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Author Topic: Why do superheaters work?  (Read 5304 times)

Offline Geezer

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Why do superheaters work?
« on: 12/09/2011 09:22:45 »
Steam turbines seem to make use of "superheated" steam. Why is that?


 

Offline CZARCAR

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Why do superheaters work?
« Reply #1 on: 12/09/2011 16:20:40 »
dunno. my pressure cooker purports to cook with 1/3 the energy
 

Offline MikeS

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Why do superheaters work?
« Reply #2 on: 13/09/2011 08:00:56 »
Because it contains more energy and can do more more work more efficiently.
« Last Edit: 13/09/2011 11:49:49 by MikeS »
 

Offline CliffordK

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Why do superheaters work?
« Reply #3 on: 13/09/2011 10:12:18 »
Your pressure cooker presumably cooks at a higher temperature,for less time, and thus saves energy.  Completely different.

At 100°C, the steam pressure would be 1 ATM, or essentially the same as atmospheric pressure, and would do very little additional work.

The hotter the temperatures, the higher the pressure.

In fact, a low temperature engine (say under 110°C) would be very inefficient, as the heat of vaporization would be far greater than the additional heat to raise the temperature an additional 10°C to increase the pressure.

As mentioned above, the energy density is greater at the hotter temperatures, which likely means that the generator can be made much more compact.  And, for a large turbine, that might be both a significant cost savings for construction, as well as being smaller, simpler, and more efficient.
 

Offline Geezer

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Why do superheaters work?
« Reply #4 on: 13/09/2011 18:01:00 »
A superheater doesn't do anything for thermal efficiency. It actully reduces it, so what's really going on?

(BTW, by thermal efficiency, I'm referring to the thermodynamic ratio between heat in and work out.)
« Last Edit: 13/09/2011 18:06:32 by Geezer »
 

Offline Geezer

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Why do superheaters work?
« Reply #5 on: 13/09/2011 18:25:10 »

Your pressure cooker presumably cooks at a higher temperature,for less time, and thus saves energy.  Completely different.


Unless you manage to convert all the water in it into steam ;D. At that point, it becomes a superheater of sorts. It's also likely to explode!
 

Offline CZARCAR

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Why do superheaters work?
« Reply #6 on: 13/09/2011 18:56:15 »
http://en.wikipedia.org/wiki/Superheater   .....seems its about avoiding rust?
 

Offline CliffordK

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Why do superheaters work?
« Reply #7 on: 14/09/2011 01:46:57 »
Your pressure cooker presumably cooks at a higher temperature,for less time, and thus saves energy.  Completely different.
Unless you manage to convert all the water in it into steam ;D. At that point, it becomes a superheater of sorts. It's also likely to explode!

All boilers, including pressure cookers should have some kind of pressure relief valve. 

Running out of water means over heating (no longer limited to the boiling temperature of water at whatever pressure you have it).  I've seen aluminum pans with the bottoms burnt out, but generally not as heavy as pressure cookers.  Assuming your cooker has proper safety equipment, I doubt much would happen to the cooker other than charred remains of your meal inside, and potentially a charred gasket.

Air, however, isn't as good of conducting temperature as water or steam, so it really would reduce to just a stovetop oven.

Perhaps you are onto a new cooking utensil though...

The Geezer Super Steamer!
 

Offline CliffordK

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Why do superheaters work?
« Reply #8 on: 14/09/2011 01:59:15 »
http://en.wikipedia.org/wiki/Superheater   .....seems its about avoiding rust?

Excellent wikipedia article.

I don't think it is about rust though.  With modern materials, stainless, brass, bronze, various ceramics, and plastics, one can choose other ways to fight rust. 

The article talks about dry steam and wet steam.  I presume the wet steam is only partially vaporized (having suspended droplets), and dry steam is 100% vaporized.  Since the water is already at or above 100°C, this second step to achieve 100% vaporization of the steam should be relatively cheap in energy.

The end of the wikipedia article notes:

The main advantages of using a superheater are reduced fuel and water consumption

So, engine efficiency apparently is affected.  A mobile application such as a locomotive engine would certainly benefit from reduced water consumption. 

However, I would assume a large nuke plant would have a voracious appetite for water.  So, reducing water consumption might be important, even for a fixed application.
 

Offline Geezer

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Why do superheaters work?
« Reply #9 on: 14/09/2011 02:38:48 »
http://en.wikipedia.org/wiki/Superheater   .....seems its about avoiding rust?

Excellent wikipedia article.

I don't think it is about rust though.  With modern materials, stainless, brass, bronze, various ceramics, and plastics, one can choose other ways to fight rust. 

The article talks about dry steam and wet steam.  I presume the wet steam is only partially vaporized (having suspended droplets), and dry steam is 100% vaporized.  Since the water is already at or above 100°C, this second step to achieve 100% vaporization of the steam should be relatively cheap in energy.

The end of the wikipedia article notes:

The main advantages of using a superheater are reduced fuel and water consumption

So, engine efficiency apparently is affected.  A mobile application such as a locomotive engine would certainly benefit from reduced water consumption. 

However, I would assume a large nuke plant would have a voracious appetite for water.  So, reducing water consumption might be important, even for a fixed application.

A boiler produces saturated steam, meaning its temperature is determined by the boiling point of water at the pressure in the boiler. A superheater takes saturated steam from the boiler and raises its temperature beyond the boiling point to produce superheated steam (or superheated H2O gas if you prefer).

However, the overall thermal efficiency is actually lower (ratio of work done to heat applied) when you superheat the steam. (I can post the numbers if you don't believe me.)

The reduction in water consumption does not seem too surprising. What does seem surprising is that the fuel consumption improves despite the fact that the thermal efficiency is lower.   
 
 

Offline peppercorn

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Why do superheaters work?
« Reply #10 on: 14/09/2011 17:39:39 »
In the main wiki article on Boilers the section on Superheated Steam Boilers says:
"[A superheater] provides steam at much higher temperature, but can decrease the overall thermal efficiency of the steam generating plant because the higher steam temperature requires a higher flue gas exhaust temperature."

Does this mean it is a fundamental property of superheating steam that TE will go down or does it just mean it involves a more complex design of generator?
I think it's got to be dependant on the fuel your using hasn't it? If I'm superheating wet-steam with oxy-acetylene I'm going to have no trouble pushing the steam temps into the superheated region at almost any pressure, no?


From what I read, Wet-steam is better for heat transfer (as you'd expect) and SH steam is better for driving machines (turbines or pistons).

So do the losses in Thermal Efficiency become outweighed (with good design) by the gains in efficiency from having a large increase in kinetic energy?  (aero is a square law, isn't it?)
 

Offline Geezer

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Why do superheaters work?
« Reply #11 on: 14/09/2011 20:07:58 »
The whole business seems to be a bit sketchy. In practice they do seem to work in certain situations despite the fact that a simple thermodynamic model indicates otherwise. It's possibly more to do with keeping the cylinders and pistons hot so that the next shot of steam that goes into the cylinders does not cool so quickly. That's why compound expansion works, but I'm not sure if its why superheaters work or not. Or, it could be more to do with the reduced water consumption than anything else.

Thermodynamics is so counter-intuitive that it's quite normal to get a result that's completely unexpected.
 

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Why do superheaters work?
« Reply #11 on: 14/09/2011 20:07:58 »

 

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