Evaporation in a Vacuum

No I'm not too sure why... please explain :) Does evaporation occur faster or slower in a vacuum? Also - can heat penetrate and enter a vacuum?

Also - can heat penetrate and enter a vacuum?

Yes, if it didn't then the heat from the sun couldn't escape and would boil the planet and everything on it (which is bad).

Not to mention the fact that earth would be an icy mass of stone where no life could exist.

lynlynch: Not to mention the fact that earth would be an icy mass of stone where no life could exist.

AKA Moscow

" Does evaporation occur faster or slower in a vacuum?" Faster. Do you know why?

Radiant heat can pass through a vacuum. As in energy from the sun that strikes the earth. Conductive and convective heat cannot pass through a vacuum.

Do you know the difference between radiation, conduction and convection?

Your assignment is to define the different types of heat transfer.

We have rules about homework here. One of those rules is that we don't just give you the answer. You have to think.

Also, define sublimation.

QUOTE :) Does evaporation occur faster or slower in a vacuum? Also - can heat penetrate and enter a vacuum?

the surface of a liquid or solid is a barrier to the gas state. To escape from this state each molecule must have enough velocity to do this. The rest of the slower ones left behind have faced the loss of some of their faster brethren = a drop in average temperature. This is the latent heat of vaporization, which is a standard value for each molecular species. In a vacuum the escaped molecule is gone quickly, it has an infinite 'mean free path'. In a non vacuum it is hindered by collisions with other molecules, so it has a smaller 'mean free path'. For a detailed explanation of this go to the kinetic molecular theory.

http://en.wikipedia.org/wiki/Kinetic_theory

Thus evaporation is faster in a vacuum.

A vacuum prevents convective heat transfer = a vacuum = nothing to carry heat away. A vacuum does not impede radiative heat transfer. The nature of the surface radiating into the vacuum does affect it. This is the term emissivity(usually written ε or e). A black body had an e of 1, a perfect reflector has an e of 0. Real materials are in between.

Read here.

http://en.wikipedia.org/wiki/Emissivity

It boils faster and at a lower temperature. see post #23....others have already answered it, but this will give you extra links that may assist you

Yes it can penetrate vacuum, heat is radiation, But Vacuum is a good insulator of sound.

p911

Because boiling point will be much lower when the matter is subjected to vacuum.

But If a sudden change from atm. pressure to high vacuum, what will happen to the subjected liquide - say water or moisture . Will it boil or it become ice?

I wish to get this clarified by somebody.

C K ManojKumar

"Yes evaporation and sublimation do both occur in a vacuum. Do you know why?"

Unless yours is a trick response, I am inclined to answer that if there is a perfect vacuum, then by definition, there is nothing there to evaporate.

If it is not a trick, then I'm inclined to suggest that there is a differential of some kind between that which is there and that which is not.

L.J.

Let me repeat the original question.

"Can Evaporation occur in a Vacuum?"

At no point, until you introduce it, has the word "perfect" been used.

Why wouldn't it?

yes, a fluid has a vapor pressure and if placed in a vacuum it will boil into the vacuum, and cool itself in the process.

Suggest you look here.

http://www.google.ca/#hl=en&safe=off&q=vacuum+distillation+of+water&aq=f&aqi=g1&aql=&oq=vacuum+distillation+of+water&gs_rfai=&fp=a962a42b56405c7b

Yes, for every liquid there a specific boiling point against different temperature.

We know water boils at 100 degree C at atmospheric pressure. And same water boils at higher temperature in pressure cookers (as well in industrial boilers), that is why hard stuffs get cooked well, which do not cook at open vessel (at atmospheric pressure).

The same way, at the lower pressure than atmospheric (partial vacuum) water would boil at a lower temperature than 100 degree C. Interestingly, if you keep applying vacuum to a partially filled vessel, the water (or any liquid) would start boiling at room temperature at one particular negative pressure which is addressed as the 'vapour pressure' of this liquid for that particular (in this case, room temperature) temperature.

the water (or any liquid) would start boiling at room temperature at one particular negative pressure

1) try to give as little hint as possible.

2) If I replace Water with Hg (the conditions met being a liquid) it will not boil.

