

[Peter_1]

.

Are you using a vacuum pump with gas ballast for this exercise?

If not, I have an idea that the vapour will partially condense in the oil in the pump. As soon as the oil in the pump becomes saturated with water, the pump efficiency falls away rapidly as the pump is now trying to clear it own water-logged system.

You may prove this with either preserving the vacuum in the subject chamber and doing an immediate oil change or substituting another pump without breaking the vacuum and noting any changes.

.
It are all pumps with a gas ballast but I tried another pomp without releasing the vacuum.
Also changing the oil.
And the problem arises real fast, aftersome minutes.

But, if oil gets contaminated with water, then I shouldn't hold the initial low vacuum point I had at the startup.
Or is this a wrong statement?

[Peter_1]

You will never be able to vacuum a bootle with water. At 25°C the saturated water vapour pressure is 23.76mmHg or 31 mbar or 3100 Pa. As soon as pressure drops below 3100Pa new water will evaporate to keep the system in balance, until no water remains.

The boiling you saw in the early stages is just air diluted in water.



The pump is reaching real fast 45 microns or 0.06 mBar, and then more slowly (+/- 3 minutes) to 35 microns so far below the saturated pressure at 20°C.

The heat we added was with a heatgun on the bottom of the glass bottle, right on the water.

That was also what I thought, diluted air and other gasses like nitrogen.

[Peter_1]

The other thing with water is the surface tension which has to be broken to allow the molecules to jump about towards the vacuum pump inlet hose.

Under 'normal' conditions you need 500 microns just to break that surface tension.

We even shaked continuous the bottle - well at least for a minute - because my son had read something about teh surface tension. Result was teh same.
Have others tried this already because this is something basic, something we do each day without thinking if it actually works fine?

[US Iceman]

Hi Peter,

Another fascinating question. What better way to teach, than to show.

Here is what I found...




Chemical Concept Demonstrated: Pressure's effect on the boiling point of a liquid
Demonstration:

A few boiling chips are added to a flask half-filled with water.
The flask is stoppered and a vacuum hose is inserted in the stop****.
Turn on the vacuum pump.http://chemed.chem.purdue.edu/demos/demosheets/graphics/boil.gif
Observations:
Without the aid of additional heat, the water inside the flask begins to boil. After a certain time, the boiling may cease.

Explanation:
The boiling point of a liquid is defined as the temperature at which the vapor pressure of the liquid is equal to the external pressure. When the pressure above a liquid is reduced, the vapor pressure needed to induce boiling is also reduced, and the boiling point of the liquid decreases. At 250 C, water boils at an external pressure of 24 mmHg. As the water boils, heat is lost due to the heat of vaporization of water, which is 40.88 kJ/mol. Eventually, the water may cool to a temperature at which the vapor pressure is less than the external pressure provided by the pump, at which point the water will stop boiling.


I hope that helps.

[Peter_1]

Well, the pressure we reach is far below the equivalent temperature of the actual water temperature, especially when we heat it with a heatgun.
If it stops boiling at a certain pressure, then it must be at that corresponding temperature, in our case far below freezing temperature.
That's the reason why they install cold traps at -100 °C (-148°F) in vacuum chambers, to hold trap the last water molecules 'flying ' in the chamber.

If I find some time, I will repeat the test and make a small movie of it and post it somewhere

As some know, we service some coldtraps in huge vacuum chambers. They reach a vacuum from which we only can dream of.

If there are sometimes water droplets on the window of the chamber, then it goes sometimes so fast that the water at a sudden freezes.
Mostly you see the droplet expanding in a few seconds and then 'exploding' and vanishing.

I've asked the operator to put once a glass of water in the chamber just in front of the water and vacuum it once and look what happens.
This operator isn't aware of the boiling process at lower temperatures
The last time I was there, he was afraid to do it because he thought it could harm the machine :D

But, perhaps this could be a very interesting test and that is running once an empty chamber only with a glass half full of water.
I will ask it to his chief if they can do it.
http://www.lsbu.ac.uk/water/phase.html

[pendlesteve]

We actually carry out this demonstartion here. What you find is the oil is of course ruined after just one demo' and has to be drained. However there is always contaminated oil that lodges in the pump body so we rinse out the pump a few times with new vacuum pump oil then refill and everythings fine. Also remember any components between the glass bottle and the pump (line, manifold, vacuum gauge etc) will be contaminated with moisture.
If you're going to do this demonstation, buy plenty of oil.

[Peter_1]

A agree that water diluted in the oil will rise vapor pressure but the pressure we see on our digital and analog vacuummeter is far below the vapor pressure at 20°C (actual water temperature)

So, even if there's water in the oil, the pressure we have should normally low enough to let the water boil.

For me, it's the pressure above the surface that counts, no matter with what means this vacuum is reached.

The water in the bottle with a vacuum above the surface, low enough to let it boil doesn't know that there's water in the pump further in the circuit.

Or is this wrong and if so, why is it wrong?

Pendlesteve, how much water you put in the reservoir?

I don't know the answer but it seems that the right answer is not that easy.

[monkey spanners]

I've always wondered how milking parlours wash out all the milk lines with water at 95C and not have all the water vaporise, but none of the parlour fitters i've asked can give a satifactory answer. You do get huge clouds of water vapor out the vac pump exhausts though. I think they have to circulate the water for 5 or 10 minutes at about 85C. Some of the bigger parlours use compressed air to chase the last of the milk or water out.

I went to the yellow jacket tools day at the wholesalers and the guy from yellow jacked said it can take two or more flushes of oil to get your vac pump working properly after bad contamination, as he then demonsrated on Pete's pump which took three changes to get a good reading on the vac gauge:eek:

How many people are going to be trying to boil water with their vac pumps this weekend:D

Jon

[Peter_1]

The technician did at the plant with the vacuum chamber at a very low vacuum did a test for me.
He placed in front of the window a glass bottle, half full of water.

He started the pumps and watched what happened.
Soon after starting up (+/- 30 seconds), the water started to boil.

Then it started boiling so heavy that water splashed out the bottle so that only 1/3 was left in the bottle.
Some of the ice droplets on the window vaporised.
But after a minute, the droplets had formed ice and remained ice.
The water in the bottle turned almost immediately in ice.

What remained in the floor fooze then almost immediately and the level didn't decrease, even after 30 minutes of pumping.

So, the pump I use isn't going for some reason not low enough, altough the digital gauge indictates a low enough vacuum level.

He suggested that we should repeat once the test when I'm there again (+/- 250 km or 155 miles from here) Perhaps I then have to make a small movie of it.