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Re: superheated steam



After reading the last few posts about superheated steam, I have a couple
of questions (...that are probably far removed from the original inquiry).

On Sun, 23 May 1999, brian whatcott wrote:

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 the container truly was capped so no air could escape, then I suspect
the water wouldn't boil at all (although it would still evaporate). After
all, the air pressure would always be greater than the saturation vapor
pressure (since water vapor would be continually added to the air that is
already there) thus squashing any vapor bubbles in the water. Can someone
set me straight on this?

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)

My understanding is that, at regular temperatures (say, less than 100F),
water vapor makes up only a small percentage of the air. This is because
any pressure greater than a few percent of an atmosphere would be
supersaturated and the excess would condense. However, at 212F and above,
the water vapor can exist at pressures greater than 1 atm and thus can
displace the air (and oxygen) out of the region. Again, I'd appreciate if
someone can let me know if this is correct.

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 wonder if anyone else is confused between water vapor and steam. My
guess is that...

water vapor = the gaseous phase of water when the temperature is less than
212F. Water vapor can exist at any temperature, but the amount that can
exist depends on the temperature, e.g.,
at 212F, 1 atm can exist
at 0F, 0.006 atm can exist
(relative humidity is the ratio of how much vapor is present relative to
how much can exist at that temperature).

steam = the gaseous phase of water when the temperature is greater than
212F?

superheated steam = steam at pressures below its saturation pressure, (or
at temperatures above its dewpoint)? For example, if the temperature is
212F, more than 1 atm can exist; if the gas is allowed to expand such
that the pressure remains at 1 atm, there is less steam than that required
for saturation.

Is steam synonymous with water vapor except for the arbitrary line of
distinction at 212F? Again, I'd appreciate if someone can either confirm
these definitions.

By the way, what do we call the "fog" we see above a boiling kettle of
water? According to the above definitions, we can't see water vapor nor
steam (they are gases). Thus, we see the water droplets that form as the
steam/vapor cools below its dewpoint.
Personally, I'd prefer to call that "fog" the steam and leave water
vapor as the word to describe the gaseous phase of water no matter what
the temperature. At least that way we it would be easier to explain to
students that clouds are made up of water droplets, not water vapor (or at
least not any more water vapor than is present down here under the
clouds).

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| Robert Cohen Department of Physics |
| East Stroudsburg University |
| bbq@esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
| **note new area code** |
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