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Red Squirrel 03-14-2011 03:53 PM

Steam engines
 
I was thinking, the way a typical steam engine works, there is exhaust steam. This is still very hot. Would it work to just feed that steam into another steam engine, and keep going for a bit, maybe 3, even 10 engines in serries? I imagine eventually the steam is no longer hot enough, but could'nt you just pass it through multiple engines to try to extract every bit of heat out of it? these separate engines would have to work inderpendantly, but if you are doing something such as generating electricity, then it can always be combined after.

all this reading on the Japan nuclear issues and steam engines (which is basically what is generating the electricity in a nuke plant) got me thinking. though from my understanding of nuke plants the steam is just recycled back after being cooled. Is this more efficient? The cooling process is just outputting that heat into the air though.

I can't be the first person to think of this so my gut feeling is that it would not work, but why?

Daniel Holzman 03-14-2011 03:58 PM

This is a very old idea, and is very commonly used in steam turbine generating electric plants. However, the "used" steam is normally utilized to generate hot water or low pressure steam for heating, rather than being used in a second steam turbine, because the efficiency of the second turbine is going to be very low due to fundamental principles of thermodynamics (see Carnot equation for a full explanation). As for steam engines, which you asked about, they are rarely used anymore, except for a few curiosity locomotives and perhaps in some third world countries, they have been replaced almost entirely by gasoline and diesel engines, and turbines as I mentioned above.

Leah Frances 03-14-2011 04:04 PM

In this house we obey the laws of thermodynamics - Homer Simpson.

52Caddy 03-16-2011 09:29 PM

Love the Homer quote!

That's what is done with superheated steam. Heated above the saturation temperature/pressure and it's used more than once, running a turbine and then exhausted and used in another turbine. I'm pretty sure there's not much energy left in the steam when they're done using it. Actually that is what they accomplish through a turbine, each subsequent blade/stage is larger since the steam has lost energy across the first set.

I don't think any nuke plants use superheated steam. Conventional plants use the combustion exhaust to heat the steam above the saturation temperature. In a pressurized water reactor, you get the primary water hot and then generate steam in a steam generator, but you don't have any way to heat it more. Same thing goes for a boiling water reactor.

At least that's what the Navy's nuke program taught me!

ptarmigan61 03-17-2011 01:43 PM

What you describe is a compund steam engine. In the golden age of rail, steam locomotives used compund cylinders for this reason. If you look carefully at some of the period photos, you can see that there are actually two cylinders with pushrod arrangements for the drivers (drive wheels); high and low pressure cylinders. the 4-8-8-2 cab forwards and the 4-8-8-4 Big Boy (7000 hp steam locomotive) run by the Union Pacific are great examples.

nap 03-17-2011 03:04 PM

Quote:

Originally Posted by Red Squirrel (Post 609258)

all this reading on the Japan nuclear issues and steam engines (which is basically what is generating the electricity in a nuke plant) got me thinking. though from my understanding of nuke plants the steam is just recycled back after being cooled. Is this more efficient? The cooling process is just outputting that heat into the air though.
?

As others have explained, with a turbine, there are typically multiple stages within what you see as one turbine to take advantage of the remaining force of the steam as it passes through the turbine. If they could toss on another stage and improve horsepower, they would.

this is a crude drawing but it shows exactly what you are talking about doing:
http://www.ephf.ca/electricity/turbine-generator/

http://www.ephf.ca/media/public/images/coal4.jpg

dberladyn 04-24-2011 11:39 AM

Steam engines are very efficient engines are they not? I always loved the idea of using steam to power some kind of generator... isn't the true waste of energy simply creating the steam?

Daniel Holzman 04-24-2011 05:49 PM

Steam engines which use pistons (this is NOT a steam turbine) are not very efficient. They lose efficiency in two distinct thermodynamic ways. The first loss occurs because the heating of the water using either wood (the old engines) or coal at best extracts about 50% of the theoretical heat in the wood or coal. Additional losses occur due to Carnot inefficiencies in the engine itself. For a theoretical Carnot engine, the maximum theoretical efficiency is E = (T1 - T2)/T1 where E is the efficiency in percent, T1 is the gas temperature at the inlet, T2 is the gas temperature at the outlet. All temperatures must be in degrees Rankine (Fahrenheit) or Kelvin (Centigrade).

The problem with steam engines is that the steam must be exhausted at no less than 212 degrees F (672 degrees Rankine), else it condenses to water and you cannot exhaust it. Typically, you can't heat the steam much past about 500 degrees F (960 degrees R), so this creates a maximum theoretical efficiency of (960 - 672)/960 = 30 percent, exclusive of the heating losses, creating an inefficient engine.

dberladyn 04-24-2011 05:59 PM

I believe you, I just remember reading years ago that they were very efficient at turning heat energy into mechanical... obviously I mis-read or mistook whatever the point was. I can see how burning wood or coal to heat water, only to be jetisoned as hot steam would be an inefficient conversion of energy.


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