Re: Coal to Oil Conversion of Naval Vessels

Dennis <tsalagiNOSPAM@xxxxxxxx> wrote:

With about 3 times greater thermal efficiency to boot.

Wiki says that for <= 1000 HP, recips are much more efficient than
turbines. It says this in discussing a recip engine at a recent solar
thermal plant in Australia. Can anyone give more data on lower power,
turbine vs. recip. for steam?

Yes well - I am sure the wiki writer was well versed in
thermodynamics. There are a number of efficiencies involved in power
producing machines.

Mechanical efficiency is basically the heat losses in an engine due to
friction and other minor factors. Here a turbine and a reciprocating
engine can be equal or the one slightly better than the other
depending on design.

Thermal efficiency however is the ratio of heat input to the
mechanical power recovered for the entire system. In a standard
locomotive, which is what was being discussed, you heat water to
steam, drive a piston and then blow the steam out of the chimney. The
average steam loco is effectively around 7 or 8 % thermally efficient.
If you were to recover some of the heat, by using a condensor for
instance, you could increase this efficiency. For instance the SAR&H
ran a number of condensor locos at one time - about 90 or so I
believe. The rationale for these machines was water saving through
our dry semi-desert areas. However they also reduced coal consumption
to some extent because hot feedwater was reducing the amount of heat
that was needed to convert the water to steam.

If you look at a graph of how much heat it takes to heat water from a
starting temperature to boiling point (sensible heat) and then how
much heat it takes to turn that boiling water to steam (latent heat)
you will see that latent heat is by far the more important figure.

In dry steam at normal atmospheric pressure the ratio is roughly
something like 10% sensible heat to 90% latent heat.

In other words if you add 10 kilojoules to the water to get it to boil
you have to add another 90 kilojoules to turn it into steam.

In all steam plants this latent heat is totally lost on each cycle.
Only some of the sensible heat can be recovered. No matter how
efficient the system condensing the steam back to water loses an
enormous amount of heat which is not being used to make mechanical
energy. If only steam could be pumped efficiently this would make
steam plants much more efficient - but that is not easily achieved

The added advantage of a turbine and condensor is that the condensor
is usually kept at quite a high vacuum - which aids the turbine
efficiency as well as the steam moving through the turbine is heading
for a low pressure area - in effect being sucked - which assists the
flow.

Eugene L Griessel

Love does not make the world go around, just up and down a bit.

- I usually post only from Sci.Military.Naval -
.

Relevant Pages

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