Re: superheated steam

[John Denker <jsd@MONMOUTH.COM>]

Hi Kathy --

I understand that firefighters must be keenly interested in what happens
when people etc. come into contact with hot air, hot water, and steam.

So let me discuss that topic in general, rather than "superheated steam" in
particular.  Looking up that term in thermo books and steam tables is
probably barking up the wrong tree.  A simple reason is that firefighters
are unlikely to encounter anything significantly different from atmospheric
pressure, whereas typical references cover the vastly more general and
complicated cases of changing pressure.  A more serious reason is given in
item (7) below.

Here are some easy-to-understand facts that I think are relevant to the
real world of firefighting:

0) This discussion assumes near-atmospheric pressure, unless explicitly
stated otherwise.  This is because ordinary building structures, even those
that are considered "closed structures" in the vernacular sense, are not
closed enough to support a thermodynamically-significant pressure
difference.  If your buddy really is interested in things like the
thermodynamics of high-pressure tanks, please clarify the question.  That
would be a really unusual subcategory of firefighting.

1) Below 212F, you never have pure steam, but rather a mixture of steam and
air, which is normally just called humid air, or if you prefer, steamy air.

2) Above 212F, you might have pure steam, but more likely another mixture
of steam and air.

3) If you take dry air and cool it from 500F (a possible fire temperature)
to 100F (skin temperature) it will give up a certain amount of energy, as
shown in the following graph.


    Temperature  |
             500 |     /  (start here)
                 |    /   (cooling is down and to the left)
                 |   /
                 |  /
             100 | /
                 |_^____^___________________________________________
                  after before
                          energy content

The slope of this curve is set by the universal gas constant: 8.3 J / mole
/ K.

4) In contrast, if you take pure steam and cool it through the same
temperature range, it will give up a vastly greater amount of energy, as
show in the following graph.


    Temperature  |
             500 |                                             /
                 |                                            /
                 |   __________________...___________________/
                 |  /
             100 | /
                 |_^___________________...______________________^______
                  after                                        before
                          energy content


The length of the flat spot on this curve is set by the latent heat of
condensation of the steam: 4500 Joule per mole.  Notice the huge increase
in the difference between the "before" and "after" energy contents.  The
"..." is a warning that this curve is not even to scale;  the latent heat
is about 25 times greater than the total energy given off by the cooling of
dry air from 500F to 100F.

5) Let's now discuss steam/air mixtures, which are vastly more common than
pure air or pure steam.

Whereas a single number (temperature) pretty much suffices to describe the
energy content of pure air, to describe a mixture you need two numbers: the
temperature and the dewpoint.   Dewpoint is measured in degrees, just like
temperature.  Dewpoint is related to our everyday notions of humidity, but
it is a vastly more sensible and clear way to think about it.  Some examples:
     a) Suppose on a spring evening the wind is nearly zero, the
temperature is 70F, and the dewpoint is 65F.  As the temperature falls
during the night the dewpoint will remain pretty much constant, so long as
the same number of water molecules are in the air.  Any object that cools
below 65 will almost certainly have dew form on it.  If the air itself
cools below 65, fog is likely to form, if there are sufficient condensation
nuclei.  When the temperature and the dewpoint are equal, this is what we
call 100% relative humidity.
    b) The pure steam above a pot of boiling water has a dewpoint of 212F.
    c) Indeed at any temperature where pure steam exists, it has a dewpoint
       of 212F. (We continue to assume atmospheric pressure.)
    d) A 50/50 mixture (by volume) of steam and dry air has a quite
       high dewpoint, probably up around 190F but I haven't worked it
       out exactly.

Why is this relevant?  Because for a steam/air mixture, the flat spot in
the cooling curve occurs at a temperature equal to the dewpoint.

6) The safety ramifications of this are clear:
  a) Steamy air has a very, very great power to heat up any surface (e.g.
skin) that is cooler than the dewpoint of the steamy air.
  b) In contrast, steamy air is no more effective than dry air at the same
temperature at heating up an object if the object's temperature is already
at or above the dewpoint of the steamy air.

7) One can easily imagine that the topic of "superheated steam" would
attract a lot of attention because the words sound sensational all by
themselves.  But the facts are much less sensational.  Superheated is a
technical term that refers to a situation where you put relatively more
heat and *less* moisture into the boiler.  The steam that comes out in that
case has a relatively high temperature and a relatively low dewpoint.  You
can then run it into your steam engine, or your steam-heat coils, or
whatever, and it will (for a while) remain steam rather than condensing.
Superheated steam is dry steam, the opposite of wet steam.

If we extend the terminology to steam/air mixtures, we see that a
superheated mixture is relatively dry (relatively low dewpoint) so it is
actually much less dangerous than a high-dewpoint mixture at the same
temperature.

I sympathize with firefighters' need to have an attention-grabbing term to
describe something as dangerous as high-dewpoint steam, but ironically
"superheated" is exactly the wrong term.

I've been trying to come up with candidates for a replacement term:
  * "high-enthalpy steam" is correct but sounds a bit too pretentious, and
is awkward to pronounce.
  * "high-energy steam" is also correct but maybe not sensational enough.
  ? Anybody else got a suggestion?  I'll continue to think about it.

8) I haven't said anything about the pressure differences caused by boiling
and condensing steam.  They exist, but I don't think they are of
particularly great interest to your buddy.

======

So, is this even close to what was desired?

Cheers --- jsd


At 06:30 PM 5/12/99 -0500, Kathy Daniel wrote:
>        I have a volunteer firefighter freind who is trying to educate
> himself
> and others concerning the conditions necessary in a closed structure for
> the formation of superheated steam. He has some engineering background
> and is willing to purchase a good thermodynamics text which might
> address this and other related questions which he has regarding
> firefighting and thermodynamics. Can someone help with info for a text
> or for an information source which he might obtain.