Re: superheated steam

[brian whatcott <inet@INTELLISYS.NET>]

At 18:30 5/12/99 -0500, Kathy wrote:
> Hi all,
> 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.
>
> Thank you.
>
> Kathy Daniel

Kathy received rather few responses to her question.
I can confirm it is not only firefighters who could wish for some
insightful explanations of thermodynamics, which is often
considered to be a 'difficult' topic.
 Kudos to Daniel Schroeder who recommended a 'good read' on the topic.
This must be one of the few.

What facts would be helpful to know, if I were firefighting?
Something about latent heat, and about superheat, certainly.
  Here, I will attempt to touch on these two topics.

If a lump of ice in a sealed box is steadily heated by air just
 a little warmer than the ice's surface temperature, the ice
temperature rises until it approaches its melting point, at which
 time, its temperature holds steady and it starts to melt, then
its temperature rise resumes until it boils. This heating with no
temperature rise is called Latent Heat. Then because the container
 volume is constant, the pressure starts to rise, and the boiling
temperature rises with it. While there is still water left, the
steam is called saturated, and by knowing its temperature, its
 pressure can be read off e.g. from a Steam Table (which is
actually a slim book in size)

If we let a little of this steam escape through a throttle valve,
this steam becomes superheated; that is to say, it is hotter than
saturated steam would be at that reduced pressure.
  When all the water has finally evaporated (which takes seven times
the energy to melt ice) further heating superheats the steam.
   At this point, knowledge of pressure doesn't necessarily tell
you temperature and vice versa. You can only say that the temperature
 can be higher, perhaps much higher, than saturated steam at that
pressure (saturated steam is that which is produced over its water
of formation)

This modest description has already departed from the short hand
definition to be found in a science dictionary: you can see that
steam at a temperature higher than 212F is not necessarily
 superheated, but steam at a pressure as low as one atmosphere
may well be...

A firefighter may  have good reason to be interested in steam,
 because this gas has been used as a fire extinguisher in closed spaces.
 It has a helpful property: it displaces oxygen and won't support
combustion, it is searching so it fills small spaces without
extensive water damage but it has unhelpful aspects too: it turns
to water vapor in a dense fog cloud which hinders vision and returns
the latent heat needed to initially vaporize it, providing a severe
burning hazard.

I would want it pointed out to me that the very best place to start
an enquiry into steam properties is with the Enc. Britannica.
 Reviewing all the article pointers there would provide a more rounded
and comprehensible start than any thermodynamics text is likely to
provide (in my view)


brian whatcott <inet@intellisys.net>
Altus OK