Steam turbine efficiency?

    --ISTR reading somewhere that a steam turbine doesn't surpass the efficiency of a reciprocating steam engine until it's big enough to produce something like 50hp. Can someone cite a reference for this? I've head this figure often but someone said 'prove it' the other day so I'm hoping someone knows how..

steamer wrote:
Sorry, that answer doesn't help. But I was interested in that question and thought that the first boat with a steam turbine was called "Turbina", But I found this: <http://www.carweb43.ch/Fiat/F_Typ_Turbina_s3.htm
It's name was Turbinia, by Parson. But that doesn't help either.
Nick

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Yikes!!!! 300hp and a gas turbine only slowed by drum brakes......
Dave

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Have a llok at
http://www.rover.org.nz/pages/jet/jet5.htm
I remember as as schoolboy listening to reports from Le Mans as the Rover turbine came past with the most distinctive whine.
As I rember the main problems were awful throttle lag - 3 seconds delay between depressing the pedal and anything happening, and the problem of getting rid of exhust heat.
Jet 1 was on display at the Science Museum for many years.
Doesn't answer the question but very evocative!

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Well...as you scale things up and down the balance of forces changes, but I suspect the turbine should always be more efficient with steam because it is like a compound engine with each ring of blades being like another cylinder. This means each ring of blades should settle at a fairly fixed temperature and minimse the power-sapping condensation that is associated with each cylinder of a steam engine, where expansive working causes heating and cooling and hence condensation (and hence the old curse that 'expansive working is expensive working').
I have no in-depth knowledge of these things, but I reckon that the modern turbine is the ultimate extension of the compound engine with as many compound cylinders as there are sets of turbine blades and nothing like the amount of mechanical losses associated with having very many cylinders. I was surpised to find in old texts on heat engines just how much steam condensation impairs the efficiency of a steam engine. Its not down to lagging, it down to expanive cooling.
I have been thinking that highly superheated steam through a flash steam boiler might be the answer, as this is the route to getting high temperature difference and hence theoretically high efficiency, but it is tough on the materials on the engine. There are accounts of flash steam engines in model boats glowing red - see the book "Experimetal Flash Steam" for details.
No doubt other more experienced guys here will point out the error in my reasoning.
Steve

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I haven't seen any figures, but those numbers feel reasonable. Bear in mind that a 50hp steam turbine would be a tiny thing. In comparison, we have a typical 8000 hp/7000rpm boiler feed pump turbine from a 600MW set in the shop at the moment and the bladed part of its rotor is about 18" diameter by 3' long. This is a turbine that normally runs in parallel with the IP turbine, so has an inlet pressure of about 500 psi and exhaust pressure of about 100 psi. There is a single row of main steam pressure blades for starting, they have blade heights of about 1/4". This is a very small and not particularly efficient turbine.
Mark Rand RTFM

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I don't have the answer to your question (any more than the other respondents) but a few facts may help put things in perspective.
In the mid 90s (when I was involved as a consultant in the power industry) the maximum thermal efficiency of very large coal-fired steam turbine generating plants was around 36%. I doubt if this has increased much. These large (say 100-200 MW per genset) stationary plants have every advantage they could have - high temperature, high pressure, high mass:surface area ratio, condensing etc. On general principles, any smaller turbine plants will have lower efficiencies.
The theoretical maximum figure for a reciprocating steam engine on the Rankine cycle, operating on 20 bar (290 psi) steam and 400 deg C superheat, non-condensing, is 20.7%. On a condensing cycle, this rises to 29.4%. It seems that even the best practical (full size) locomotive designs do not achieve more than 90% of the maximum non-condensing efficiency, and condensing only gives about 50% of the theoretical benefit, making these figures say 19% and 23%. (Source: Andre Chapelon, "La Locomotive a Vapeur", English edition, p 72.)
The typical overall thermal efficiencies of the very best entrants in the IMLEC competition for miniature steam railway locomotives (as far as memory serves, I'm not about to go find the reports) was in the region of 2-3%.
For comparison, I have seen specifications for very large reciprocating internal combustion engines with efficiencies of around 45%. The most efficient heat engines I have seen are the latest combines cycle gas turbine (CCGT) gensets, which combine a gas turbine and a steam turbine operated from the gas turbine exhaust; these have efficiencies (at least according to the manufacturers) of around 60%. The laws of thermodynamics will not allow much further progress, unless someone can make materials capable of operating at very much higher temperatures.
A note of caution, I am not entirely convinced these different sources are comparing like with like - the Chapelon figures seem to be based on a percentage of the heat content of the steam (i.e. ignoring boiler losses). The power generation figures are though overall thermal efficiencies (electrical energy output versus energy content of the coal or gas fuel) as are, AIUI, the IMLEC figures.
As I say, this does not answer your question, but it may spur someone else to add useful details.
David

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Sorry, combined cycle gas turbine.
David