Re: Superheated Steam

[brian whatcott <inet@INTELLISYS.NET>]

At 15:03 8/27/99 -0500, John D Wiseman wrote:
> ... I am especially interested in knowing if there is any research
> that can explain the difference between clockwise rotation of a
> fog nozzle versus a counterclockwise rotation.

I had not previously heard about the different effects of opposite
rotations of a fog nozzle.  Your description is most interesting.


 Also a respected fire instructor told
> me that steam is not superheated unless
it is pressurized. Is this correct?

Superheated steam is defined as the gas phase of water at a
temperature higher than its boiling point.
(This is what the Steam Tables call 'The Vapor Phase'.)

 That means it has somewhat more capability of heating objects which are
below its boiling point than the gas phase which starts out only from
the local boiling point (which in this case is called 'saturated steam').

 But your interest will be more focussed on steam/air mixtures, I'm sure.

I expect your fire instructor was aware that water boils off
at 32 degF when the pressure is as low as 0.089 psi
and at 98degF at a pressure of 0.89 psi.    You may recall that the
method of vacuum desiccation has been used to preserve biological
specimens, even defunct pets. (Though it's not usually thought of
 as 'boiling'.)

   In practical applications, steam usually exists under some pressure
at or above atmospheric, so that at a pressure as high as 3090 psi for
example, superheated steam will exist at some temperature above the
saturation value of 700 degF.

>    A second safety factor involves the expansion of water to steam
> by a ratio of 1,700/1 at 212oF.

This ratio is given in my Steam Tables (Keenan et al) as 26.80/0.01672,
the ratio of the specific volume of the saturated liquid to the
saturated vapor. This works out to 1603/1.


> ... Iowa State University... Keith Royer
> and Bill Nelson. They discovered that the best way to use a fog nozzle on a
> confined fire is to rotate the nozzle rapidly in a clockwise manner as
> viewed from the nozzle position. This clockwise rotation is far superior to
> a counterclockwise rotation ...


This is, as I say, a fascinating result.

>   In this analysis of water behavior and fire behavior, no mention has been
> made of superheated steam. It is not necessary to do so since both
> evaporation and condensation of water occur at a T of 212oF. It is a fact
> that superheated steam is much less dangerous to firefighters than high dew
> point steam, or steamy air. "Superheated steam" is a technical term used to
> describe a steam-air mixture with a relative high temperature and a
> relatively low dew point, compared to a normal steam-air mixture.

This paragraph is not quite correct. Superheated steam is the gas phase
of water at temperatures higher than the local boiling temperature.
It is not, strictly, a mixture with air.

 All the same, it is clear you are referring to a mix of steam and air.
 The safety issue is that the higher the proportion of steam versus air
the more the potential for burning exposed flesh. Your sentence quoted
beneath describes this danger.


> It is the transfer of heat by
> condensation at the dew point that is the real danger to firefighters.



brian whatcott <inet@intellisys.net>
Altus OK