GAS-L: Enthalpy, Extraction and Reheat

[Peter Singfield <snkm@btl.net>]

>Peter,
>You said the boiler had to put "ALL"  1491 BTU/lb in to make the 
>steam, however, the water being boiled already has some enthalpy.
>So your total heat input would be reduced thus your efficiency 
>would increase.

>Gary Harmond

Hi Gary;

Great question. Lets delve into it --

Of course I am pulling all those figures from my trusty old steam charts.
Here is some more info that will throw some lite on your question.

To date -- I have been using just the superheat figures.  Now -- let's look
at the broader picture.

Taking our two examples -- that is a condenser pressure of 2 psi and a
boiler pressure of 400 psi.

Temp.  Press.  Sat. Liquid   Evaporation   Sat. Vapor
               Enthalpy      Enthalpy      Enthalpy  

100    .9492   67.97         1037.2        1105.2

445    400     424           780.4         1204.4

Pressures in psi abs. Heat measured as btu per pound.

The Saturated Liquid Enthalpy represents the heat invested above 32F to
reach that pressure steam.

The Evaporation  Enthalpy represents the heat needed to accomplish the
change of state (Liquid to gas -- vaporization).

Saturated Vapor Enthalpy represents the total heat invested to reach that
state of steam quality.

To go one step further:

400 psi steam superheated to 750 F = 1389 btu/lb

186.6 btu/lb invested in super heat.        

While heating feed water using waste heat (such as flu gasses -- condenser
temperature waste heat not being high enough) does improve over all plant
efficiencies (Your boiler "economizer") -- it has no effect on steam
turbine/engine efficiencies regarding energy in and energy out.

However -- there is a way to "cheat".

This is done by extraction and reheating. This allows turbines (and
compound steam engines) to break the theoretical efficiency levels and is
responsible for those impossible to believe efficiency rates achieved in
the very large power plants.

In simple terms -- it is simply superheating the exhaust steam from one
stage before it goes to the next. 

This breaks the change of state lock -- as water is not being boiled to
steam -- just steam being heated. Ergo -- theoretical efficiency of heat in
to gas expansion is 100% -- not the 19.65% as in my citrus cogeneration
model -- as example.

But to answer your question in a direct manner -- the turbine/engine does
not care what the feed water temperature is in regarding efficiency of its
operation.

Simple -- so many btu's come in. From this amount, so much mechanical
energy is extracted -- the rest goes out the exhaust as waste heat.

In the next posting we'll discuss extraction and reheat (superheat) and
also the problems inherent in achieving this goal -- or why only the really
large plants can afford to play with it. Also -- how I designed an
economical solution to this problem -- which has never been adopted.

But maybe, before that -- we had better start discussing the "theory" of
the Tesla Turbine? 

I have a number of very pertinent point to demonstrate in that domain and
wish to find out just how aware the people involved with Tesla Turbine
technology are of the mechanical theory of a disk turbine. 

For now -- it would appear to me that a Tesla Turbine must be exactly
"tuned" to steam quality and quantity to achieve the proper mechanical
efficiencies. I see no problems with doing this -- and will be advancing
some math models on how to proceed in that direction -- if you all have not
already been there??

We'll get into extraction and reheat during that discussion. 

Now -- can we not define a Tesla Turbine as an impulse turbine with
multiple disks without the buckets??

Impulse Turbines were pretty well the most common type during the time
Tesla designed his disk turbine.



Peter in Belize


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