3) More for the OP to study - study triple points and how the phases are affected as each of the paremeters of the triplepoint are affected.

Hg is not ordinary liquid, it is the only metal in liquid phase at room temperature.

Still, still Hg will also emit it's vapour subjected to high vacuum!

I remember to have heard from my professor saying, "the empty space at the top of Torricelli's barometer is not true vacuum, there would have few Hg vapour in it". I believe him.

quote 2) If I replace Water with Hg (the conditions met being a liquid) it will not boil.

well mercury will 'boil' in a vacuum in free-fall.

boiling that we know, mainly with water, is the response to a heat input and when vapor pressure of the fluid is above the pressure on the surface where the heat is being applied it will boil, ie bubbles will rise from where it is heated and escape towards the surface, and if they do not run through cooler water which will condense them, and we have all seen incipient boiling with vapor bubbles budding off the bottom and collapsing as they rise. In time the entire volume is brought to the boil and the bubbles form on the bottom/sides, etc and transit to the top.

Now mercury will do this in free fall with heat input, but at room temperature the vapor pressure of mercury is low, so it may not look like water boiling, instead it will show up as an increase in volume of the mercury mass. This mass may well detach from the bottom as the heated bottom forms a vapor piston to raise the mass, since with no convection in zero G no bubbles will rise.

The more correct term is phase change.......

I remain unphased

I'll have the admin's turn up the thermostat then......

Question for you:

If you have cylinder that doesn't distort, collapse or release any materials, and pull the plunger until you develop vacuum, but still have 100 meters of bore to keep pulling, would it be any increase on negative pressure?

Yahlasit

Vacuum values are a bit misleading, once you suck all of the air or whatever out of your container you get 1 atmosphere of vacuum. The subsequent pumping by cryo pumps or similar devices are scavenging any remaining molecules out of the container & this makes no measurable change to the pressure.

Dear Yesyen,

What will happen if i create a vacuum higher than the vapor pressure of water- before it boil compleatly? Will it continue to boil or it become ice?

If you heat (say, at the rate of Q joule) some quantity, say one liter of water in a bowl of it boils out completely in certain time. If you raise the heating rate, say to 2Q, don't you expect the time of boiling-out completely would reduce to half the time? Same thing can be expected if you increase the vacuum. Immaterial of any process, either you apply more heat or more vacuum, the boiling will be accelerated.

Hi..

Iam using fibar glass tap and fibare glass bush in a in a high vacuum chamber . What will be the degassing possibility and vacuum deterioration after some time, say after 10-12 months?

C K ManojKumar

This thread is very old.

I suggest you start a new thread.

Most of liquids boils (evaporates) when subjected to vacuum pressure. The same problem occurs at suction of pumps, when the liquid pressure decreases due to suction and reaches its vapour pressure, hence the liquid starts to boils (evaporates) which lead to the famous problem called "cavitation".

Cavitation is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure, and collapsing of these bubbles at impeller blades.

Vacuum evaporation is a technique that is frequently used by manufacturers to preserve food and beverage products. When food or drinks are vacuum evaporated, they can usually be kept for longer periods of time without going bad. Vacuum evaporation may also be used to concentrate these products. For example, evaporated milk is essentially fresh milk that has been vacuum evaporated, resulting in a richer, creamier product that has a longer shelf life than fresh milk. In other words, evaporated milk is a concentrated dairy product.

Also used to deposit alminum on glass to make mirrors for telescopes.

oilcan13

Trick question -Once something evaporates into it, it's no longer a perfect vacuum.

Trick Answer - Nothing / Nobody is perfect.

Also the evaporation in vacuum will be there for only for a moment.

Immediately afterwards there will be a layer of vapour above it and as it moves away more and more vapours will be formed.

First Mr OP.

Please define and understand what is evaporation.

A liquid evaporates under the following conditions and requirements

a)

b)

c)

In vacuum the following conditions are met...

However the following are not met.

These (not met criterions) - are mandatory / not mandatory for evaporation to take place.

Hence the liquid will/ will not evaporate in vacuum.

part 2-

The heat transfer from one point to another ...

