Subject: A little help with thermo....

From: [email-masked] (Steve S)
Date: Mon, 02 Sep 1996 23:22:27 -0600

After 20 years my thermo is a little rusty. I'm trying to relearn it from
my college textbook (Thermodynamics, JP Holman, 1974) to understand this
solar/steam stuff a little better. Can someone tell me if I understand the
following stuff correctly?

1. Internal energy, u. - basically a measure of the thermal energy - the
energy stored in the random kinetic motion of the molecules.

2. enthalpy, h. -  the internal energy plus the work available because the
gas is compressed.
To determine the work available between two states, compare the
enthalpies. (I'm thinking in terms of moving a piston here.)

If I'm wrong, correct me in public so no one follows me down a dark road.

Please e-mail me if you'd be willing to answer an occasional question. I
won't ask till I've thought.

Thanks in advance,

Steve S

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From: All locked up and nowhere to go <[email-masked]>
Date: 3 Sep 1996 15:41:55 GMT

>energy stored in the random kinetic motion of the molecules.

Close enough for government work.

>2. enthalpy, h. -  the internal energy plus the work available because the

Enthalpy = internal energy plus the product of pressure and specific volume.
(Multiply pressure in N/m^2 by volume in m^3/kg, you get N-m/kg, or specific
energy.)

>enthalpies. (I'm thinking in terms of moving a piston here.)

Not quite.  Enthalpy is used when the fluid crosses a boundary, e.g. steam
flowing out of a boiler.  The pressure-volume product is a transfer of
energy which has to be accounted for.  In hydraulic systems, nearly all of
the energy transfer is from the pressure-volume product, a negligible amount
is from internal energy changes.

If you are heating, expanding, cooling, and compressing a fluid inside a
closed cylinder, you have nothing crossing any boundaries such as nozzle
apertures and you'd use the internal energy throughout.
 
>If I'm wrong, correct me in public so no one follows me down a dark road.

I'm always happy to help put the public discussions on a firmer foundation.
I post publicly when I have the ability and time.