On 23/04/17 16:39, Chris Hogg wrote:
On Sat, 22 Apr 2017 22:46:33 +0100, Tim Streater
wrote:

In article , Johnny B Good
wrote:

On Sat, 22 Apr 2017 12:13:31 +0100, charles wrote:

In article , Tim Streater
wrote:
In article , The Natural Philosopher
wrote:

I suspect that the boilers and engines you are familiar with are
more nineteenth century technology than twenty-first century. A
titanium tube flash boiler can supply steam at up to 4,000 psi and
650C. Ceramics allow engines to work at those temperatures and
pressures and achieve efficiencies not even dreamed of with
conventional materials.

Exatamondo. I cant remember what the law is called, but the hotter the
working fluid (steam in this case) is to start with, and the colder
the final exhaust, the more efficient is the engine.

Which is why a combined cycle gas turbine that starts with 1000C gases
in the jet engine, then heats a boler to get steam, and has a final
after condenser temp of around 50C, willnet you over 60% thermal
efficiency.

One of the laws of thermodynamics IIRC - efficiency is related to the
difference between input and output temps - in degrees K, not C.

my elementary physics suggests that if you are only refering to a
temperature difference the answer is the same in °K and in °C.°

The main benefit of using deg K instead of Deg C is the complete and
utter absence of negative numbers by which to confuse the mathematics. :-)

It's true enough that as far as steam engines are concerned, this is
unlikely to effect calculations involving deg C, but there are other heat
engines designs based on fluids with much lower freezing and boiling
points than zero deg C (the triple point of water to within an accuracy
of one decimal place).

The formula for efficiency *requires* K.


+1

From the wiki entry for the Carnot cycle I posted elsewhere in this
thread, the theoretical maximum efficiency of a heat engine is given
as 1 - Tc/Th, where Tc is the lower temperature and Th is the higher
temperature, both in kelvins http://tinyurl.com/alle4tg (Equation 3).
For a heat engine where the operating temperature is say 600°C (873K)
and exhausting into ambient of say 20°C (293K), you get very different
results if you attempt the calculation in °C compared to calculating
in kelvins, 0.97 for the former, 0.66 for the latter. It is the latter
which is correct.

A quote from that article:

"Carnot realized that in reality it is not possible to build a
thermodynamically reversible engine, so real heat engines are even
less efficient than indicated by Equation 3. In addition, real engines
that operate along this cycle are rare. Nevertheless, Equation 3 is
extremely useful for determining the maximum efficiency that could
ever be expected for a given set of thermal reservoirs".

Note that the basis for the formula depends on a relationship between
energy and temperature that is not necessarily obeyed if you go through
a phase change.


--
Truth welcomes investigation because truth knows investigation will lead
to converts. It is deception that uses all the other techniques.