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Unread postAuthor: psycix » Thu Oct 30, 2008 9:23 am

If you want to drink the water like when making tea, then you should not vacuum it. Boiling doesnt kill bacterias, but the temperature does. So besides boiling, you need to achieve certain temperatures anyway.

Btw, if you got a partial vacuum steam vapor on about 90 degrees C, it will condense immediately as soon as you open it up and let the pressure rise.
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Unread postAuthor: mega_swordman » Thu Oct 30, 2008 10:22 am

My original intent was to get water at a boiling temperature quickly. I believe I used to term "boiling" too freely. In short, yes, the vacuum is OK because I want the temperature, not the gas.

In regards to the partial vacuum, if you remove air, then boil the water, won't the steam just return the pressure to sea level status? Thus as far as a creating a source of steam for a gun application, it would be useless to have a vacuum in the first place.
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Unread postAuthor: Ragnarok » Thu Oct 30, 2008 10:50 am

mega_swordman wrote:My original intent was to get water at a boiling temperature quickly. I believe I used to term "boiling" too freely. In short, yes, the vacuum is OK because I want the temperature, not the gas.

Well, a vacuum WILL lower the boiling temperature. If you need water at a high temperature, it would be wise to add a little pressure, and certainly don't take it away.

To give an example, at 1/2 atmospheric pressure, water will boil at roughly 81 degrees C.
If you increase the pressure to 2 atmospheres, water will boil at about 120 degrees.

In regards to the partial vacuum, if you remove air, then boil the water, won't the steam just return the pressure to sea level status?

At 100 degrees C, and only at 100 degrees, yes. Above that, pressure will exceed atmospheric. Below, it will fall below atmospheric.
100 degrees C is when the vapour pressure of water is one atmosphere, thus allowing it to convert to a gas under regular air pressure.

Take away pressure, it boils at lower temperature. Add pressure, it boils at a higher temperature.

It's the basic principle of the pressure boiler of a steam locomotive. Trap the steam, keep shoving the temperature up, you can easily get hundreds of psi.
Now - take away some steam to run the locomotive. This drops the pressure, but the water in the boiler is already hot enough that some more flashes to steam, replacing it. Clever :)
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Unread postAuthor: mega_swordman » Thu Oct 30, 2008 6:38 pm

Ragnarok wrote:It's the basic principle of the pressure boiler of a steam locomotive. Trap the steam, keep shoving the temperature up, you can easily get hundreds of psi.
Now - take away some steam to run the locomotive. This drops the pressure, but the water in the boiler is already hot enough that some more flashes to steam, replacing it. Clever :)


Interesting. If I am understanding this correctly, with the design I presented, I I first pump most the air, boil the water, and put in a check valve to move the steam out of this system, I could essentially create a steam generator (or a device that generates steam).

This project is turning out to have a lot more potential than I first thought.
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Unread postAuthor: Ragnarok » Thu Oct 30, 2008 7:40 pm

mega_swordman wrote:I could essentially create a steam generator (or a device that generates steam).

Well, yes... you would expect that from boiling water.

There is no need to pump out the initial air, and in fact, doing so is quite moot, even if you want to get pure steam from the generator. Water has a lot of dissolved gases in it which come out on heating (or over time. This is why you can get "stale" water). They didn't have to do these things on steam engines.

The best solution if you absolutely need PURE steam is just to boil it up and accept you lose the first X amount of steam to vent the air out. If you don't, don't fuss with it, just boil it up. It doesn't matter if the pressure in the boiler is steam pressure or air pressure.

~~~~~

But back to the boiler - that's actually what they did with fireless locomotives (for working in flame risk area). Fill them up with water at a couple of hundred degrees C or so, then send them off. As they went, the steam they used was replaced by some of the water flashing off, so they could actually keep moving for a considerable time with no additional energy input.

Just remember you will need a lot of temperature to achieve something similar, so plan for that - for example, don't be a half wit and make your boiler out of PVC. Also, don't touch it when it's running. (Warning: Boiling water is hot!)
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Unread postAuthor: mega_swordman » Fri Oct 31, 2008 12:28 am

Ragnarok wrote: (Warning: Boiling water is hot!)


Gee, Imagine that :roll:

Anyway, thanks for the help, I really appreciate it.
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Unread postAuthor: psycix » Fri Oct 31, 2008 5:47 am

Besides being hot, steam and water burn deeper in your skin then oil or something solid, as your skin can allow water to get in it (as a sponge).
Thats why boiling oil can cause less severe burns then boiling water, even if its way hotter.
And on top of that: Boiling water is indeed hot, but steam under pressure can be way hotter and make sure you do not get a flash of steam in your face when venting off pressure or something.
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Unread postAuthor: jimmy101 » Fri Oct 31, 2008 11:04 am

psycix wrote:Besides being hot, steam and water burn deeper in your skin then oil or something solid, as your skin can allow water to get in it (as a sponge).
Thats why boiling oil can cause less severe burns then boiling water, even if its way hotter.

I believe the difference between steam (not hot water) and oil is that the heat content of steam at 1 ATM and say 120C is much greater than is the energy content of something like oil at the same temperature. The oil would be a liquid with a heat capacity some thing like 1 cal/g/degree C. The heat capacity of steam is only something like 0.5 cal/g/degree C. But, and this is the big but when working with steam, the heat of condensation of steam is huge, roughly 540 cal/g (2260KJ/Kg). So steam doesn't really burn because of the energy associated with cooling off the steam, it burns because of the huge amount of energy the steam releases when it condenses to liquid water.

In steam engines that vent the steam after the power stroke the steam must condense to liquid during the power stroke otherwise the efficience of the engine is extremely poor, down around a few tenths of a percent. The big energy store in steam is the heat of vaporization, not the change in temperature.

A similar situation occurs with ice. If you want to store "negative energy" in water by freezing it, the big energy storage is in the heat of fusion (melting) of the ice and not in changing the temperature of the ice or the resulting water. So when using ice for cooling the important aspect is that it is ice. It doesn't matter much if the ice is at -20C or -1C, the amount of energy it will absorb is about the same.

Going back to steam, the same thing applies. Steam at 110C contains darn near the same amount of energy as does steam at 100C. The heat capacity (~0.5cal/g/C) tells you how much energy you get in cooling the steam from 110C to 100C. For 1g of steam you would get 5 calories of heat as it cooled from 110C to 100C. When the steam condenses (the temperature stays the same as it condenses) you get 540cal/g. For 1g of steam you get 540 calories as it condenses. So the condensation released about 100 times more energy than did cooling the steam from 110C to 100C.
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Unread postAuthor: psycix » Sun Nov 02, 2008 10:34 am

Good point.
The energy required to boil water into steam is indeed incredibly much.
We once did an experiment at school, where we heated up water with a constant rate, and measured the temperature every x amount of time.
Taking the water up to 100* C was done within a few minutes. Waiting it to boil up and thus all convert to 100* C steam took well over 20 minutes I believe.
This made me look like "wtf" and that way I realized how big the heat of vaporisation/condensation is.

And all the energy one has put in, will indeed get out as it condenses.
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