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Author Topic: By how much does water expand when turning from liquid to gas?  (Read 9256 times)

Offline barneyboy

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when water boils it turns to vapour. In simplistic terms (as I am but a simple man :o), what is the factor of the increase in volume and is there any other liquid or solid, natural or man made that gives similar results at a lower temperature. ?
« Last Edit: 02/02/2014 22:47:25 by chris »


 

Offline alancalverd

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Re: vapour pressure
« Reply #1 on: 06/12/2013 22:43:50 »
About 3000:1 at atmospheric pressure - and pretty much the same for all low-density liquids. Alcohol, ether and petrol are familiar liquids that boil at less tha 100 deg C.
 

Offline barneyboy

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Re: vapour pressure
« Reply #2 on: 06/12/2013 22:48:24 »
why do we not use a liquid that boils at a lower temperature to power our turbines then rather than one that requires more energy being put in to get the same results? (I have a feeling I should have maybe posted this in a different section, sorry :-(
 

Online chiralSPO

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Re: vapour pressure
« Reply #3 on: 09/12/2013 20:39:35 »
In theory you could use any volatile liquid for this process. I suspect the reason people do not is a combination of cost and risk. Water is very cheap compared to liquids like ethanol and methylene chloride (or any other liquid I can think of). There are also no risks associated with flammability, toxicity, carcinogenicity etc.
 

Online Bored chemist

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Re: vapour pressure
« Reply #4 on: 09/12/2013 21:07:54 »
Also, using a liquid with a lower boiling point wouldn't actually help.
 

Offline SorryDnoodle

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Re: vapour pressure
« Reply #5 on: 09/12/2013 23:06:19 »
Also, using a liquid with a lower boiling point wouldn't actually help.

Why not?
I haven't thought about this before, so from my uneducated  point of view it seems logical, Lower boiling point: less heat used = more "steam" = more energy ?

I assume this is not the case, how come?
Genuinely intrigued.
 

Offline alancalverd

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Re: vapour pressure
« Reply #6 on: 09/12/2013 23:15:17 »
Apart from being cheap, nontoxic, nonflammable, and fairly noncorrosive, water has an anomalously high latent heat of vaporisation, so steam contains a lot more available energy per unit mass than most other gases.
 

Offline CliffordK

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Re: vapour pressure
« Reply #7 on: 09/12/2013 23:40:34 »
I was thinking of the heat of vaporization of water. 

The pressure of a gas should essentially be a function of moles of the gas and temperature.

Water (H2O) is actually a very small/light molecule which certainly would have benefits for material handling.  However, one should look at the latent heat of vaporization in moles, rather than in mass or volume, which makes it not too different from many of the other compounds. 

There may, in fact, be benefits of a lower latent heat of vaporization though, as less energy is wasted in converting from a liquid to a gas (and then lost again when condensed).

In some cases, water can be safely released into the atmosphere, and cheaply replenished.  All other chemicals would require condensing for reuse.  This also limits the liquids chosen to allow condensing at ambient temperatures.  No sense in using liquid Nitrogen as the energy to condense the liquid would be similar to that gained from boiling, although the BP can be altered somewhat by pressurizing the entire system.

Many steam turbines run at very high temperatures, and thus minor differences in boiling points and latent heat of vaporization can be minimized.

I have wondered about using a low bp molecule such as pentane BP (36C, 97F), in a water condenser for secondary power generation, but the energy recovered would be minimal.
 

Offline alancalverd

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Re: vapour pressure
« Reply #8 on: 10/12/2013 08:14:08 »
Quote
However, one should look at the latent heat of vaporization in moles, rather than in mass or volume,
Clearly the words of a chemist, not an engineer!

The problem is to transfer as much power as possible from the boiler to the turbine via the smallest volume and/or mass of pipework. Water does this better than anything else I can think of.
« Last Edit: 10/12/2013 08:16:35 by alancalverd »
 

Offline evan_au

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Re: vapour pressure
« Reply #9 on: 10/12/2013 09:57:58 »
why do we not use a liquid that boils at a lower temperature to power our turbines?

There have been proposals to use ammonia as the working fluid in ocean-based turbines using the difference in temperature between the deep ocean (about 4C) and tropical surface waters (around 27C), for a temperature difference of around 23C. For this application, you would like a liquid that boils below 27C, and condenses above 4C - water would not fit these requirements as well as ammonia.

As mentioned in previous posts, there are economic and ecological factors in the selection of the working fluid - a big ammonia leak would cost money to replace, kill the fish, and stink out any nearby tropical resorts.

There are also efficiency factors - turbines are driven by temperature differences - and the wider the temperature difference between the hot and cold parts of the system, the more efficient your turbine can be, as proven by Carnot - one of the earliest results in thermodynamics. 

Water can be obtained from rivers and the sea at "room temperature", and can be heated well above 100C by keeping it under pressure. The temperature can be raised even further with a superheater, allowing a temperature range of over 350C, achieving much higher efficiencies than turbines driven by ocean temperature gradients. All of this can be done at temperatures and pressures that can be contained by common steel alloys.
 

Offline barneyboy

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Re: vapour pressure
« Reply #10 on: 12/12/2013 22:44:05 »
thank you all for your replies, I will give it some thought  :D
 

Offline MrVat7

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Re: vapour pressure
« Reply #11 on: 21/01/2014 23:03:09 »
P= P1 + ( P2-P1 ) x2
 

Offline rasymus

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I don't have all the technical engineering knowledge, but I do work in the industry. Water expands on the order of 1600 times as it becomes vapor.  We send the steam out to do work, as it cools it condenses back to liquid (condensate) and is returned to the boilers to be reused, repeatedly. think of it this way, a boiler producing 20klbs/hr requires 20klbs of water an hour which is app 2,410 gallons of water (2410 x 8.3). So a 2500 gallon tank of water converted to steam will fill miles of steam line before it loses heat and returns to liquid state.  We don't get it all back, so we have a source of "make up water" that is a fraction of the total steam produced daily.  Hope this helps from the practical standpoint.  One more curiosity, water hammer occurs when the steam in a vessel loses enough heat that it spontaneously condenses, this creates a type of void that is instantly filled with more steam and or water.  The results can cause a great deal of damage if left unchecked.  The opposite of this happens when a boiler surface becomes overheated and liquid water contacts it instantly expanding 1600 times often producing boiler explosions. Hope I did not bore all you technophiles.

borrowed from an outside post,

At 212F, 14.7 psia, liquid water has a specific volume of 0.016716 ft3/lbm and steam has a specfic volume of 26.80 3/lbm, which is a volume ratio of ~1603 : 1 of steam:water.


 

Offline joeweller

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I think it will be 1 to 3000
 

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