Re: GTs and Submarines

On Fri, 28 May 2010 21:42:17 +0100, "Keith Willshaw"
<keithnospam@xxxxxxxxxxxxxxxxxxxxx> wrote:



"Peter Skelton" <skeltonp@xxxxxxxxx> wrote in message
news:j970069lt7l5gquofpv2vvf309pq5kq93r@xxxxxxxxxx
On Fri, 28 May 2010 13:48:04 +0100, "Keith Willshaw"
<keithnospam@xxxxxxxxxxxxxxxxxxxxx> wrote:


They are only comparable in efficiency at full load. One of
the problems with gas turbines is that they are very inefficient
at light loadings. This is one of the reasons CODAG and
CODOG systems are so popular for ship propulsion.

That's not quite right. GT's have a peak efficiency which, in
aircraft applications is for loads near the cruising speed of the
aircraft, one hell of a lot less than full load take-off. Of
course, a GT cannot sustain take-off load for ever. THere's a
smaller full load rating for the load the engine can sustain for
long periods.

In stationary applications (marine are similar) it may not make
sense to design the gearbox to handle the highest power the
turbine can produce briefly. It may also make sense to tweak the
compressor for a flatter power curve. Back in the day, they could
make a turbine that had reasonable efficiency from half to full
sustainable power (peak around 80% IIRC, and I may not)

If you mean 80% efficiency you do not. The very best gas turbines
in a combined cycle installation can hit around 60%. Simple
gas turbines come in at 30-33% and the best large simple cycle
installations come in at around 45%
+

OFCS, I mean power output. That is very clear froim the context.


or one
that had good efficiency from 2/3 to 90%. Quite likely they can
do better now, but even those old engines had a power range the
utilities would have found attractive.


Indeed because utilities can typically run them at much higher
load , the first major use was of simple cycle engines to
meet peak loads


A wardhip that can make 30 knots might cruise at 18, which would
be 1/3 power or less, well below half power, hence CODAG COCOG
and fancy gearing systems to send one engine's power to both
shafts. The new electric power systems may end this.


It will end the fancy gearing systems but we know that a
number of modern designs use diesel generators for low speed
cruise and cut in the gas turbines for high speed dash.

Will it? What power output cannot be rached efficiently running
some combination of three turbines? The constraint that made the
complications necessary was the need to power both shafts.

Peter Skelton
.

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