(repeat the sequence of part-1)

"Trick Answer - Nothing / Nobody is perfect."

My mum told me I was perfect.

Yes, most definitely.

Alot of refinery's use this concept of vacuum to lower the boiling points to which evaporation (phase change, from a liquid to a gaseous state) can occur at a lower temperature and use less energy for distillation purposes. an example, water which boils at 212 degrees F at sea level and pulling about 28 Hg of vacuum will boil at approx 113 degrees F .

A good WASP (Water and Steam Properties) program will have this and with the phase change point. Heres one as an example

A really good program will have different liquids but you may have to enter some of the thermal properties.

p911

http://en.wikipedia.org/wiki/Triple_point#Triple_point_of_water

Can Evaporation occur in a Vacuum? So long as nothing from nothing equals one. Right?

Evaporation is a big problem in hard vacuum systems, try googling 'outgassing'. Even materials that you would normally consider to be solid can evaporate. We pull vacuum at around 1x10-10 torr & bake the vacuum chamber to 350°C, at these values, for instance, we can't use brass components because the zinc will come out of solution & coat everything inside the chamber.

Dear Sir,

We are facing defficulty of vacuum deterioration in a vacuum chamber. The entair chamber is made out of SS304 material and need to maintain 1-5 mtor vacuum inside this chamber for longer period - say 1-5 years. Initialy we do the vacuum process and seal the chamber at 1 mtor. But gradualy the vacuum deteriorates to aprox 50 mtor in 4-8 months.

Other than the SS304 meterial, a viton "O"ring, 3-4 mtr length of fibre glass tap, one small FRP bush and aluminium foils and insulation papers are used in this vacuum chamber.

We do eleminate moisture compleatly during the vacuum process.

If i consider, there is no vacuumleak ( we conduct healium leak test of this chamber before vacuum process), then do you think any of the above mentioned components including the SS304 material will cause a degasing - If i consider the room temperature of about 40-50 degree celcious and high vaccum that present inside the chamber?

(C K ManojKumar)

There are many ways in which the vacuum can degrade. The viton seal will leak over time (as will any flexible seal). The materials you mention could outgas, do you bake the chamber & contents to drive off impurities before you seal off? Obviously the viton will limit the bakeout temperature. If you have surfaces that fit closely together without providing escape routes for gasses, you can get 'virtual leaks' which will slowly release molecules.

Could you fit a small ion pump to the chamber so that it continuously maintains vacuum?

"If I consider, there is no vacuum-leak (we conduct helium leak test of this chamber before vacuum process)"

You would have blinded (isolated) the vessel/chamber for this vacuum test, then it become a closed system. But during operation, the vessel/chamber is no longer a closed system. Liquid inlet, outlet and vapor outlet, make this, a open system, so more chances for the un-study vacuum. The throughput of liquid, the temperature of liquid inlet, the amount of dissolved gas in the liquid and the ability/capacity of vacuum mechanism (pump or ejector), anything could hamper the vacuum.

I recall an occasion of vacuum pump's worn-out shaft sleeve (though with new gland packing) had substantially reduced the capacity of the vacuum pump (letting air through the gland). There by the original vacuum could not be reached until the pump's reliability was restored.

Dear Yesyen,

We do Helium leak test just before the evacuation process. Day before yesterday, i seald the chamber (of one experimental cylinder) at 02 mtor and started observing the pressure rise. Found 4 mtor increase in 24 hours.

Refering to various advises i received, now iam planing to,

1: Increase the temperature of heat impulse from 100 -150 degree celcious.

2: Arranging a clean room for the pre-assy process.

Let me try this.. hoping for a better result...

Hello Sir,

This is interesting..

In our factory - we have to achieve a vacuum of 1-2 Mbar in the interspaces of vacuum jacketed cylinder. During this process, we simultaneously do the heating of the cylinder to 150-180 degree Celsius. The vacuum chamber consists copper coil and SS tubes that silver brazed and copper MIG welded.

Referring to your explanation, is it possible to come out the zinc from the brazed joints?

Hope to get a reply..

C K M

That is a very soft vacuum & a low temperature so I would of thought it unlikely.