Re: What Future Now For The Railways?

Andrew Robert Breen wrote:
In article <drt4im$1j3j$1@xxxxxxxxxxxxxxxxx>,
Charlie Hulme <info@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
Brimstone wrote:

So steam is not a gas?
Yes it is, but a 'steam turbine' is not a 'gas turbine'. A gas turbine, in its normal meaning, uses combustion products directly to drive the turbine.

Stick with this distinction or you will confuse everyone.

"Gas turbine" is the colloquial term used for a Joule- or Brayton-cycle
engine.

Not quite. The Joule/Brayton cycle implies a closed thermodynamic cycle where the compressor delivery gas is heated, and the turbine exhaust is then cooled before return to the compressor. A gas turbine is an open circuit device in which air is compressed, a fuel is burned in the compressed air (at nearly constant pressure), and the combustion products are expanded in a turbine to atmospheric pressure, using the work liberated to power the compressor, with the excess available for various tasks.

A genuine closed cycle employing a perfect gas as a working fluid has been proposed for a nuclear power cycle in South Africa, using He as the working fluid.

"Steam turbine" is the colloquial term used for a rotary machine
expanding a continuous flow of steam (though the gas phases of other
liquids can be used) via a Rankine cycle.

One difference is that, while "gas turbine" generally means the whole compression-combustion-turbine machine, "steam turbine" often means just the turbine bit. In its most common application this involves use in a rankine type cycle, although steam turbines have been employed in open circuit steam plant (eg the LMS pacific, yay, on topic)

In the "steam turbine" the combustion process is separate from the engine, in the "gas turbine" they're combined.

One other point to bear in mind is that in many steam turbines, condensation will occur at the low pressure end, so you will have both gas and liquid phases present. Steam turbines of course can be used with a non-combustion heat source, such as a nuclear reactor.

Of course the issue of super-critial steam plant is a nice and muddy one. When does a closed cycle GT with heat input from a non-combustion source (as per Joule/Brayton) become a steam plant?

OK, you can wake up at the back there, the thermodynamics is done now.

Robin
.

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