steam requirements for an engine

There isn't any external energy input during the expansion.

Oh my, we are dividing the hairs into tiny filaments. I think the problem is more one of nomenclature than thermodynamics. Most modern texts use "isentropic" but the older stuff uses "adiabatic". I've seen "reversible adiabatic", "adiabatic-isentropic" and "reversible adiabatic isentropic", too. And I'm an old fart who uses old texts.

Dunno, there was no arithmetic in my post, just a lookup of tabulated data from a 1916ish "Mechanical Engineers' Handbook". BTW, the table was calculated assuming adiabatic expansion, and in the index of its

2000odd pages there's no mention of "isentropic".

Regards,

David P.

In an engine there certainly is external energy input/output... It's what makes the engine go :-)

They are very different things.. see below for terminology. Trust me I'm an electrical engineer :-).

More gobbledegook:-

The end temperature of an isentropic expansion is lower than that of an adiabatic expansion due to the work that has been extracted from the steam.

Although efficiency is usually stated as a % adiabatic efficiency, this is treating the system (steam plus turbine/engine plus load) as a whole rather than just the steam. As far as the steam itself is concerned, if the expansion were adiabatic it would be because there was no external work done by the steam e.g. by expanding it through the regulator or turbine governor valves.

Another way you can look at it is that a perfectly efficient engine (i.e. as good as Carnot's law will allow) will extract its output power from the steam without adding or subtracting any other heat from the steam. This could be described as "remove the work from the inlet steam _and then_ adiabatically expand to the exhaust condition", but it's more accurate to describe it as exhaust entropy=inlet entropy.

The HP steam turbines that we were producing before we became a French/Swiss/not_very_British organisation were 94% efficient in these terms. We have now improved the current designs to the point where they are about that efficient as well :-|

Have a good one.

regards Mark Rand RTFM

Read my post again. The second sentence. It's repeated just a few lines up. The external energy input is at constant pressure. Then the inlet valve closes and we have a fixed mass of gas doing expansion. No energy input during expansion.

So am I. Never trust an EE. :))

snip

Snipped, I know how to suck eggs. Try to understand that the usage and meaning of words changes over time. You appear to think that an adiabatic process precludes external work output. I'll give you a (obviously true) quote that illustrates my use of "adiabatic". "Statement: Of all the possible adiabatic processes which can be executed by a system starting at a fixed initital state and ending at a fixed final pressure , the reversible one gives the maximum external work".

-Spalding and Cole, "Engineering Thermodynamics" 1958, a book that has been my reference of choice for over 45 years.

Time to shut this down, I'll agree to disagree with you.

Interesting, any idea of the steam conditions at the hot end? I got out when I discovered that the frogs were coming - I'm ex English Electric / GEC.

Regards,

David P.

In article , Mark Rand writes

With some trepidation at butting into someone else's catfight, there seems to be more heat than light here, and I hope a few comments from a chemist will be helpful.

An adiabatic process is defined as one where no heat is transferred in or out. Enthalpy is not the same as energy, and an adiabatic process may result in a change in the free energy of the system, and hence produce (or absorb) some work; in general, the work from an adiabatic process is given by C(v)*(T2 - T1), i.e. the specific heat at constant volume times change of temperature.

All mechanically reversible adiabatic processes are isentropic. Of course, in reality processes are never absolutely reversible*, therefore processes are never exactly isentropic, but many well-designed adiabatic processes approach perfection quite closely. Thus for a turbine, the theoretical maximum work that can be obtained is that for an isentropic process, and the turbine efficiency is defined as W(real)/W(isentropic). This figure is usually about 0.7-0.8, maybe a little more for really good big turbines. Note though that turbine efficiency is only part of the story for any heat engine, and engine efficiencies are invariably lower than this (60% for the very best combined cycle gas turbine electricity plants, less or much less for pretty well everything else).

*To be reversible, a process must be frictionless, must never be moved more than infinitesimally from equilibrium, must be driven by infinitesimal forces, and can be reversed at any point by infinitesimal change in external drivers to give an exact repeat of its previous condition. No difficulty there for real-life design then....

The end temperature of a process is whatever it is; whether it is adiabatic or reversible adiabatic (isentropic) depends entirely on the way it is carried out, and the amount of work obtained depends entirely on how close to isentropic it is (see above). In fact, an isentropic process producing the same energy as a non-reversible (and thus non-isentropic, i.e. real life) process would of necessity leave the steam at a higher temperature (for given final pressure). Otherwise it would be less efficient, not more.

I hope you can now see this is incorrect.

I guess that 94% is what I described above as the turbine efficiency - had not realised they got quite so high - but see my caveat about confusing this with overall efficiency.

Sorry guys, I think you are both getting your Y-fronts in a twist over nomenclature and concepts here; hope the above helps.

David

Note to self:- Write out 100 times, Adiabatic doesn't imply isenthalpic. Isentropic doesn't preclude adiabatic.

Mea Culpa, I'm wrong :-(

I think I've puzzled out where my misconception went haywire... When working out the steam temperature at an HP turbine inlet, one takes the before stop valve pressure and temperature and the after governor valve pressure and calculate the temperature assuming an adiabatic expansion.

The logic behind this is that the steam after the valves won't be well mixed and a thermocouple pocket would be unlikely to give a representative temperature reading.

The other confusion I was labouring under comes from the fact that with an HP turbine, where there is no steam bled off for feedwater heating part way along, and the exhaust steam is still superheated, one can calculate the efficiency without knowing the load or the steam flow. One just compares the enthalpy change of the steam with the enthalpy change for the same pressure drop and starting temperature with zero entropy change. (if I remember it right)

I agree with you... I was wrong!

IIRC still not much different from 585C/160bar (1085F 2300psi) Supercritical is still the exception rather than the rule, but more and more is from heat recovery boilers on gas turbine exhausts at about 400C.

Where from? I'm still hanging on at the Willans works, but spend my time herding computers these days.

Regards Mark Rand RTFM

snip water under the bridge.

Thanks. I might know a bit about the gt stuff. :)

About 20 miles East of you, down the 427, our turbines run on hot air. Lots of fun with PDP8, then PDP11 and 8085 etc for real time stuff, and a pair of elderly KDF9s to do the math.

Regards

David P.