steam in a vacuum?

Dodge M4S · 8th November 2009, 11:24 AM

OK, I know that waters boiling point is lowered when in a vacuum, but does it still create steam?

I had this pretty weird dream that I built a machine with a boiler and and the outlet of the boiler was going to a generator. The machine was placed in a vacuum and the water started to boil wich made steam to turn the generator.

A couple of days ago I was watching a doco on nuclear reactors and i guess some of it got stuck in my head.

Id google it, but I keep getting stupid steam cleaning products.

Foliage · 8th November 2009, 11:35 AM

The energy to create the vacuum and get the water in there would be more than the energy produced by the turbine, but yes in a vacuum you have no surrounding pressure to keep the water together so I imagine it would 'boil'.

SLATYE · 8th November 2009, 1:10 PM

Yes, it does - but it's still a vacuum.

If you have a bottle half filled with water and you gradually decrease the air pressure, eventually (at some pressure P) some of the water will vaporise to make steam. However, making steam obviously increases the pressure, so the pressure will rise until it reaches P again. It won't go above P, because when it reaches P the pressure is too high for water to vaporise.

This makes it useless for powering a generator. Steam in itself has no useful properties here; it only works in a normal generator because the steam is at a far higher pressure than its environment. In this case the pressure of the steam is always P (at least until you run out of water to boil). This will be less than air pressure, because obviously water doesn't boil at air pressure.

Therefore the only way to use this steam in a generator would be to make a better vacuum on the other side of the generator. But then you might as well just use normal air at the inlet - no point in having steam.

Dodge M4S · 8th November 2009, 2:19 PM

Just palm slaped my self lol. I dont know why i didnt think of the steam creating preasure.

Thanks for that

McDaddy_ · 14th November 2009, 10:35 AM

In power stations they pull vacuum on the steam after it has passed through the turbine for a couple of reasons:
1) It increases the effeciency of the steam turbine process as the work done by the turbine a fucntion of the pressure ratio across it. The condensor will naturally reduce the pressure of the steam by cooling it but vacuum pumps are normally also used.
2) It prevents water droplets from forming in the turbine stage of the process. The blades at the LP end can be quite large in radius ~1.3m and spining at 3000rpm which means any water droplets would pit or even pierce the blades.

Dodge M4S · 18th November 2009, 12:43 AM

Thanks I didnt know that

UncleRes · 18th November 2009, 8:44 PM

When I use the vacuum vessels at work, you can literally see the water in the air instantly turn to steam. Its quite a cool effect.

Atleast, I think its steam

8th November 2009, 11:35 AM	#2
Foliage Member Join Date: Jan 2002 Location: Sleepwithyourdadelaide Posts: 23,641	The energy to create the vacuum and get the water in there would be more than the energy produced by the turbine, but yes in a vacuum you have no surrounding pressure to keep the water together so I imagine it would 'boil'. __________________ I like to construct strong views on random things, and then argue for absolutely no reason about them. Last edited by Foliage; 8th November 2009 at 11:39 AM.

8th November 2009, 1:10 PM	#3
SLATYE SLATYE, not SLAYTE Join Date: Nov 2002 Location: Canberra Posts: 25,773	Yes, it does - but it's still a vacuum. If you have a bottle half filled with water and you gradually decrease the air pressure, eventually (at some pressure P) some of the water will vaporise to make steam. However, making steam obviously increases the pressure, so the pressure will rise until it reaches P again. It won't go above P, because when it reaches P the pressure is too high for water to vaporise. This makes it useless for powering a generator. Steam in itself has no useful properties here; it only works in a normal generator because the steam is at a far higher pressure than its environment. In this case the pressure of the steam is always P (at least until you run out of water to boil). This will be less than air pressure, because obviously water doesn't boil at air pressure. Therefore the only way to use this steam in a generator would be to make a better vacuum on the other side of the generator. But then you might as well just use normal air at the inlet - no point in having steam. __________________ Main system: Phenom II X4 920 \| 8GB (4x 2GB) DDR2-800 \| Gigabyte M57SLI-S4 v2.0 \| Leadtek Geforce 9600GSO 384MB \| Enermax Modu82+ 525W \| 1TB Hitachi HDD \| 3.5" + 5.25" FDD Laptop: Compal EL80 \| C2D T7200 \| 320GB Fujistu HDD \| 2GB DDR2-667 \| GF Go 7600

14th November 2009, 10:35 AM	#5
McDaddy_ Member Join Date: Sep 2001 Location: Newcastle Posts: 612	In power stations they pull vacuum on the steam after it has passed through the turbine for a couple of reasons: 1) It increases the effeciency of the steam turbine process as the work done by the turbine a fucntion of the pressure ratio across it. The condensor will naturally reduce the pressure of the steam by cooling it but vacuum pumps are normally also used. 2) It prevents water droplets from forming in the turbine stage of the process. The blades at the LP end can be quite large in radius ~1.3m and spining at 3000rpm which means any water droplets would pit or even pierce the blades. __________________ "True ignorance is not the absence of knowledge, but the refusal to acquire it." - Sir Karl Raimund Popper Last edited by McDaddy_; 14th November 2009 at 10:38 AM.

18th November 2009, 8:44 PM	#7
UncleRes Member Join Date: Nov 2002 Location: Adel Posts: 3,735	When I use the vacuum vessels at work, you can literally see the water in the air instantly turn to steam. Its quite a cool effect. Atleast, I think its steam