Re: Superheated Steam

[Philip Zell <Zell@ACT.ORG>]

To PHYS-L Members,
        I am trying again to transmit my article on Superheated Steam. Sometime
ago Kathy Daniel secured help from some of you about steam and
firefighting. I want to ask your help to be sure that the physics is
correct. 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. Also a respected fire instructor told
me that steam is not superheated unless it is pressurized. Is this correct?

The Myth of Superheated Steam
by John D. Wiseman, Jr.





> 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 for the following reasons

        >(1) Clockwise rotation is safer because it drives smoke, gases, and
> flames
> away from the nozzle. Counterclockwise rotation does just the opposite.
        >(2)Clockwise rotation produces steam with an active rolling action.
> Counterclokwise rotation produces steam with an inactive and lazy action.
        >(3)A clockwise rotation produces a faster knockdown time.

> The scientific reason for these differences is not known.


     What type of pumps are used to supply water?  Don't centrifugal pumps
     supply rotation to the water in the hose?  If centrifugal pumps are
     used, could the preference for one sense of rotation be a resonance
     effect?  That is, if the water in the hose possesses rotation, then
     must the nozzle rotate in the same direction for the water to leave
     the nozzle with the highest possible speed?

> "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. A good example is a heating system
> with a steam boiler. A major difference in this system is that it operates
> at a much higher pressure than atmospheric pressure.


     It sounds like you are saying that superheated steam is used for
     heating systems.  Steam boilers that are used primarily to produce
     heat typically do not employ superheated steam; rather, the steam is
     at low pressure (say 5 to 15 psi gauge pressure) and saturated.
     Superheaters are found in electrical utility and industrial boilers
     used to make steam for power production.  Wet steam would quickly
     erode the blades in turbine generators. Superheating avoids erosion
     and permits a high rate (lbs/hr) of steam injection into turbine
     generators.



     Philip Zell
     zell@act.org