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Author Topic: How do steam engines work?  (Read 4904 times)

system

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How do steam engines work?
« on: 21/06/2011 19:01:02 »
This week we are in Cambridge’s Museum of Technology to explore the engineering of an iconic bit of coal-fired power – the steam engine...
Read a transcript of the interview by clicking here
or Listen to it now or [download as MP3]
« Last Edit: 21/06/2011 19:01:02 by _system »


 

Offline Geezer

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How do steam engines work?
« Reply #1 on: 21/06/2011 19:29:16 »
They missed a couple of points.

Because the boiler is under pressure, the temperature of the water and steam in the boiler is much greater than 100°C, and because the steam engine is a heat engine, the higher that temperature is, the more efficient (and dangerous) the engine becomes.

Multiple expansion stages (compounding) improves the efficiency of the engine, but the reason why is not necessarily obvious. If it was simply a case of allowing the steam to expand, the same effect could be obtained from a single large cylinder instead of going to the trouble of having multiple cylinders.

The reason multiple stages is effective is because of the cooling effect. The steam is still quite hot when it leaves the first, high-pressure, cylinder, so the metal in the cylinder and piston is also hot. This reduces the cooling effect on the next shot of steam to enter the cylinder, allowing it to do more work as it expands. 
 

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« Reply #2 on: 21/06/2011 20:08:05 »
The reason multiple stages is effective is because of the cooling effect. The steam is still quite hot when it leaves the first, high-pressure, cylinder, so the metal in the cylinder and piston is also hot. This reduces the cooling effect on the next shot of steam to enter the cylinder, allowing it to do more work as it expands.

Modern steam turbines sometimes employ reheaters before the steam goes into an intermediate pressure turbine stage.  Were such methods used in steam engines as well?
 

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« Reply #3 on: 21/06/2011 22:43:58 »
The reason multiple stages is effective is because of the cooling effect. The steam is still quite hot when it leaves the first, high-pressure, cylinder, so the metal in the cylinder and piston is also hot. This reduces the cooling effect on the next shot of steam to enter the cylinder, allowing it to do more work as it expands.

Modern steam turbines sometimes employ reheaters before the steam goes into an intermediate pressure turbine stage.  Were such methods used in steam engines as well?

I'm not sure if they were. It would not be very easy to organize something like that on a locomotive, but it ought to be possible on a stationary engine. If it works with a turbine it should also work with a piston expander.

Locomotives started to use superheaters extensively around 100 years ago, so that would help, but compound expansion pretty much died out, in the UK at least.

BTW, there were several attempts at building locomotives with steam turbines rather than pistons, but they did not seem to catch on. One example was the LMS Turbomotive  http://en.wikipedia.org/wiki/LMS_Turbomotive
 

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« Reply #4 on: 22/06/2011 03:55:19 »
Spoke too soon! The French engineer Chapelon actually did reheat the steam -

"Subsequently, in order to maintain a more constant temperature throughout the cycle, Chapelon successfully applied re-superheating between HP and LP stages plus steam-jacketed cylinders to an a "test-bed" freight locomotive, 160 A1 (tested 1948-51)."

http://en.wikipedia.org/wiki/Compound_locomotive#The_thinking_behind_compounding
« Last Edit: 22/06/2011 03:57:59 by Geezer »
 

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« Reply #5 on: 22/06/2011 17:27:27 »
It would not be very easy to organize something like that on a locomotive, but it ought to be possible on a stationary engine. If it works with a turbine it should also work with a piston expander.

I never even occurred to me that anyone would be bonkers creative enough to stuff a reheater into a steam loco but it seems they did!

BTW, there were several attempts at building locomotives with steam turbines rather than pistons, but they did not seem to catch on.
One word: Gearing! :D
Turbine-electrics have had a little more success I believe, but of course these were Gas turbines.
 

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« Reply #6 on: 22/06/2011 20:59:44 »
Sounds like the Turbomotive worked rather well. I think it fell victim to the approaching end of steam more than anything else. Steam engines were incredibly inexpensive to build compared with diesel locomotives, but they were very labour intensive to operate.
 

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« Reply #7 on: 22/06/2011 21:12:22 »
Sounds like the Turbomotive worked rather well. I think it fell victim to the approaching end of steam more than anything else. Steam engines were incredibly inexpensive to build compared with diesel locomotives, but they were very labour intensive to operate.

What they needed was fly-by-wire steam trains! ;D
 

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« Reply #8 on: 22/06/2011 21:16:41 »
Sounds like the Turbomotive worked rather well. I think it fell victim to the approaching end of steam more than anything else. Steam engines were incredibly inexpensive to build compared with diesel locomotives, but they were very labour intensive to operate.

What they needed was fly-by-wire steam trains! ;D

Guess what! http://en.wikipedia.org/wiki/SR_Leader_class
 

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« Reply #9 on: 22/06/2011 21:26:48 »
+1 Good Link :)

Should be known as fly-by-gear...
 

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« Reply #10 on: 23/06/2011 01:53:43 »
+1 Good Link :)

Should be known as fly-by-gear...

It used a chain drive  ;D
 

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How do steam engines work?
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